3 Refraction of light 3.1 Refraction of light 3.2 Laws of refraction 3.3 Snell’s law and refractive index 3.4 Refraction through a block 3.5 Refraction.

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3 Refraction of light




3.1 Refraction of light

3.2 Laws of refraction

3.3 Snell’s law and refractive index

3.4 Refraction through a block

3.5 Refraction through a prism

3.6 Real depth and apparent depth

3.7 Total internal reflection and critical angle


3.8 Total internal reflection in prisms

3.9 Optical fibres

3.10 Formation of a mirage

Summary

When light travels from one medium to another, it is

bent or refracted. This is because light travels at

different speeds in different media.


Light travels faster in optically less dense media and

slower in optically dense media.

The car hits the grass at an oblique angle. As the car changes speed, it changes direction.


The car hits the grass at right angles. There is no change of direction.


Describing refraction.


air

glass

incident ray normal

refracted ray

(

angle ofincidence i

(angle ofrefraction r

1 Rays of light travelling from air into glass are refracted

towards the normal.

2 Rays of light travelling from glass into air are

refracted away from the normal.

We can conclude that light bends towards the

normal in optically denser material.


Experiment 3.1 Laws of refraction

1 The incident ray, the refracted ray and the normal

are in the same plane.

2 The ratio of the sine of the angle of incidence to

the sine of the angle of refraction is a constant,

this is called Snell's law.

The laws of refraction state that


sin i

sin r= constant

A graph of sin i against sin r.


This ratio of sin i to sin r for light rays passing from

air to a medium is called the refractive index of

that medium or nm. That is


nm =sin i

sin r=

sin

sin ma

where a is the angle of incidence in air and m is the

angle of refraction in the medium.

Material Refractive index n

Vacuum

Air

Water

Perspex

Glass

Diamond

1.00

1.0003 ((at 20ºat 20ºC)C)

1.33

1.50

1.50 – 1.70

2.42

Refractive indexes of some materials.


Refraction by a rectangular glass block.

3.4 Refraction through a block

A D

B C

(

(

(

(

incident ray

abc

d

emergent ray

lateraldisplacement

Deviation of light by a prism.

3.5 Refraction through a prism

B C

A

incident ray emergentray

angle ofdeviation

The apparent depth is less than the real depth

because rays of light are refracted from the normal

as they leave the water.


The light rays appear to come from a point I which

is a virtual image.

Experiment 3.2 Apparent depth

The real and apparent depth of a swimmer.


real depthreal depth

I

apparent depth

virtual image

The inside of a glass block can act like a plane mirror.

We can learn more about total internal reflection

and the critical angle C in the following experiment.


Experiment 3.3 Critical angle

The angle of incidence is small.


ray box

semi-circularglass block

very strongrefracted ray

very weakreflected ray

The angle of incidence is equal to the critical angle C.


strong refracted ray

strong reflected ray

C

The angle of incidence is larger than the critical angel.


very strongreflected ray

Prismatic periscope.


Experiment 3.4 Construction of a prismatic periscope

Two prisms being used in binoculars. Notice that the light rays are turned through 180 º after two internal reflections.


45o 45o

A bundle of glass fibres. Notice the total internal reflection in the fibre.

3.9 Optical fibres

emergent light beam

single glass fibre

bundle of fibres

light beam

Sometimes, on a hot day, we see mirages. For

example, a driver may see what looks like a large

pool of water on the road. In fact, the road is dry.


The road seems wet on a hot day. This is called a mirage.


cool air

warm air

hot airtotal internal reflection herelooks like a pool of waterlight from the sky

A mirage is formed because there are hot layers of air

near the hot road and a cooler and denser layer above

it. Cool air has a greater refractive index than hot air.


Summary

• Total internal reflection and critical angle

9 Total internal reflection takes place when

• the angle of incidence is greater than the

critical angle.

• light rays are travelling from an optically

denser region to a less dense region, and

110 The refractive index =

sin C

or C = sin-1 ( )1n

3 Refraction of light 3.1 Refraction of light 3.2 Laws of refraction 3.3 Snell’s law and refractive index 3.4 Refraction through a block 3.5 Refraction.

Documents

angle of refraction

total internal reflection

critical angle c

refraction of lightthe

laws of refraction state

oblique angle

light bends

apparent depththe light