laser and optics lab manual

8/12/2019 Laser and Optics Lab Manual

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Laboratory Manual

Laser and Optics Section

Project QCC TechASCEND

This material is based upon work supported by the National Science Foundationunder Grant No. 0206101

Any opinions, findings, and conclusions or recommendations expressed in thismaterial are those of the author(s) and do not necessarily reflect the views of theNational Science Foundation.


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Table of Contents

Topic PageIntroduction 2

#1 Alignment Skills 3

#2 Refraction and Total Internal Reflection 10

#3 Lenses, Image Formation, and Telescopes 18

#4 Reflection of Polarized Light 25

#5 Spectroscopy 35

#5B Monochromaters 47

#6 Analog Oscilloscopes 54

#7 Prisms 56

#8 Beam Expanders and Spatial Filters 60

#9 Holography 66

#10 High-power Laser Demonstration 74

References 76

Budget 77


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Introduction

This lab manual is designed for a program which originated at Queensborough

Community College to introduce you to careers in technology in the areas of lasers and

fiber optics. You will be working with state-of-the-art equipment that is both costly and

fragile. Please be careful when operating any of the equipment set up for your

experiments because if you are not careful you could be injured and the equipment

could become damaged. The following guidelines will help you to use the equipment

properly and safely (See reference #5):

*When you enter the room and find the equipment set up on the lab bench, DO NOT

TOUCH anything until the instructor has explained the experiment and the safe use of

the equipment

*If you want to try out something with the equipment, please ask your instructor for

advice.

*Do not force anything to move beyond its normal operating range. In other words, if a

knob doesnt seem to want to turn anymore in one particular direction, do not try to

force it

* You will receive and sign a separate sheet dealing with laser safety, but it is so

important that it will be repeated here: NEVER AIM A LASER AT ANOTHER PERSON

Please be ready to begin as soon as the session begins. If at all possible,

everyone should arrive at the lab promptly. Otherwise, you will miss the explanation

of the lab exercise for that week.

Finally, please try to attend each session.

We are here to help, so please, ask questions and enjoy the project!


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Information about careers in technology can be found at:

http://www.qcc.cuny.edu/physics/lfot/LFOT_FAQs.asp

Laboratory #1: Alignment Skills Exercise

Whats Cool About Alignment Skills?

It may not sound like much, but being able to align a laser beam is something

that most people cant even imagine doing. Surveyors do it to measure distances much

more accurately than they could with the old equipment. Anyone working in a laser

company, building, repairing, testing, and installing laser systems must have good

alignment skills. Alignment skills even come into play if you work for a gun company

that uses laser guided scopes! The military uses lasers to accurately aim at their

targets, so their equipment must be built and maintained by someone who knows how

to align a laser beam. Being able to align a low-power laser can also be used to get a

high-power laser up and running. Basically, almost any job in the laser field requires

good alignment skills. This lab will show you how to get started.

Background

Laser beam alignment, and alignment of optical components in general, is a very

important skill for anyone wishing to work with optical systems. Alignment means

getting the laser beam to follow the path that you need it to follow in order for a device

or experiment to work properly. For example, when a laser is being built, usually

another laser is used to align the new laser in order to get it running for the first time.

Alignment is also important for people using surveying equipment which can contain

lasers in order to make a measurement, the laser must be aimed at the correct target.

Finally, if you are setting up an optical experiment involving many mirrors, beam

splitters, lenses, and other components, they all must be aligned so that the laser used in

the experiment will be properly directed through the various mirrors and lenses.
http://www.qcc.cuny.edu/physics/lfot/LFOT_FAQs.asp


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Optical alignment requires skill, but also patience and steady hands. It requires

the use of a specific method, not just random adjustments in the hope of stumbling onto

the right position. It also requires very small corrections when the system gets close to

final alignment, so it is important not to get impatient and tweak things too much whenyou start getting close! If you follow the method described in this write-up, you will

begin to develop good alignment skills. It, of course, takes a lot of practice to get

proficient at it, but every time you do it, it gets a little easier than it was the previous

time. If you have trouble, dont give up! Take a break, then ask for help from the

instructor.

Procedure

In this exercise you will learn to use mirrors to direct a laser beam along a

specific axis and through two pinhole apertures. This is very much like the process

used to align a new laser that is being built. This experiment is divided into three parts.

In part one, you will align the laser beam to pass through two pinholes set up in a Z-

pattern. In part two, you will adjust the path of the laser beam to not only pass through

the pinholes, but to follow a straight-line Z-pattern mapped out by masking tapebetween the laser, mirror, and pinholes. In part three, an extra pinhole and mirror will

be added and you will align the laser beam to pass through all three pinholes.

The experimental setup is shown below in Figure 1:

HeNe Laser mirror 1

mirror 2

pinhole 1

pinhole 2white card


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Figure 1 Experimental Layout for Part OnePart One

As shown in Figure 1, you will need to manipulate two mirrors in order to make

the laser beam pass through both pinhole apertures. This is because you need to control

both the position and the angle of the laser beam. Do not make any adjustments on the

laser itself this was set by the teacher and should be left alone. Also, do not move the

pinholes.

First, you will rotate the first mirror holder gently clockwise and counter-

clockwise to make the laser beam pass through the first pinhole and strike the second

mirror. You may also slightly tilt the first mirror within the holder to make the laser

beam go up and down a little bit, but you do not have a lot of play in this direction. If

the laser beam is set at the correct height for the pinholes, you should not need to tilt the

mirror too much. By looking at Figure 1, you can see that the horizontal adjustment of

mirror 1 will control the position of the laser beam as it strikes the first pinhole, and the

horizontal adjustment of mirror 2 will control the position of the laser beam as it strikes

the second pinhole. You must alternate between adjusting the position of both mirrors

until the beam is lined up with the both pinholes the vertical height will only be

slightly adjusted by slightly tilting the mirrors in their holders, but again, they cannot

and should not need to be tilted too much if the laser height is set correctly. Alternate

back and forth between rotating the mirrors clockwise and counter-clockwise to get the

laser beam through both pinholes. You may also have to slide the mirrors left and right

a little to keep the laser beam on them. If you have any question as to the vertical

alignment of the laser with the pinholes, ask your instructor. Do not attempt to move

the laser yourself. Hints about alignment will be given in class and are shown below,

although you only really have to worry about it in one direction: horizontal.


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As an example, let's say that your laser beam is misaligned horizontally as follows, and,

lets insert a second pinhole to demonstrate the alignment procedure:

The beam is passing through the first pinhole, but is not passing through the second.

The way to fix this alignment problem is to first raise the position of the laser beam byrotating mirror 1 counter-clockwise and then tilt the laser beam downward by rotating

mirror 2 clockwise to obtain a level beam which should be horizontally lined up with

the bottom pinhole:

**Note that the first step of raising the position of the laser beam appears to make the

alignment problem worse, but it is the only way to fix the alignment problem.** Keep

this example in mind as you work with your mirrors - when something is misaligned,

you must adjust both position and tilt. With your equipment, you can do both by

rotating the mirror mount clockwise or counter-clockwise, but you may occasionally

have to also slide the mirror left or right to keep the laser beam on the next mirror.

Remember to always be aware of where laser beams go when you change their

path and do not ever look straight into the path of a laser beam, even a weak one. Make

mirror 2

Raise: Tilt:


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sure your beam is not shooting across the room or onto anyone elses table. If it is, use a

book or anything handy and safe to block the stray beam, which is not part of your

experiment, form going off of your table or assigned work area. Finally, the slight tilt in

the vertical direction is not what is meant by tilt here this is for the horizontalalignment only as said before, if the set-up was initially correct, there should be no

need for massive vertical alignment.

