Refraction

Bending of light when it travels from one medium to another

Speed of light differs in different mediums:Vacuum = 3.0 x 108 m/sAir ~ VacuumWater = 2.26 x 108 m/sAcrylic = 1.76 x 108 m/s

Refraction

Rules of Refraction:

1. Incident ray, refracted ray and normal all lie on the same plane

2. Light bends TOWARDS the normal when speed of light in 1st medium > 2nd medium (i.e. Slows down)

3. Light bends AWAY from the normal when the speed of light in 1st medium < 2nd medium (ie. Speed up)

Refraction

If SLOWING down when entering another medium bend TOWARDS the normal

Normal Line

Incident Ray <i

Medium A (Faster)

Medium B (Slower) <R

Refraction

Refracted Ray

If SPEEDING UP when entering another medium bend AWAY from the normal

Normal LineIncident Ray <i

Medium A (Slower)

Medium B (Faster)

<R Refracted Ray

Refraction

Partial Reflection & Refraction

Refraction is often accompanied by reflection = partial refraction & reflection

Ex: Transparent window

Silvered two-way mirror:Ex: mirrored sun glasses,Glasses on buildings

Here most lights are reflected,

while few are refracted

Partial Refraction & Reflection

If ray of light is going from vacuum (or air) to a slower medium, then….

We can use the angles given to calculate index of refraction using the formula

sin <i .= n sin <R

where n = Index of Refraction

Calculating Index of Refraction (n)

Example: A light ray travelling from air into acrylic has an angle of incidence of 480. The angle of refraction is determined to be 300 . What is the index of refraction of acrylic?

Normal LineIncident Ray

<i = 48o

Air (Faster)

Acrylic(Slower)

n = sin 480 sin 300

n = 0.743 0.5

n = 1.49

<R = 30o

Refracted Ray

Another way to calculate n is by using the ratio between the speed of light in vacuum (c) and the speed of light in the medium (v)

n = c . Where c = 3.00 x 108 m/s v

NB: Index of refraction for air is almost the same as for vacuum. For our calculation purposes they are the same.

Calculating Index of Refraction (n)

Example 1: Speed of light in air is 3.00 x 108 m/s. Speed of

light in salt is 1.96 x 108 m/s. Light enters from air to salt. Which way does the light bend? What is the index of refraction?

Answer:Light travels from air (faster) to salt (slower), thus

refracted ray will bend TOWARDS the normal.

Index of refraction (n) = c = 3.00 x 108 m/s = 1.53 v 1.96 x 108 m/s

Calculating Index of Refraction (n)

Example 2: Index of refraction of quartz is 1.46. If the angle

of incident ray from air is 600, what is the angle of refracted ray in quartz?

Calculating Index of Refraction (n)

Total Internal Reflection

Normal Line

Air (FASTER)

Water (SLOWER)

Recall that refraction is often accompanied by reflection = Partial Refraction & Reflection

Incident ray

Refracted ray

Reflected ray

Normal Line Air (FASTER)

Water (SLOWER)

Critical angle = when <R is equal to 900

Q: What happens when we keep increasing the angle of incidence?

Incident rays

Refracted rays

Reflected rays

Air (FASTER)

Water (SLOWER)

Total Internal Reflection

• Occurs when <i is GREATER than critical

angle.

• NO light is being refracted.

ONLY occurs under 2 conditions:

1. Light is travelling more slowly in 1st medium than in 2nd medium (going from SLOWER to FASTER)

2. Angle of incidence is GREATER than critical angle

Total Internal Reflection

Critical angle is a physical property of medium High index of refraction means a very small

critical angle.

Example:Diamond n = 2.42 critical angle = 24.40

Any light coming in at angle greater than 24.40 will be internally reflected “sparkling” effects

Total Internal Reflection

Total Internal Reflection

Effects of refractions

Apparent DepthMirage

Apparent Depth To draw diagram to show this phenomena:1. Draw the 2 different mediums & eye2. Draw the object in slower medium

AIR

WATER

Apparent Depth3. Draw an incident ray coming from a point

on the object, the normal line and the refracted ray.

4. EXTEND the refracted ray BACKWARDS

AIR

WATER

Apparent Depth5. Draw a second incident ray from the

same point, but at different angle, show the new normal line and refracted ray.

6. EXTEND the refracted ray BACKWARDS

AIR

WATER

Apparent Depth

7. Where the two dashed lines meet is where your image will be.

AIR

WATER

The mirage is a virtual image of the sky.

n of cold air > n of warm air

Light bends AWAY from normal as it goes from cold to warmer air

Water on pavement - Mirage

Eventually once you reached the lowest (and hottest) air layer, total internal reflection occurs

Light ray now travels UP through the layers.

Water on pavement - Mirage

Water on pavement - Mirage

Refracted ray reaches your eyesYour brain automatically extends the ray BACKWARDS. Image of the sky appears to come from the ground ahead of you