Refraction

• Refraction is the bending of light when it enters from one transparent medium into another.

• As a light ray travels from one medium into another medium where its speed is different, the light ray will change its direction unless it travels along the normal.

• When light slows down when it moves from medium A to medium B, we say that the medium B is optically denser than medium A.

                                                                               

Using a laser pen, a beam of light is sent from water into air.

laser pointer

When light strikes such transparent boundary, both reflection and refraction occur.

Refraction of Light

water

What do you think will happen to its path of travel as it strikes the air-water boundary?

reflection

refraction

air

glass

reflection

refraction

refraction

reflection

Glass is optically denser than water hence light bends more when it enters glass compared to water.

Refraction Terminology

Direction of bending of light• When light enters an optically less dense medium, the

light ray bends away from the normal.• When light enters an optically denser medium, the light

ray bends toward the normal.

air

bends away from the normal

bends towards the normal

air

water water

air

glass

air

glass

i

                                      

r

from an optically less dense medium to an optically denser medium, i > r

from an optically denser medium to an optically less denser medium, i < r

bends towards the normal

bends away from the normal

i

r

Direction of bending of light

Refraction of Light

                                                                               

air

glass

The more optically dense the material, the slower the speed of light in that material

Air has lower optical density, light travels faster.

Glass has higher optical density – light travels slower, bend towards normal

Air has lower optical density, light travels faster, bend away from normal.

The change in speed at the transparent boundary between two media causes light to change direction.

Optical density of glass > water > air > vacuum

Refractive Index, n

                                                                               

airglass

c

v

cFor example, the refractive index of glass at the air-glass boundary is given as:

cn =

The refractive index of a medium is the ratio of the speed of light in one medium relative to the speed of light in the other medium.

where c and v are the speed of light in air and glass respectively.

v

Refractive Index of Various Medium

Laws of Refraction

• The incident ray, the refracted ray and the normal all lie on the same plane.

• For two particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant (Snell’s Law)

sin i = constantsin r

air

glass

                                                                               

2

1

1

2The ratio of the two sin s gives the same refractive index , and this relationship is called Snell’s Law.

sin 1

sin 2

n =

Then:

if 1 represents the angle of incidence in the less dense medium, and 2 represents

the angle of refraction in the denser medium;

Snell’s Law

Refractive Index

• When light passes from vacuum (or air)

into a given medium (eg. water), the

constant ratio of is known as the

refractive index, n, for that medium.r

i

sin

sin

r

in

sin

sin

Angle of incidence

Angle of refraction

                                                                               

A ray of light approaches a glass-air boundary at an angle of incidence i = 30. What is the refractive index of the glass if the angle of refraction r = 49 ?

air

glass

30

49

sin 1

sin 2

n =

sin 49

sin 30 =

= 1.5

Sample Problem 1

angle in less dense medium

angle in less dense medium

water

35

air

                                                                               

A ray of light approaches a water-air boundary at an angle of incident i = 35.

(i) Complete the path of the light as it crosses the water-air boundary.

(ii) What is the angle of refraction if the refractive index of water n = 1.3?

sin 1

sin 2

n =

sin 35

sin r = 1.3

= 26 r

Sample Problem 2

r

Daily Phenomena of Refraction

• Swimming pool and ponds appear shallower than it really is.

• Object is at a deeper depth than where it appears to be.

• Bent objects in liquids

                                      

The picture below be easily reproduced with a laser pointer and a transparent semi-circle glass block in a darkened room. As light enters the glass block, it bends at the surface instead of traveling its original path.

What do you think will happen if the angle of incidence in the glass block is increased gradually?

Total Internal Reflection

                                                                               

A light ray from water is incident on the water - air boundary. The angle of incidence is gradually increased.

(i) Calculate the angle of refraction if the refractive index of water is 1.3. (ii) What do you observe about the angle of refraction as the angle of incidence is gradually increased?

57o

air

glass

40o

air

glass

30o

41o

20o

air

glass

26o

What is the maximum angle of refraction that can be produced?

Total Internal Reflection

As the angle of incidence increases, angle of refraction increases as well.

At a certain critical angle c, a maximum angle of refraction = 90 is produced.

A weak reflected ray is also produced.If the next incident angle is greater than c, no more refraction will occur.

The reflected ray becomes very strong and intense.

                                                                               

90air

glass

c

airglass

i> c r

if incident angle > critical angle:

no light ray from the optically denser medium will be refracted.

all light rays will be totally internally reflected into the optically denser medium.

cweak reflection

strong reflection

Total Internal Reflection

Critical angle and Total Internal Reflection

                                                                               

Total internal reflection only occurs when:

a light ray is travelling from an optically denser medium to an optically less dense medium.

Critical angle c is defined as the angle of incidence from a denser medium which produces an angle of refraction of 90.

90air

glass

c cweak reflection

airglass

i> c rstrong reflection

the angle of incidence is greater than the critical angle.

Critical Angle

• Critical angle is the angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90o.

nc

1sin

Refractive index

Critical

angle

                                                                               

A laparoscope is a medical equipment inside a hollow, thin tube. It is connected to a camera and a high intensity light for doctor to see the structure inside our body.

Perform an internet search to find out how total internal reflection plays a part in different fields.

Be prepared to share with your classmates what you have learnt from your research next week.

Total Internal ReflectionEnrichment