Part Two

Once the horizontal alignment is completed, you must refine it by making sure

the laser beam follows a taped pathway that your instructor will have on the table

before the lab begins. The taped pathway will trace out a Z-pattern from the laser to the

white card at the end of the set-up. Not only must your laser beam pass through the

two slits, but it must also always be directly above the masking tape on the table. This

requires you to refine your adjustments a little, gives you some more practice, and

makes sure everyone in the lab group gets a turn. Laser beam alignment is not difficult,

but it is tedious and it gets MUCH easier if you practice it a lot. In the beginning, it will

seem to take a long time, but if you put in the effort, soon, you will be able to align a set-up like this in minutes. Practice is the key, so if you get frustrated, remember, everyone

does at first, and you will get better if you do not give up. Take a break, let someone

else have a turn, then try again, but before this lab is over, make sure every group

member is capable of getting that laser beam through both pinholes. You will need

these alignment skills for future labs, and especially if you would like to become an

optical lab technician!

Part Three

If you have successfully completed parts one and two of the experiment, try this:

add a third mirror in place of the white card, and a third pinhole after that mirror. Place


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the white card at the end. Now, repeat the above procedures to make the laser beam go

through all three pinholes and strike the white card at the end. Make sure everyone in

the group can do this, and before you begin, mess up the alignment so you must start

from scratch with the first two pinholes. Again, practice, practice, practice!

Equipment List

From the Daigger Sci-Ed Warehouse Catalog, 2005

www.daigger.com, 1-800-621-7193

Catalog Location Number Name Price Quantity

p. 75 DH3529A HeNe Laser, 0.5 mW $410.00 (1)p.75 DH3361A Light Box & Optics Set $85.09 (2)

p. 76 DH3509 Economy Optics Kit $24.00 (4)

From the Edmund Optics America Optics & Optical Instruments Catalog, 2005

www.edmundoptics.com


p. 144 G56-879 4x4 pos. lift $146.00 (1)

This equipment list provides the optics holders (Economy Optics Kit), round mirrors,

laser, white card with holder, slits (Light Box & Optics Set - slits can be taped to form

pin-hole apertures and held with holders from Economy Optics Kit). This will allow the

students to position the mirrors, in the horizontal plane only, to see how difficult it is to

get the laser to make the z-path through the slits, which are mounted on the same type

of mount as the mirrors and are therefore at the same height. This experiment can be

done for $750.18, but many of the parts are needed for other experiments, and the bulk

of the cost is the laser, which is needed in almost every other experiment, so the price

should not be fully considered until the total equipment list is finished.


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Laboratory #2: Refraction and Total Internal Reflection

Whats Cool About Refraction and Total Internal Reflection?

Any time a beam of light passes from one material to another, refraction occurs.

Refraction is the basis for how lenses direct light, so you must understand refraction in

order to understand how lenses work. Most optical systems that use laser beams use

lenses, and in order to figure out what the lens will do to the laser beam, you need to

know about refraction. Refraction also happens in our atmosphere, so military planes

and spy satellites using detectors and guidance systems must take refraction into

account. Total internal reflection (TIR) is the reason laser light is able to travel through

optical fiber if you want to be a fiber technician, understanding TIR is the first step. It is

a way to make a mirror of sorts out of almost any material, depending on the angle of

the light and the material the light is coming from. Its not hard to understand, but

understanding it can lead you towards a career in fiber optics. TIR can also be used to

re-direct laser beams without using a regular mirror. You will do this in this lab with a

regular piece of plastic.

Refraction - Background

In this exercise you will learn how light bends when it goes from air into another

material, like glass or plastic. This bending is called refraction its the reason why, if

you are standing in a pool of water and you reach for something at the bottom, like a

coin, the coin isnt sitting where you think it the light bends, but your brain assumes

the light is traveling in a straight line (see Figure 1). Refraction is important because

without it, optical fibers would not work. What you will learn in this experiment

relates directly to how optical fibers are designed to transmit information.


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Figure 1 Illustration of how light bends when it crosses the boundary between twomaterials

Materials have what is called an index of refraction which tells us how much,

and in what direction, the light will bend when it enters that material. The index of

refraction in a perfect vacuum (totally empty space) is 1.000. In air it is close to, but not

exactly, equal to one it is 1.0003, but for practical purposes we can use 1.0 for the index

of refraction of air. In glass, the index of refraction is usually around 1.5, and in water,

it is 1.33. Notice how the index of refraction for various materials is greater than 1. This

is because the index of refraction is defined as the ratio of the speed of light in vacuum

to the speed of light in the material. Light travels faster in vacuum than in any other

material, so that means the index of refraction must always be greater than 1 for all real

materials.

If light hits a boundary between glass and air, as shown in Figure 2, the light will

bend as it crosses the boundary it will change directions, just like it did in the

swimming pool example. The direction of the light is described by the angle at which it

strikes the surface, measured with respect to the surface normal ( a line perpendicular

to the surface), as shown in Figure 2.

air

water

your view ofthe coin

coin is hereyou think thecoin is here


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Figure 2- Light coming from air and entering a piece of glass gets bent at the surface.

The real path of the light in the glass is shown by the solid line, while the path your eye

thinks the light takes is shown by the diagonal dotted line.

There is a simple way to predict how much the light will bend when it crosses

the boundary between two materials. If the light is incident from a material with indexof refraction n1 at an angle of q1 and, after it crosses the boundary into a material with

index n2 it is at an angleq2, the equation relating all these quantities is called Snells

Law:

n1sin1 = n2sin2In this exercise you will figure out the index of refraction of a piece of glass by tracing a

light ray through it and measuring the angles q1 and q2.

q1

q2

n1 air

n2 glass

surface normal

light goeshere

your eye thinks thelight goes here


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Part I Refraction - Procedure

In this part of the lab, you will draw a diagram like that of Figure 2 using a piece of

lucite (well call it glass to make it simpler) and a laser beam. Since it is not possible to

trace the path of the laser beam with your pencil, you will first mark the path with

pins, then remove the pins and draw straight lines through the pin holes in the paper.

Follow these step-by-step instructions:

1. Place the glass plate on a sheet of paper over the cork board. Trace the outline of

the glass on the paper.

2. Turn on the laser and aim it at the glass so that it is passing through the front and

back surface of the glass at some angle other than straight-on.

3. Mark the beam path on the paper by inserting pins along the beam path on both

sides of the glass. Two pins on each side will be enough for you to mark the path

of the laser beam.

4. Remove the pins and the glass block and draw straight lines through the pin

holes up to, but not through, the glass block outline.

5. Now draw the laser beam path through the glass by connecting the entry and

exit points (connect the two lines you drew in step 4).

6. Draw a line perpendicular to the glass surface at the point where the beam

entered the glass and again where it exited the glass. Your paper should now

look something like this:


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Measure the anglesq1 and q2 and, using n1 = 1.0, use Snells Law to calculate the index

of refraction of the glass.

Total Internal Reflection - Background

As you can see from Part I, when the light entered the glass, its angle with

respect to the surface normal decreased in other words,q1 >q2. As you can see from

your drawing, when the light exited the glass on the other side, its angle with respect to

the normal increased compared to when it was inside the glass:q1


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exit the glass it will be reflected back into the glass as if the glass-air boundary were

like a mirror. This is called total internal reflection (TIR) and it is the reason that

optical fibers are able to transmit light efficiently. The core of the fiber contains a

high-index material and the surrounding cladding is made of a low-index material the light therefore gets trapped in the core and totally internally reflects as it travels

down the fiber.

The condition for TIR comes from Snells Lawwe simply plug inq2 = 90

degrees and solve forq1, which is then called the critical angle it is the angle above

which TIR will occur. So,

sinqc = 1/n2

if we are going from glass into air, and n2 is the index of the glass. This means that any

light which strikes the boundary at an angle greater thanqc will be reflected back into

the glass. If it strikes the boundary at an angle less thanqc, it will still be able to cross

the boundary.

