review notes for optics

8/14/2019 Review Notes for Optics

1/23

D'Amato PTHS 20071

How we see

Vision happens when a ray of light enters an eye

Without a source of light, there can be no vision atall

Rays of light travel in straight lines


2/23

D'Amato PTHS 20072

Luminous and illuminated objects

Objects that create light are called luminous objects.

Examples include the sun, a candle, a light bulb Other objects do not create light but instead are

illuminatedby other light sources

(Draw a ray diagram showing how a candle allows

you to see your hand in a dark room.)


3/23

D'Amato PTHS 20073

Each point on an object emits or reflects light rays in every

direction

This is NOT the way it works

How does the shadow of the

ball appear?How does this support the

correct idea?

Each point sends light rays in

every direction


4/23

D'Amato PTHS 20074

An experiment that shows how each point sends light rays in all

directions

What do you see when you perform the experiment

below? Why does this happen?


5/23

D'Amato PTHS 20075

Remember the basics about angles

You need a corner between two lines to have an angle

An angle is measured at the corner where two linesmeet

When two lines form an angle of 90 they are said tobe perpendicular or normal to each other.


6/23D'Amato PTHS 2007

6

Circle facts

If point O is the center and

point A is on the circle: Line CD is tangent to the

circle at point A

Line OB is normal to thecircle at point A

O

A

BC

D



7

Reflection from a smooth surface

AO is the incident beam

OB is the reflected beam OC is the normal, an

imaginary line drawn 90

from the surface at thepoint of reflection

Law of reflection:

AOC = BOC

Angle of incidence = angle

of reflection

Smoothsurface,i.e.M

irror

A

O

B

C

Incident

Beam

ReflectedBeam



8

Reflection from a smooth surface

If a surface is smooth at the

microscopic level, like amirror, the normal lines at

different locations are

parallel to each other.Thus, parallel incident rays

are reflected at the same

angle.This type of reflection is

called specular reflection.

Mirror

Incident rays



9

Reflection from an irregular surface

Ordinary surfaces are rough and

bumpy at the microscopic level,

and the normal lines point in all

different directions. Thus,

parallel incident rays will be

reflected in all directions.A beam of light will be reflected

diffusely in all directions, so

this is called diffuse reflection.

Ordinary surface

Incident rays



10

Refraction

When light travels from one medium into

another medium (like going from air

into water) its path changes when itcrosses the surface. This is called

refraction.

A normal line is imagined, perpendicular

to the surface between the two media,at the point the light ray crosses.

Going from air into something else, the

ray crosses the normal line and bends

towards the normal in the othermedium.

Going from something else into air, the

ray crosses the normal line again and

bends away from the normal.

air something else air



11

Seeing an image

The direction of the light ray

that reaches your eyedetermines where you see

the image

In this example, twoobservers would agree that

the image of the candle

flame is behind the mirror


12/23

D'Amato PTHS 200712

Reflection from a concave mirror

Reflections from a curved

mirror also obey the law ofreflection

The normal to a circular

shape always goes throughthe center O of the circle

incident beam

reflected beam

O

Normal


13/23

D'Amato PTHS 200713

Definition of the focal point

Light rays that are incident

parallel to the main axiswill be reflected through

the mirror's focal point

The focal point is halfwaybetween the center O and

the surface of the mirror

O

focal point center

main axis


14/23

D'Amato PTHS 200714

Rays through the focal point

Reflection works the same

way in reverse:Rays that pass through the

focal point will be reflected

parallel to the main axis

O

focal point center

main axis


15/23

D'Amato PTHS 200715

Other easy rays to reflect from a concave mirror

Any ray that passes through

the center will be reflectedstraight back

Can you explain why?

O

center


16/23

D'Amato PTHS 200716

Locating an image on a concave mirror

To find the image of a small

object, draw some easyrays from the object and

see how they are reflected

The image is located at theplace where observers

would see the reflected

rays diverging from

Of

small object

image location


17/23

D'Amato PTHS 200717

Which is NOT a condition required for you to see object X?

1. A light ray must reach your eye

from object X

2. Object X must emit light rays

3. There must be an uninterrupted

straight line between your eye and

object X4. All of the above are required for

you to see object X

5. More than one of the above are

NOT required

The best answer is 2. You can

also see an object thatonly reflects light rays


18/23

D'Amato PTHS 200718

If Tim is standing outside in the bright sun, why is his shadow

not completely black?

1. Light rays bend around

Tim

2. The sun is larger than Tim

3. The grass in Tim's

shadow holds light fromthe sun

4. Other objects are

reflecting light

5. The sun is the only

luminous object in this

situation

The best answer is 4. Light

rays are reflected fromother objects and

illuminate the area in

Tim's shadow


19/23

D'Amato PTHS 200719

Which term does not belong with the others?

1. Perpendicular

2. Tangent3. Normal

4. At right angles5. 90

The best answer is 2. All the

other words are about 90angles


20/23

D'Amato PTHS 200720

A laser beam is aimed at the top of a tank of water. The beam

makes an angle of 60 with the surface of the water. What is the

incident angle of the beam on the water?

1. 60

2. 903. 30

4. 05. There is not enough

information to say

Make a sketch and remember

the definition of "angle ofincidence". The answer is

3.

water

beam

30

normal

60


21/23

D'Amato PTHS 200721

Which of these statements is TRUE about a surface that exhibits

specular reflection?

1. All reflected light rays are

parallel, no matter where they

come from

2. Normal lines are parallel at all

points on the surface

3. The light ray bends toward thenormal when it reflects

4. Normal lines point in different

directions from the surface5. More than one of these is true

The best answer is 2.


22/23

D'Amato PTHS 200722

Which of these statements is TRUE about a flat surface that

exhibits DIFFUSE reflection?

1. Parallel incident rays will

produce parallel reflected rays

2. An incident beam of light will

be reflected in all directions

3. The incident angle of each ray

is not equal to the reflectedangle

4. Even if the surface looks rough

to the naked eye, it is perfectlysmooth at the microscopic

level

5. More than one of these is true

Both 2 and 4 and correct, so 5

is the best choice


23/23

D'Amato PTHS 200723

Which is a good explanation for why you cant normally see a

laser beam from the side in midair?

1. A laser beam is made of many

light rays, and light rays

always go straight

2. You need light in order to see,

but there is no light in the

middle of the beam3. A laser beam is a special kind

of light that goes in only one

direction

4. If there was chalk dust in the

air, you could see the laser

beam

5. None of these is a goodexplanation

Some of these are true

statements, but none ofthem are good

explanations of why you

can't see the beam inmidair.

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