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 of incidence i ( angle of refraction r

1Rays of light travelling from air into glass are refracted towards the normal. 2Rays 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

1The incident ray, the refracted ray and the normal are in the same plane. 2The 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 n m. That is 3.3 Snell’s law and refractive index nmnm = sin i sin r = sin where  a is the angle of incidence in air and  m is the angle of refraction in the medium.

MaterialRefractive index n Vacuum Air Water Perspex Glass Diamond 1.00 (at 20ºC) (at 20ºC) – 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 AD BC ( ( ( ( incident ray a b c d emergent ray lateral displacement

Deviation of light by a prism. 3.5 Refraction through a prism B C A incident ray emergent ray angle of deviation

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 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-circular glass block very strong refracted ray very weak reflected 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 strong reflected ray

Prismatic periscope 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 45 o

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 Formation of a mirage

The road seems wet on a hot day. This is called a mirage Formation of a mirage cool air warm air hot air total internal reflection here looks like a pool of water light 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 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 1 10The refractive index = sin C orC = sin -1 ( ) 1 n