Part II TIR - Procedure

You will use TIR to find the index of refraction of a glass semi-circle and an acrylic one,

and you will see if there is any difference in your calculations.

1. Place the semi-circle made of glass on a sheet of paper over the corkboard and trace

its outline. Remove the plastic temporarily. Mark the center of the flat side of the

semi-circle. Draw a normal (perpendicular line) through that center. Put the plastic

back on its outline.

2. Aim the laser through the semi-circle from the curved side and try to hit the center

of the flat side. You should get something which looks like this:


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Note that, even before you reach the critical angle, there will be a reflected ray from the

flat side and from the curved side. This is normal light always reflects a little bit when

it hits a piece of plastic or glass. This is NOT TIR though as long as the refracted ray is

still coming out on the other side.

3. Increase the angle of incidenceq by rotating the paper and the semi-circle together

until the refracted ray is parallel to the flat side of the semi-circle. Make sure the

laser is still hitting the center of the flat side as you rotate! The point at which the

refracted ray starts to disappear (when it is parallel to the flat side) is the onset of

TIR. At this point, stop rotating, mark the incident ray with pins, and remove the

semi-circle.

4. Connect the incident ray to the normal on the flat side and measure the angle that

the incident ray makes with this normal (the angle marked q in the drawing

above). This is the critical angleqc. Use it in the equation

sinqc = 1/n2

incidentray reflected

rays

refracted

ray

q


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to calculate the index of refraction (n2) of the glass semi-circle.

5. Repeat this procedure with the acrylic semi-circle again, this will give everyone more

practice and will allow you to see the difference in working with different materialswhich may look the same, but are not scientifically the same.

Equipment List




p. 76 DH9283A Prism & Lens Set, Glass $61.00 (1)p. 76 DH9283B Prism & Lens Set, Acryl. $51.00 (1)

This set, together with the laser from lab #1 and the rectangular piece of lucite from the

Light Box & Optics Set from lab #1, contains the rectangle and semi-circle required to

perform all aspects of this experiment. It also adds a prism, so this experiment may be

expanded and the prism used to further study geometrical constructs of Snells law (see

lab #7). You still have to find some corkboard, paper, and pins somewhere optics

catalogs tend not to stock such things, but you can purchase them at Staples stores or

any office supply store. This lab adds another $112 and change to the equipment total,

but also provides components needed for the lenses lab and the prisms lab.


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Laboratory #3: Lenses, Image Formation, and Telescopes

Whats Cool About Lenses, Image Formation, and Telescopes?

Lenses re-direct light and it would be very difficult to form images or focus light

at all without them. You could do it with mirrors, but lenses work in almost the same

way, so once you understand one, you understand the other. Image formation is

interesting because it relates to things like holography, cameras, detectors for military

applications, photocopiers, laser printers, and of course TV and movies! This is just a

partial list of things that use lenses and images, but any kind of movie or photo

projector can be understood once lenses and image formation is understood. Finally,

telescopes use lenses (and mirrors) to help us understand our universe. Galileo created

the first telescope using two lenses, which is something you will also do. Without lenses

and image formation, our information about our world and our universe, and our

ability to record that information would not exist, and the entertainment industry

would almost not exist either! Business would not be able to print and copy documents

with the quality we have today, and life as you know it would be very different.

Knowing about how lenses form images is a very powerful skill that can help you

become an optical technician.

Background

In this exercise you will learn how lenses are used, individually and in

combination, to form images. You will also build a simple telescope and see exactly

what it does to the image of the original object. Lenses can be either converging or

diverging, depending on what they do to light rays which hit them. If parallel rays

strike a converging lens, they get focused to a point by the lens, as shown in Figure 1. If

parallel rays strike a diverging lens, they spread out after passing through the lens, but

they look like they came from a single point in front of the lens, as shown in Figure 2.


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Figure 1 Image formation by a converging lens. This image is a real image.

Figure 2 Image formation by a diverging lens. This image is a virtual image.

The equation which tells us where an image is formed for a given lens is:

iof

111+=

Figure 3 Diagram illustrating the quantities given in the lens equation.

lensobject

o i

image

incominglight rays

image formedhere

incominglight rays

image formed here


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wheref is the focal length of the lens, o is the objects distance from the lens, and i is the

images distance from the lens. If you place the object very, very far away from the lens,

thenowill be very large and 1/o will be very small, so the image distancei will equal

the focal distancef. This is the example shown in both Figures 1 and 2 an object whichis very, very far away sends light rays to the lens which are almost parallel and can be

considered parallel.

The equation above can be used for both converging and diverging lenses. If the

lens is converging, the focal length gets plugged in as a positive number if the lens is

diverging, the focal length gets plugged in as a negative number. Then, if the image

distancei turns out to be positive, the image is located on the opposite side of the lenscompared to the object. This is called a real image. If the image distance iturns out to be

negative, then the image is located on the same side of the lens as the object. This is

called a virtual image. The difference between a real image and a virtual image is

simple: a real image has real light rays passing through it, whereas a virtual image is an

image that appears to be at a certain point in space, but the light rays dont actually go

to that point. Figure 1 shows an example of a real image, whereas Figure 2 shows an

example of a virtual image the rays do not actually converge on the opposite side of

the lens, but they appear to converge on the object side of the lens.

We will use different lenses and different object distances to find the focal

lengths of some individual lenses, and then we will use some lenses in combination to

build a simple telescope. The way to deal with combinations of lenses is simply to take

them one at a time. You apply the lens equation to the object and the first lens, and you

find the image location as if the second lens werent there. Then, you use the imagefrom the first lens as the object for the second lens, and you apply the lens equation

again. This means that you must measure the distanceo from the second lens to the

image from the first lens (which is now your new object). The value for ithat you get

this time is the location of the final image for the two-lens combination. If you had more


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lenses in combination, you would just keep on applying the lens equation to each one in

turn. Figure 4 illustrates the two-lens example.

Figure 4 Illustration of a two-lens combination.

A simple telescope can be made with a two-lens combination. A telescope takes parallel

input rays (because it looks at stars that are very far away) and magnifies them,

producing parallel rays at the output. Because the output rays are parallel, your eye

does not visualize the image from a telescope as being at a specific location rather, it

looks lie the planet or star is floating out there somewhere. You will not see this effect

in the lab demonstration because the telescope you will build will be used to view an

object close by, but if you ever look through a telescope at a planet, you will notice that

you have no idea how far away the image of the planet is!

The way to construct a two-lens telescope is simple remember that we said that

parallel input rays focus at the focal point of the lens? Well, if you apply that theory to

both ends of the telescope, you will see that you will get parallel rays out for parallel

rays in when the lenses are separated by the sum of their focal lengths. This is

illustrated in Figure 5.

image from

lens 1object

o i

lens 2object forlens 2

o i

finalimage


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Figure 5 A two-lens telescope. The lenses have focal lengths f1 and f2 they are

separated by a distance f1 + f2 to produce parallel output rays when parallel input rays

come into the telescope.

Procedure

Single Lens

1. Place your light source (candle or flashlight) as far away from the lens as you

can on the optical rail. Use an f = 15 cm double-convex lens and find a clear

image of the light source measure the distance of this image from the lens it

should be 15 cm, the focal distance, because o is large in this case. How close

is your measured value to this theoretical value? Is the image rightside-up or

upside-down? Take notes in the spaces provided here.

2. Put the 15 cm lens on the rail and put the light source 20 cm in front of it.

Find the sharpest image and measure its distance from the lens.

3. Use the lens equation with o= 20 cm and f= 15 cm to calculate whati, the

image distance, should be. Compare this with your measured result.

