LIGHT

by: Chris Lembalemba(chrislembalemba@yahoo.com)

Light : What is Light?

Light carries energy and travels as a wave.

Light travels at 300 000 000 m/s or 300 000 km/s (much faster than sound).

Light waves travel in straight lines.

Light waves undergo reflection, refraction diffraction and interference.

Light waves are transverse

Reflection

Reflection

• Law of Reflection

– The angle of incidence equals the angle of reflection [ <i = < r ]

- The incident and reflected rays and the normal lie in the same plane.

Reflection: the bouncing back of light as it strikes a barrier (mirror).

Definitions:

1. Incident ray (i): the ray of light that strikes the mirror (barrier)

2. Normal (N): a line drawn perpendicular to the mirror drawn at the point where the incident ray strikes the mirror.

3. Angle of incidence (<i): the angle between the incident ray and Normal.

4. Reflected ray (r): the ray of light leaving the mirror surface.

5. Angle of reflection (<r): the angle between the reflected ray and the Normal.

Reflection: Image location using a plane mirror

1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it.

Place a pin as the object in front of the mirror.

2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for 3 more positions of the ruler.

3. Remove the mirror and ruler. Where the lines cross is the image position.

I

Object

Image

What is the distance between the mirror and object?What is the distance between the mirror and image?What is your conclusion?

1. Draw a line and label it ‘mirror’.

2. Draw a dot on top of the line and label it as ‘O’

3. Draw a line (incident ray) from the ‘O’ to the mirror and label it i1.

4. Draw a ‘Normal’ perpendicular to the point where i1 meets the mirror and label it N1.

5. Use a protractor to measure the angle of incidence (<i1).

6. Use the formula: angle of incidence = angle of reflection, and draw the reflected ray (r1).

7. Label the angle of reflection as <r1

Reflection: Image location by construction

8. Repeat the steps 3 to 7 for i2, <i2, N2, <r2 and r2.

9. Extend r1 and r2 backwards as dotted lines. They meet at the Image location.

10. Measure the distance from the object to the mirror.

11. Measure the distance from the image to the mirror.

(draw diagram on the board)

Question:

What is your conclusion?

CONCLUSION

When a plane mirror forms an image:

1. The image is the same size as the object.

2. The image is as far behind the mirror as the object is in front.

3. A line joining equivalent points on the object and image passes through the mirror at right angles.

Diffuse vs Specular Reflection

• Diffuse Reflection

– Light incident upon a rough surface

– Law of reflection still holds; Normals are not parallel.

• Specular Reflection

– Mirror like reflection

– All Normals are parallel

REFRACTION

Refraction : Bending light

The speed of light waves depends on the material they are travelling through.

If the light waves enter a different material [e.g. from glass into air] the speed changes.

This causes the light to bend [or refract].

Air = Fastest Diamond = slowestGlass = slower

Glass

Refraction: the bending of light as it moves from one medium to another due to

change in wave speed.

Refraction

• In both cases the speed of the wave has decreased. This is indicated by the decrease in wavelength!

Refraction : Investigating Refraction

Angle i

Angle r

Definitions:

1. Incident ray (i): the ray of light that strikes the boundary.

2. Normal (N): a line drawn perpendicular to the boundary drawn at the point where the incident ray strikes the boundary.

3. Angle of incidence (<i): the angle between the incident ray and Normal.

4. Refracted ray (r): the ray of light in the other medium.

5. Angle of refraction (<r): the angle between the refracted ray and the Normal.

Speed of light

• v is the speed of light in the new medium.

• c= 3.0 x 108 m/s

• As the index increases the speed decrease.

n is the absolute index of refraction, Refractive index. This is a measure of optical density. n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium.

Refractive indexMedium Refractive index

Diamond 2.42

Glass (crown) 1.52

Acrylic plastic (Perspex) 1.49

Water 1.33

Exercise:Calculate the speed of light in;(a) Diamond(b) Glass (crown)(c) Acrylic plastic (Perspex)(d) Water

SNELL’S LAW

• In 1620, Willebrord Snell the Dutch scientist discovered the link between the two angles: their sines are always in proportion.

• When light passes from one medium into another:

sin i is a constant

sin r

That is:

refractive index = sin i

sin r

Refraction: Determining the refractive index of glass

1. Place a glass block on an A4 paper and draw around it.

2. Place 2 pins on one side of the glass block and label them p1 and p2.

3. Place 2 more pins on the other side of the block such that they make a straight line with the images of p1 and p2 in the block. Label them p3 and p4.

4. Remove the glass block and the pins.5. Draw a line passing through p1 and p2

up to the upper bounder of the block.6. Draw another line passing through p4

and p3 up to the lower boundary of the block.

7. Join the upper line to the lower line.8. From the upper line, draw a normal and

measure the angles of incidence and refraction.

9. Calculate the refractive index of the glass block.

x p1

x p2

x p3

x p4

Refraction : Summary

When light bends this is called refraction.

Refraction happens because the light changes speed [or velocity].

When light enters a more dense medium [e.g. glass], it bends towards the normal.

When light enters a less dense medium [e.g. air], it bends away from the normal.

If the incident ray hits a surface at 0º, no refraction occurs.

Remember that the angle of reflection [r] and angle of refraction [r] have the same symbol.

In reflection, <I = <r

In refraction, <I <r

Consequences of Refraction of light1. Apparent depth

R – Real Depth

A – Apparent Depth

If the chest is 20m below the surface, calculate its apparent depth.

Exercise

1. Calculate the angle of refraction if light (in air) strikes water at an angle of incidence of:

a. 24

b. 53

2. In an experiment, it was discovered that light travels at 1.9 x 1o8m/s in a certain material. Calculate

a. The refractive index of the material

b. The angle of refraction given the angle of incidence as 38o.

CRITICAL ANGLE

• Critical angle is an angle of incidence that gives an angle of refraction = 90o.

ic

ic = critical angle

To find the critical angle, we use:n = sin 90

sin ic

= 1 .sin ic

Or ic = sin-1 (1/n)

Exercise:Calculate the critical angle of a. Diamondb. Crown glassc. Perspex plasticd. Water

INTERNAL REFLECTION

• Whenever light moves from medium A (water) to medium B (air), there is a weak ray of light that is reflected back into medium A. This phenomenon is called Internal Reflection.

TOTAL INTERNAL REFLECTION

• When the angle of incidence is greater than the critical angler, light undergoes Total Internal Reflection.

APPLICATIONS OF TOTAL INTERNAL REFLECTION

1. Periscope- light incident at 0o to the normal is not refracted.- in a periscope, light is incident at 0o to the normal (i.e through the normal) on the glass blocks.- the refracted ray passes through the normal of the other side of the block.

Reflecting prisms

-Light entering the prism on side ‘a’ is at 0o

(i.e. through the normal) is not refracted.-The ray of light reaches side ‘b’ at an angle greater than the critical angle.- The ray of light is totally internally reflected.-The ray of light leaves the prism through the normal.

ab

c

i. ii.

Optical fibres

• An optical fiber is a flexible, transparent fiber made of glass or plastic, slightly thicker than a human hair.

• It can function as a waveguide, or “light pipe” to transmit light between the two ends of the fiber.

1. Optical fibres1. Engineering

2. Endoscope

THANK YOU