3 refraction of light 3.1 refraction of light 3.2 laws of refraction 3.3 snell’s law and...

3 Refraction of light

3 Refraction of light

3 Refraction of light

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 Refraction of light

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.

3.1 Refraction of light

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.

3.1 Refraction of light

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

3.1 Refraction of light

Describing refraction.

3.1 Refraction of light

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.

3.2 Laws of refraction

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

3.2 Laws of refraction

sin i

sin r= constant

A graph of sin i against sin r.

3.2 Laws of refraction

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

3.3 Snell’s law and refractive index

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.

3.3 Snell’s law and refractive index

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.

3.6 Real depth and apparent depth

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.

3.6 Real depth and apparent depth

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.

3.7 Total internal reflection and critical angle

Experiment 3.3 Critical angle

The angle of incidence is small.

3.7 Total internal reflection and critical angle

ray box

semi-circularglass block

very strongrefracted ray

very weakreflected ray

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

3.7 Total internal reflection and critical angle

strong refracted ray

strong reflected ray

C

The angle of incidence is larger than the critical angel.

3.7 Total internal reflection and critical angle

very strongreflected ray

Prismatic periscope.

3.8 Total internal reflection in prisms

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.

3.8 Total internal reflection in prisms

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.

3.10 Formation of a mirage

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

3.10 Formation of 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.

3.10 Formation of a mirage

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