4. Repeat step 3 with the light source located 30 cm from the lens.

f1 + f2parallelinput rays

parallel outputrays


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Two lenses

Place the light source 20 cm in front of the 15 cm lens. Place the other 15 cm lens 30 cm

behind the first 15 cm lens. Find the sharpest image formed by the second lens and

measure its distance from the lens. Use the lens equation to determine this image

distance.

If time permits, repeat this experiment with one of the 15 cm lenses replaced by the 40

cm lens.

Simple Telescope

Use the both lenses (the 15 cm and the 40 cm) to construct a telescope using the

guidelines given in the background section. If it works, you should be able to look

through the lens back towards the light source and see an inverted image of it. Is the

image larger than the source or smaller?

Diverging Lenses

1. Use the 15 cm double-concave lens and repeat the single-lens experiment and

calculations. Did you get an image this time? If not, why not? What did the

calculations tell you? A class discussion of the differences in results between

using a double-convex lens and a double-concave lens should follow these

procedures.


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Equipment List




p. 76 DH3508 Meter Stick Opt. Bench $29.00 (2)

p. 76 DH3508A 38 mm double convex lens $5.00 (2)

p. 76 DH3508C 38 mm dbl. concave lens $5.00 (2)

All lenses have focal length of 150 mm.




p.26 G45-298 dbl.convex 40cm fl lens $33.80 (1)

The additional cost is $72.80, but the 40 cm lens is also needed for experiment #6 on

beam expanders and spatial filters and the meter stick optical bench and lenses are used

in numerous other experiments.


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Laboratory #4: Reflection of Polarized Light

Whats Cool About Polarized Light?

Polarized light is all around you every day. It is used in common items suchas sunglasses, computer screens, and wrist watches, but it also has many applications

in high-tech devices like lasers, optical radiation detectors, and telecommunications

systems. Most light that reflects off of things like car hoods, windows, and the street is

polarized, so thats why polarized sunglasses are good. They cut down on glare.

Computer screens and wrist watches that use liquid-crystal displays (LCDs) use

polarizers so that you can read the letters and numbers the polarizer creates the

difference between the black numbers and grey background on a digital watch, for

example (see Reference #4 for more information on LCD displays and polarization).

Lasers need polarizers so that the light emitted has only one direction of oscillation

(well discuss that later), which is vital for many experiments. Much research that uses

laser light requires it to be polarized. That means, not only does the laser need a

polarizer, but the optics in the rest of the experiment might need a polarizing coating on

them in order to work right. Similarly, detectors might look for light of only one kind of

polarization, which can increase their sensitivity and effectiveness, like for military uses

and other applications. Finally, fiber-optic telecommunications systems use polarized

light to cut down on loss and other complicated effects which can degrade the signal. So

you see, understanding polarization is very important for an optical, laser, or fiber-

optics technician!

Background

The purpose of this laboratory experiment is to study the reflection

characteristics of polarized light as its angle of incidence and polarization are varied.

Whenever light is incident on an optical surface there are three things which can

happen the light can be reflected, transmitted, or absorbed. For most optical surfaces

all three of these things happen, but usually one of them will dominate. For example,


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you know that a glass window transmits light, but actually it reflects 4% of the light

which is incident upon it. Similarly, a mirror is primarily a reflecting surface, but a

very small percentage of the light is transmitted through most mirrors. For purposes

of this lab, we will be ignoring absorption.

Polarization deals with which way the electric field part of the light is oscillating.

Light is an electromagnetic wave which contains both electric and magnetic fields

that oscillate (move up and down) as they travel they look like the kind of wave youd

see if you tied a rope to the wall and shook your hand up and down. This type of wave

is called sinusoidal and looks like the wave shown in Figure 1.

Figure 1 An example of a sine wave the electric and magnetic fields which make up a

light wave oscillate like this as they travel.

The wave shown in figure 1 is called a transverse wave because the direction of

oscillation is perpendicular to the direction of travel of the wave. Now, remember that

light travels in three-dimensional space, whereas the pictures we draw here are only

two-dimensional. The actual light wave can oscillate in the direction shown in Figure 1

(up and down on the page), or it can oscillate in and out of the page, or any direction in

between. If the direction of oscillation of the wave varies randomly as the wave

travels, and it does not favor any one particular direction, then the light is unpolarized.

directionof travel

directionofoscillation


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A polarizer is an optical component that restricts the direction of oscillation of

the wave to a particular direction. There are many ways to do this, but in this lab, we

will only be concerned with what is called linearly polarized light, which means that it

is either vertically or horizontally polarized. If the light is vertically polarized, forexample, it can only oscillate in the vertical direction. If it is horizontally polarized, it

can only oscillate in the horizontal direction. In this lab, you will see that polarization

affects the way light reflects off of a piece of glass and we can use this property to turn

unpolarized light into polarized light. Lets now take a look at what happens when

light reflects off of a boundary between two different materials, for example, glass and

air.

The amount of reflection and transmission which occurs at the boundary

between two materials is determined by their indices of refraction (recall from lab #2).

For normal incidence, meaning the light strikes the boundary perpendicular to it, or

straight on, the reflectance R (defined as the reflected beam intensity divided by the

incident beam intensity) is given by

R = {[n1 - n2]/[n1 + n2]}2

Where n1 and n2 represent the indices of refraction for the two materials it does not

matter which is the "entrance" media and which is the "exit" media because, as you can

see, the equation is symmetric in n1 and n2 , meaning that if you swap them it makes no

difference. So, for the example of ordinary window glass, if we use an index of

refraction n1 = 1.5 and the index n2 = 1.0 for air, the above equation tells us that the

reflectance at the air/glass interface is 4%. This means that any time light hits regularwindow glass, 4% of it is reflected back rather than being transmitted through.

Fresnel's Laws of Reflection tells us that when the angle of incidence is anything

other than normal (zero degrees) the reflectance will depend on the polarization of the


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incident beam this means that the amount of reflected light will be different for

horizontal polarization than for vertical polarization. The way the light will behave as

the angle of incidence is changed is shown in Figures 2 and 3. Figures 2 and 3 show

reflectance as a function of angle of incidence for both s and p polarizations (this isthe short-hand way to denote vertical and horizontal polarization directions). Figure 2

represents the situation when the light is coming from air and reflecting off of the glass

surface, whereas Figure 3 shows the case where the light starts in glass and reflects off

of the boundary with the air. Notice that Figure 3 showsqc, the critical angle (recall

from lab #2). Beyond the critical angle, the light cannot escape the glass it experiences

total internal reflection.

Reflectance air to glass

0

0.2

0.40.6

0.8

1

1.2

0 20 40 60 80 100

angle (Degrees)

Re

flectance

Rs

Rp

Figure 2 Reflectance as a function of angle of incidence for the case of lightcoming from air and reflecting off of glass.


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Reflectance glass to air

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

angle (degrees)

Re

flectance

Rs

Rp

Figure 3 Reflectance as a function of angle of incidence for the case of light

coming from glass and reflecting off of the boundary with air.

Note that in both figures 2 and 3 for p-polarization there is an angle for which

the reflectance is equal to zero. This angle is called Brewster's angle and it is defined

by the equation

tanqB = n2/n1.

Recall from lab #2 the formula for the critical angle:

sinqC = n2/n1.


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We will not be studying total internal reflection (TIR) during this laboratory exercise,

but you should recognize that TIR is the reason why the curve in Figure 3 does not

extend out to 90 degrees like the one in Figure 2.

So, we see that there is an angle, Brewsters angle, at which p-polarized light can

be made to disappear when it reflects off of a surface, leaving only s-polarized light.

This property of reflected light is very useful for generating polarized light if an

unpolarized beam of light is made to reflect off a surface sitting at Brewster's angle, the

resulting reflected light will be linearly polarized. Specifically, it will be s-polarized

light. This method is sometimes used to ensure that the output beam of a laser is

linearly polarized. This property is also the reason why polarized sunglasses work

well. When you go outside and see a lot of glare, that usually comes from sunlight

reflecting off of buildings, roadways, and cars. This reflected light will be mostly s-

polarized. Polarized sunglasses are designed to block this s-polarized light, leaving only

p-polarized light. This reduces glare and makes everything easier to look at. In this lab,

you will be conducting a series of experiments that will allow you to observe and

determine Brewster's angle for a glass prism.

Procedure

You may enter your data in the tables provided below. You will do a series of

experiments to try to see the difference between reflected light when it is horizontally

polarized vs. vertically polarized. The experimental apparatus should look like Figure

4. The linear polarizer is there to make sure the polarization is vertical or horizontal, as

needed.

HeNe laser

glass prism

rotationstage

linearpolarizer


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2. Repeat step 2 of Part I.A.

Part II. Reflection as a Function of Polarization Angle at Fixed Angle of Incidence

1. Set the rotation stage of the prism at an angle of 57 degrees. Leave it at this setting

for this part of the experiment.

2. Rotate the polarizing film from 0 to 45 degrees in 5-degree increments and record the

power readings of the reflected beam. You may enter your results in the table below.

Your instructor will have to rig up a way to measure how much the polarizing film is

being rotated this depends on the equipment available at your school.

After you have taken all of this data, have your instructor switch the glass prism out

and out the acrylic prism in and repeat your measurements. Note any differences and

discuss them. Do you expect the acrylic to perform perform the same way as the glass?

Why or why not? A class discussion should be held after all the data is taken and the

groups have time to analyze it., this can be a two-week project, with the glass prism

being used one week, the acrylic started the first week and then finished the second

week, and a discussion of the results the second week. Again, the purpose of using the

two different prisms is not only to see the difference (if any) in resulted, but to get

more practice at alignment, data collection, data analysis, and discussion of results.

Being able to draw conclusions from your results is as important as being able to

obtain them is!

Results

Your experimental results should look like the results shown in Figure 2 the s-

polarized light power should gradually rise with increasing angle of incidence. The p-

polarized light reflected power should go to zero at some angle this is Brewsters

angle. Calculate what is should be from the formula above, using 1.5 as the index of


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refraction for the glass and 1.0 for air, and compare your calculated value to your

experimental value. What should the data from part C do? Well, since it contains an

equal mix of s- and p-polarized light, it should look like it would fit in between the

curves of Figure 2. Finally, the data from Part II should show you that s-polarizedlight will reflect off of the glass prism when it is set at Brewsters angle, but p-

polarized light will not. This means your reflected power when the half-wave plate is

set at zero degrees should be high and the reflected power when the half-wave plate is

set at 45 degrees should be close to zero. What happened with the acrylic prism?

Table for Results of Part I (glass, above, acrylic, below):


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Glass

Angle ofincidence

Power -Part I.A.

Power -Part I.B.

Power -Part I.C.

5

10

15

20

25

30

35

40

45

50

5560

65

70

75

80

Acrylic

Angle of

incidence

Power -

Part I.A.

Power -

Part I.B.

Power -

Part I.C.5

10

15

20

25

3035

40

45

5055

60

65

70

75

80


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Table for Results of Part II (glass, left, acrylic, right):

Angle ofincidence

Power

5

10

15

20

25

30

3540

45

Angle of

incidence Power

5

10

15

2025

30

35

40

45


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Equipment List


www.edmundoptics.comCatalog Location Number Name Price Quantity

p. 133 G53-027 Rotary Assem. w/hold. $174.10 (2)

p.120 G56-929 Bench Plate $60.00 (1)

p.128 G03-649 Post Holders 6.0 $23.10 (2)

p. 128 G36-499 Mounting Posts 4.0 $10.00 (2)

From the Daigger Sci-Ed Warehouse Catalog, 2005Catalog Location Number Name Price Quantity

p. 76 DH15012 Light Meter $174.00 (1)

Use the above equipment with the prisms from the Prism & Lens Set from Lab

#2. This adds an additional $474.30 to the equipment total, but the rotary holder,

bench plate, posts and post holders are also needed for thee spectroscopy lab.

The instructor will need to acquire some polaroid vertically polarizing slides and

a method to mount and rotate them in measured 5 degree increments (or do the

best you can to simulate this), which should be available already in the HS

science lab.


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Laboratory #5: Spectroscopy

Whats Cool About Spectroscopy?

Spectroscopy is the science of breaking multi-colored light down into its

individual, pure component colors (wavelengths) so that you can see exactly

which colors are present in the light you are examining. Whats the big deal

about that? Its amazing, because there is so much you can do with this skill. This

is how forensic scientists identify unknown substances at crime scenes this is

how astronomers can tell us what gases are contained in the atmosphere of Mars

(or any other planet) this is how scientists can identify the chemicals present in

any unknown sample given to them, if they have spectroscopy available to them.

This is such a powerful tool, it has allowed us to learn about the nature of the

universe, how the universe was formed, how the planets were formed, what they

are made of, and from this information, scientists can figure out other

information, like why the planets contain certain elements from the periodic

table, and why they dont contain others.

Background

The reason this works is because each element in the periodic table of the

elements has a unique spectroscopic signature. Thats a fancy way of saying that

each element has its own unique finger print, and that finger print just happens

to be its spectrum, which is what the spectrometer reveals to us. Just like every

human being can be identified by either their fingerprints or their DNA, each

element in the periodic table can be identified by its spectroscopic output, which

is the unique pattern of colored bands that the chemical emits when submitted to

certain conditions. The electrons in the sample must be excited by some energy

source, which causes them to absorb energy and raise to higher energy levels,

and then, eventually, they fall back down to the energy levels where they came


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from. In so doing, they emit the energy they absorbed, and we see this energy as

light of a specific and unique wavelength corresponding to the energy difference

between where the electron was when it was excited and where it ended up after

it gave up the energy it absorbed. This energy difference, between where theelectron was when it was excited and where it went to after it emitted the energy

it absorbed, is a unique number, and there is only one number that it can be.

That one number, an amount of energy, corresponds to one unique pure color

and shows up as a colored line in the spectrum of that element. There are several

electronic energy transitions which are allowed by nature for each element, so

each element will therefore have more than one line in its spectrum, and some of

the spectral lines fall outside the visible range. Therefore, some elements, to us,will appear to have many lines and others will appear to have very few, but it

might be because we, as humans, can only view a very small portion of the total

electromagnetic spectrum and therefore we are only viewing a portion of the

spectrum that the element is emitting, but it is enough for us to be able to

identify each element. So, every element in the periodic table has a unique

spectrum and therefore, when that element is present in a sample of something

being analyzed using spectroscopy, its spectrum will show up and you will be

able to identify the element.

Thats what you are going to learn to do in this lab. You will be given 4

sealed tubes of gases which are unknown, and you will build a simple

spectroscope which will allow you to view the spectrum of each tube of gas as it

is excited by being plugged into a socket (there are safety issues here more

later). The tube will glow and have a distinctive color to it, but the color you see

coming from the tube with your eye alone is actually made up of several pure

colors mixed together, and the spectroscope will separate these pure colors for

you, you will identify their wavelengths and compare them to a chart, and by

this method, you will identify the elements contained in each tube. You will


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record the wavelength of each spectral line you view and compare it to the

standard values and see how close your values come to the true values.

From this lab, you will know, in part, how forensic scientists help policesolve crimes, how astronomers discover the nature of the universe, and how

unknown substances can be identified to determine if they are dangerous or not

by a poison control center, for example, or in a medical lab, when you send a

specimen in and the doctors are trying to figure out what is making you sick.

Todays lab teaches you a very powerful skill which is applied in so many

different fields, from chemistry, biology, medicine, astronomy, to police work

and beyond, that being familiar with it is essential if you would like to become alaboratory technician in many possible fields you may choose to study.

Procedure

A. Building the Spectroscope

Building a simple spectroscope is not as complicated as it sounds. All you need

are the following 2 meter sticks, a source to analyze, a holder for one meter stick,

a holder for a transmission diffraction grating, and the transmission diffraction

grating. You set them up as follows.

1. Using the meter stick optical bench. Place this along the end of your lab

work area. Behind the 50 cm mark on the meter stick, place the spectrum

tube power supply so that when the gas tube is in the supply, it will be

lined up exactly with the 50 cm mark on the meter stick optical bench. The

meter stick should be lower than the center of the gas tube.

2. Instructor should do this: Use the other, regular meter stick to measure,

from the base of the power supply for the tube of gas, a distance of 1


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meter, and place the diffraction grating, mounted in the rotary assembly

and secured so that it is not damaged or able to move during the

experiment, exactly at this 1 meter mark. It is the grating itself that should

be 1 meter away from the base of the supply, not the bench plate, therotary assembly, or anything else, so make sure that the distance from the

base of the supply to the actual grating is 1 meter, or 100 cm. Important:

Do NOT ever touch the surface of the diffraction grating. It is expensive

and fingerprints cannot easily be cleaned off of it, and they may

permanently ruin it. If anyone needs to handle it, it should be the

instructor, and then, the grating should only be handled by its edges,

never by touching the flat, ruled part of it. Touching anything but the thinedges of the grating can permanently damage it, so allow your instructor

to set up the grating and then do not touch it. If there is any problem

during the experiment, ask the instructor to check on it for you. Do not

attempt to make any adjustments to the diffraction grating yourself.

B. Using the Spectroscope

3. Make sure the power supply is plugged in, but turned off. Insert a tube of

gas by carefully pushing down on the spring in the bottom portion of the

holder, then sliding the tube vertically in so that it is under the top spring,

and slowly releasing the bottom spring until the top spring in the top

holder catches the top of the tube. Keep guiding the tube until it will not

go any higher. It is now secured between two conducting springs which

will supply 110 volts to the supply when the switch is turned on.

Important: This power supply should be thought of as a large wall socket

if it is touched when the power is on, it is the same as sticking your finger

into a wall socket. NEVER touch the tube or the spring holders while the

power is on. Also, the tubes get very hot while working, so when it is

time to switch to another tube, turn the power off, use a piece of paper


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towel or paper folded many times over, or some kind of cloth with which

to gently grab the fat portion of the glass tube, NOT the metal conducting

part because that will be even hotter, and not the skinny part because it is

too delicate, gently push down on the bottom spring until the top of thetube is free from the top spring, pull the tube out, and replace it in its

holder, being very careful not to break the skinny portion of the tube.

Then, place a new tube of gas in using the same procedure outlined

above. It is best to allow the instructor to perform this part of the lab, but

if the students do it, just make sure you do not touch a tube of gas that

has just been running with your bare hands or a single-ply sheet of

paper or paper towel it is very hot. Waiting a few minutes in betweenand doing some calculations is also a good idea, since, once you take the

data, you must calculate the wavelengths of the lines you saw. So, do

some calculating before you switch out tubes, but still assume they are hot

and fragile.

4. Once a tube is loaded in the supply, turn it on and turn off the overhead

lights. Make sure there is some backlight somewhere in the room so

people may move safely about, but before you turn the lights out, make

sure the aisles are clear of obstructions and nothing is in danger of being

knocked off of a lab bench. (For more tips on safe lab practices, see

reference number 5,How to Study for Success in Science, Math, and

Engineering Courses, Amy E. Bieber.) Be careful while working in the dark

be extra-alert to your movements and where things are. Look through the

transmission grating at the glowing tube of gas, and you should see

colored lines, identical on the left and right side of the light source. These

colored lines should look like they are floating above the meter stick.

Write down every color you see and the number on the meter stick where


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you see it, and write it down for the left and the right side for each color.

You are now recording the spectrum of this gas tube.

5. Repeat the above procedure for all 4 tubes of gas, but follow the safetyprecautions and do the calculations in between, as already stated.

6. You will now calculate the wavelengths of the spectrum for each tube of

gas as follows. Fill in the following tables:

Tube #1:

Leftpositionof line (S)

Rightposition ofline 50cm, thenconvertedto meters

Averageof left andrightpositions(AverageS) inmeters

22

SXg

S

+

=l

(see below)

in meters

Standardvalue ofl inmeters

% error inyourmeasurement(see below)

Tube #2:

Leftpositionof line (S)

Rightposition ofline 50cm, thenconvertedto meters

Averageof leftand rightpositions(AverageS) in

meters

22SXg

S

+=l

(see below)

in meters

Standardvalue ofl inmeters

% error inyourmeasurement(see below)


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Convert your S values to meters by dividing by 100 before entering the

data into the table.

8. Calculatelby using the formula in the table. X is 1 meter g is the numberof grooves per meter on your grating, which in this case means g = 11000

grooves/meter. When plugging the numbers into your calculator, make

sure you use parentheses to tell the calculator to use the proper order of

operations. Your instructor will explain this. It means, if you dont use

parentheses, your calculator, just for the denominator, will think you

want g* 22 SX + instead of having the S2 part under the square-root

sign too, so you have to put parentheses around everything that goes

under the square root sign, and parentheses around everything that goes

in the denominator. If you have trouble with the calculation, your

instructor will help you, and there will be enough spectral lines so that

you will get plenty of practice so by the end of the lab, you will know

exactly how to use that equation, easily!

9. Compare your value of lin meters to the standard value supplied in the

table. Calculate the % error as follows:

%100*tan

tanexp%

dards

dardserimentalerror

-=

10. The bars are absolute value signs they mean that if your answer comes

out with a minus sign in front, throw it away. We dont care if the

experimental answer was too big or too small, we only care by how much

it was too bog or small. Anything less than 20% is considered good for an

experiment using this sort of equipment.


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11. Repeat the calculations for all experimental lines you saw. Be extra careful

to look for the violet and blue lines they can be very faint and hard to see,

so if you appear to be missing some lines, go back and look for the violet

and blue ones. Also, in one case, there might be some yellow lines tooclose together to read separately, so consider them one line for calculation

purposes, but when trying to identify the spectrum by comparing it to the

chart, remember that there was a pair of yellow lines. Write it down

somewhere.

12. Once you have all of your wavelengths computed, compare your results

to the spectrum of gases chart and try to identify which gas was in whichtube. They are color-coded and your instructor will have the answers.

Note that the color code on the tube gives no hint as to anything to do

with the color or name of the gas inside!

Equipment List

From the Daigger Sci-Ed Warehouse Catalog, 2005www.daigger.com, 1-800-621-7193


p. 75 CH3531 Spect. Tube Pwr. Sply. $161.00 (1)

p. 75 DH3531G Helium Gas Tube $33.00 (1)

p. 75 DH3531M Neon Gas Tube $33.00 (1)

p. 75 DH3531H Hydrogen Gas Tube $33.00 (1)

p. 75 DH3531L Mercury Gas Tube $40.00 (1)

Use this with the meter stick optical bench from lab #3 and the diffraction

grating listed below. Another meter stick will need to be supplied by the

instructor, along with a base of some sort (a piece of wood will do just fine) to


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Laboratory #5B: Monochromators (if you have one)

Whats Cool About Monochromators?

The color of visible light depends on its wavelength. Reds and orangeshave the longest wavelengths of visible light while blues and violets have the

shortest wavelengths. But light comes with different levels of purity of color. It

can be like laser light, containing a very narrow range of wavelengths, it can be

like a neon sign, containing a larger but still limited number of different

wavelengths, or it can be like an incandescent light bulb, which contains every

color of the rainbow. How do you tell? If you need a laser beam to be extremely

spectrally pure, meaning that it contains very few different wavelengths, how

do you measure that? You can use amonochromator. It can tell you if your light

source has the spectral purity you need. Lasers usually come with a specification

sheet that tells you the spectral purity of the beam someone has to do those

measurements, which means that someone working at the company building the

lasers has to know how to use a monochromator. Researchers also use them to

test the lasers they already have, so if you want to work with a scientist in a

research lab, it would be helpful to know how to use a monochromator.

Unfortunately, monochromaters are pretty expensive. The ones available at

Queensborough are fairly simple quarter-meter monochromaters, and they cost

about $6,000 each in 2005. But if you have them available, they are very useful

tools. This lab will show you how to use one, and what the results from it mean.

Background

In this exercise you will learn to use a monochromator to determine the

wavelength of several different colors of light emitted by the helium-neon laser.

The helium-neon, or HeNe laser, is the laser used in many supermarket scanners,

surveying equipment, and some laser pointers. It most commonly emits red

light, but the ones you will use today are tunable HeNes and are capable of


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The wavelength spread contained within the laser beam will not contain

equal amounts of power at all wavelengths the power distribution will look

something like that shown in Figure 2.

Figure 2. Power as a function of wavelength for a typical laser beam.

Notice that the shape of the power curve has some labeled features on it. The

point where the power reaches its highest value is the maximum. The maximum

occurs at the center wavelength, which ideally should be the wavelength right in

the middle of the spread of wavelengths contained in the power curve. You will

look for the power maximum and record the center wavelength at which the

maximum occurs as part of this experiment.

Notice the two arrows that point to the half-maximum points. There are

two points on either side of the maximum at which the power drops to half of its

maximum value these are called the half-maximum points. Record the power

and wavelength at these half-maximum points. Then, subtract the two

wavelength values of the half-maximum points to obtain the full-width at half-

maximum (FWHM) for the laser beam. The FWHM is a measure of the

wavelen th

power

maximum

half-maximumpoints


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wavelength width of the laser beam, measured between the half-maximum

power points.

Procedure

1. The monochromator comes with several small slits which can be inserted at

the entrance and exit apertures of the device - begin by removing these slits.

**Important: the dial on the monochromator that selects which wavelength

will pass through should not be forced if it stops turning in one direction, do

not try to force it.**

2. Begin with red laser light - this should be at a wavelength of 632.8 nm (nm

stands for nanometer - one billionth of a meter). Set the dial on top of the

monochromator for 632.8 so that it will allow light of this wavelength to pass

through it. Align the monochromator with the laser so that the red laser light is

going into the monochromator and coming out the other side. Use a white piece

of paper to see if the laser beam is coming out of the monochromator DO NOT

LOOK INTO THE MONOCHROMATOR WITH YOUR EYES!

3. Align the detector with the output laser light from the monochromator. You

should see a power reading on the detector scale. If you dont get a reading, try

adjusting the scale on the detector until you get a reading. If it says 1. then it is

overloaded and you need to turn to a less sensitive scale.

4. Slowly adjust the wavelength knob on the monochromator until the output

power is at its maximum value. Write down the wavelength at which this

happens in Table 1 on the following page. Also, make a note of the maximum

power level.


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Table 1 Experimental data for monochromator using no slits.Color center wavelength

(wavelength wherepower is maximum)and max power, no

slits

two wavelengthsat which power is

half maximum

FWHM , no slits(difference

between the2wavelengths

where power is

half maximum)

red wavelength:power:

orange wavelength:power:

yellow wavelength:

power:

green wavelength:power:

Table 2 Experimental data for monochromator using 150 mm slits.color center wavelength

(wavelengthwhere power ismaximum)and

max power, 150m slits

two wavelengthsat which power is

half maximum

FWHM , 150 mslits (difference

between the2wavelengths

where power ishalf maximum)

red wavelength:

power:orange wavelength:


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power:yellow wavelength:

power:

green wavelength:

power:


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the prism, is parallel to the bottom base of the prism, then you have found

the minimum deviation angle, or something very close to it.

6. Repeat this experiment for the other prism (one is glass, the other acrylic,so they will have different indices of refraction and therefore your

answers should be different, but not hugely different).

After all the data is taken and everyone has successfully found the

minimum deviation angle, a class discussion should ensue as to why the

entrance and exit angles had to be equal for the deviation angle to be minimum,

and why the laser beam should travel parallel to the base of the prism when it isfollowing the path of minimum deviation. Think about this, use common sense,

and you should have and interesting and lively discussion with a lot of original

ideas being floated about. Also, why was there no rainbow? Because you were

using laser light, which is pure in color, and therefore the prism cannot spread

the beam out any farther than it already is. Had you used white light, you would

have gotten a rainbow, and since the optical meter stick bench kit comes with a

flashlight, you could try to use this flashlight with the prisms to create a

rainbow.

Equipment List




p. 76 DH9283A Prism & Lens Set, Glass $61.00 (1)

p. 76 DH9283B Prism & Lens Set, Acryl. $51.00 (1)

Also needed is the laser and the corkboard and pins used in the refraction lab,and the flashlight from the optical meter stick bench kit. This adds an additional

$112 to the total cost.


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Laboratory #8: Beam Expanders and Spatial Filters

Whats Cool About Beam Expanders and Spatial Filters?

Beam expanders make laser beams bigger and spatial filters make then

cleaner, meaning more uniform in intensity. After a laser beam has been

expanded and spatially filtered, the edges will be as bright as the center, which

would otherwise not be true. This is important because without an expanded,

spatially filtered laser beam, holography would be impossible. Spatial filtering

an d bea m expansion is also use d in defense / militar y app lications like opt ical

pat tern recog nit ion. It ca n be use d, for exa mp le, to take a hig h-a ltit u de pho to of a

tank on the ground and then determine if it is a friendly or enemy tank. The

same is true of any other piece of large equipment on the ground that might be

p ho to gr aphe d fro m a plane or satellite an d that nee ds to be ID d by very fie

features not detectable to the human eye. So, if you work in the aerospace or

defense industr y, you might find you rself using a spatial filter / bea m expand er.

Holography of course would be impossible without this device, but that will be

discussed in a future lab.

Background

In this exercise you will learn how to use lenses in combination to make

two very important optical components a beam expander and a spatial filter. A

beam expander uses two lenses to increase the diameter of a laser beam a

spatial filter does the same thing, but it also cleans up the beam and makes it

extremely uniform in intensity across the entire diameter of the beam. This is

important for holography, which is the lab we will do next week with the use of

the spatial filters you will set up this week. Beam expanders are important for

many applications that involve magnification, like building telescopes and

microscopes.


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distances of the lenses meet (as shown below) and its purpose is to cut off the

tails of the laser beam as it passes through. This way, when the laser beam is

expanded, it will have uniform brightness across its entire diameter because the

dim parts that were there before got filtered out by the pinhole.

Figure 2 - Spatial Filter

The reason this works is because before the laser beam gets filtered, its intensity

looks something like this:

Figure 3 - Laser Beam Intensity Profile

incomingsmallbeam

outgoingfiltered largebeam

pinholeaperture

distancecenter of

beam


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Procedure Spatial Filter

1. The spatial filter assembly is made up of the collimated lens set-up from

Part One with the addition of the pinhole inserted exactly at the spot

where the laser beam is its smallest. Determine where that spot is, insert

the pinhole into the card holder from the meter stick optical bench kit, and

try to get the laser beam to pass through the pinhole when the laser beam

is its smallest. A little bit of the laser beams edge should be blocked, or

chopped off by the pinhole thats the whole point. This weaker edge

of the beam is what we are trying to eliminate in order to clean up the

beam profile and make it have a more uniform intensity across its entire

diameter when it becomes expanded.

2. Adjust the vertical and horizontal position of the pinhole until the light

coming through is maximized. Look at the quality of the expanded light.

Is it uniform in intensity across its diameter? If not, one of two things is

wrong. Either your pinhole is not the right size, or your pinhole is in the

wrong place.

3. The easier thing to check first is whether the pinhole is in the right place.

So, repeat the step above, adjusting the vertical and horizontal position of

the pinhole, also slightly adjust the positions of the lenses if you think the

beam is not perfectly collimated, and again check the beam quality.

Remember, these three components are working together to create a

uniform beam, so they might all three need to be adjusted, and this is an

iterative process, meaning you must do the same thing over and over afew times before you get it right, like back in lab one with the alignment

procedure. If the beam is reasonably uniform, then you have built your

spatial filter. If it is not, then you have one more step to add and then

repeat the above process.


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4. At its best position, look at the laser beam striking the pinhole and

determine if you think too much of the beam is being chopped off as it

passes through, or if not enough of it is being chopped off. If too much is

being chopped off, your enlarged laser spot will be dim and will fade a lotat the edges. If not enough of the laser spot is being chopped off, your

enlarged beam will look much brighter in the middle than at the edges

and it will not look clean. By looking at the way the laser spot is striking

the pinhole and the way the output beam looks, determine if the pinhole

needs to be enlarged or made smaller, remove the pinhole, either enlarge

it or make it smaller (the pinhole is created by wrapping tape around the

slit from the Optics Set, so to enlarge or shrink the pinhole, you simplyneed to add or remove some tape), and re-insert it.

5. Repeat steps 1 - 4 until you have an intense, round, uniform laser beam

emerging.**This takes time and patience dont expect this to happen in 5

minutes!

****IMPORTANT remember to practice good laser safety habits when

removing and inserting components into the path of the laser beam.****

Equipment List

This experiment can be constructed using the parts from Lab #3 (the meter stick

optical bench kit and lenses) and lab #1 (the slits) and the following, which can

also be used in lab #7:




p.26 G45-298 dbl.convex 40cm fl lens $33.80 (1)

This adds an additional $33.80 to the equipment total.


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this works, we need to first discuss the idea of interference, which is what

happens when two or more light waves meet at the same point in space. Picture

light waves as sine waves that travel through space, like this:

Figure 1 An example of a light wave.Lets say that two such waves meet at some point in space. What happens to

them? They simply add together. If you have two waves that meet, you add

their field strengths, point-by-point, at each point in space where they meet to

form what is called a resultant wave. Now, this resultant wave can be larger than

either of the two separate waves, or it can be smaller than either of them. It all

depends on the relative phases of the two waves when they meet. Phase refers

to which part of the wave we are looking at in a particular point in space we

usually speak of the phase of one wave compared to another. For example, the

two waves shown in Figure 2 have a phase difference of 180 degrees:

Figure 2 - Two waves that have a phase difference of 180 degrees.

distance

Electric fieldstrength


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As you can see, a 180 degree phase difference means that the waves are exact

opposites of one another: the crests of one line up with the valleys of the other.

This is called 180 degrees out of phase. This means that if we were to add themtogether, they would completely cancel each other out. This is called destructive

interference.

Now, if two waves have no phase difference between them, we say that

they are in phase, which means that the crests and valleys of the two waves

line up perfectly. If we add two in-phase waves together, we get a resultant

wave that is larger than either of the two separate waves this is calledconstructive interference. If the two waves line up in any way other than exactly

in phase or 180 degrees out of phase, we get a more complicated wave pattern,

but it is still called interference.

The next thing you need to know to understand holograms is that laser

light is coherent. This is what makes it so powerful and dangerous! Coherence

means that all of the light waves in a laser beam travel together with the same

phase, meaning that they have constructive interference and none of the power

gets wasted. In an ordinary light bulb, the waves do not travel together with the

same phase all the light waves from a light bulb have different phases and they

vary randomly, which means that many of them have destructive interference

and power is wasted. This is part of the reason that a few milliwatts from a laser

can blind you but a 60 watt light bulb can be looked at briefly without irritating

your eyes.

Because a laser beam is coherent, it can tell how far it has traveled that is

something unique to laser light and it is what makes holograms, CD players, and

DVD players work, to name a few examples. A laser beam can tell how far it has


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are recorded in an interference pattern on a holographic plate the interference

pattern is simply the addition of the object and reference waves at each point in

space where they meet. The phase information (which is actually distance

information) is contained within the interference pattern itself because the phasedifferences between the waves determine what the interference pattern looks

like. For example, the interference pattern will contain spots of constructive

interference when the object and reference waves are in phase, or destructive

interference when the object and reference waves are 180 degrees out of phase.

For all other phase differences corresponding to different distances to features on

the object (ranging from 0 to 360 degrees), the interference pattern is more

complicated-looking, but it still gets recorded on the holographic plate. So, thisis why a hologram contains both distance (depth) information as well as

brightness information from the object, and therefore looks three-dimensional.

Regular pictures do not look three-dimensional because they only contain the

brightness information and no depth information. This is because a regular

camera looking at light that is not coherent has no way to record phase

information, which is what tells us about distance.

Figure 3 shows a simple set-up for recording a hologram, which is what

you will do in this weeks lab exercise. The reference beam is simply the

expanded and filtered laser beam as it hits the transparent holographic plate the

object beam is the part of the laser beam that bounces off of the object and is

reflected back towards the holographic plate. When the reference and object

beams meet, an interference pattern is formed that is recorded on the

holographic plate. This plate must then be developed using a method similar,

but not identical, to the method used to develop regular 35 mm camera film. The

details on how to develop the plates will be given in the lab when we make the

holograms.


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Figure 3 Set-up for recording holograms.

To view the holograms that we will make in class, an ordinary white light

bulb can be used, but the holograms will look even better if they are viewed with

the same laser set-up used to record them. In the lab, we will attempt to view the

holograms we make both with a bright filtered light bulb (holding the hologram

in front of the bulb and looking down through the plate to try to see the image)

and with the laser set-up. To view the hologram with the laser set-up, the plate

should be placed back on the stand where it was recorded and you should look

through the plate (away from the laser, not towards it!) to view the image, which

should look very three-dimensional and like it is sitting behind the plate.

Note that when you look at the holographic plate itself, you dont see the

image it is not like looking at a photo. The plate only contains the interference

pattern the image from the plate is seen floating behind the plate when it has

light hitting it and you look at the right angle.

filtered andexpanded laserbeam

transparent holographicplate records interferencebetween reference andobject beams

objectreference beam object beam


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References

1. Laser Academy Optics Laboratory Manual, Queensborough Community

College Physics Department, available at

http://www.qcc.cuny.edu/physics/lfot/LFOT_FAQs.asp, click button on left

side of screen that says Laser Academy lab Manual.

2. Daigger Sci-Ed Warehouse Catalog, 2005, www.daigger.com, 1-800-621-7193.

3. Edmund Optics America Optics and Optical Instruments Catalog, 2005,

www.edmundoptics.com, 1-800-363-1992.

4.Is There a Laser in the House? Understanding Your High-Tech Everyday World ,

Amy E. Bieber, Pearson Custom Publishing, 2005. Available at

www.pearsoncustom.com. ISBN 0536919038.

5.How to Study for Success in Science, Math, and Engineering Courses, Amy E.

Bieber, Pearson Custom Publishing, 2005. Available at

www.pearsoncustom.com. ISBN 0536943222.

6. Queensborough Community College Astronomy Laboratory Manual, 2003.

Available at QCC Bookstore.

7. Hofstra University Physics Department Astronomy Laboratory Manual, 2000.Available from Hofstra University Physics and Astronomy Department.
http://www.qcc.cuny.edu/physics/lfot/LFO

laser and optics lab manual

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