IGCSE Physics Waves

Lesson 7 – Refraction Aims: To describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms. To recall and use the relationship between refractive index, angle of incidence and angle of refraction n = (sin i) / (sin r). To describe an experiment to determine the refractive index of glass, using a glass block.

Introducing refraction

Refraction of light

Real and apparent depth The rays of light from a stone get bent (refracted) as they leave the water. Your brain assumes these rays of light have travelled in straight lines. image Your brain forms an image at the place where it thinks the rays have come from – the stone appears to be higher than it really is. actual location

The Archer fish The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey. image of prey prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

Refraction diagrams and law of refraction

What do you notice about the light rays either side of the glass block?

angle of incidence > angle of refraction Air to glass When light is refracted as it travels from air to glass: angle of incidence > angle of refraction  i >  r i > r As the light ray travels from air into glass it moves towards the normal. air glass In general, when light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.

angle of incidence < angle of refraction Glass to air When light is refracted as it travels from air to glass: angle of incidence < angle of refraction  i <  r air glass As the light ray travels from glass into air it moves away from the normal. In general, when light rays travel from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal. i < r If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end.

Refraction – angle of incidence What happens to light travelling from air through a glass block when the angle of incidence is 0°?  i = 0° air glass When the angle of incidence is 0 the light ray is not deviated from its path. undeviated light ray

Refraction of light Conclusion When light enters glass or water it bends towards the normal line. When light leaves glass or water it bends away from the normal line.

Refraction in water waves When waves in water travel through water of different depths they change speed. In shallow water the waves slow down; in deeper water they speed up. We can investigate this by changing the depth of the water in a ripple tank. As the water waves slow down, their direction changes due to the change of speed. This is called refraction. Perspex sheet used to change depth of water

road mud Imagine a car driving from the road into a muddy field. In the muddy field it slows down as there is more friction. mud road If it enters the field at an angle then the front tyres hit the mud at different times. tyre 2 tyre 1 Tyre 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal. When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal.

If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time. The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

Formula and examples

Travelling through different materials If you were running along a beach and then ran into the water when would you be moving slower – in the water or on the sand? In the water. In a similar way, as light moves from one medium to another of different density, the speed of light changes. Do you think light moves faster or slower in a more dense medium? Light moves slower through a more dense medium.

This causes the light to bend or refract. The speed of light waves depends on the material they are travelling through. air = fastest glass = slower diamond = slowest If light waves enter a different material (e.g. travelling from glass into air) the speed changes. This causes the light to bend or refract. air glass

The speed of light Light travels at 300,000 km/s in a vacuum. As light enters denser media, the speed of light decreases. From this bar chart, which material do you think is denser, glass or water? Glass must be denser than water because light travels more slowly through glass than water.

Glass and water Glass is denser than water. Light travelling through glass will be refracted more than light travelling through water.

Refractive index = speed of light in air The speed of light We can study refraction of light by comparing its speed in air to that in a different material. A number called the refractive index is the ratio of these two speeds: Refractive index = speed of light in air speed of air in material Example: The speed of light in air is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water? 1.33

Calculating refractive index The speed of light in air is 300,000,000 m/s. The speed of light in crystal is 150,000,000 m/s. What is the refractive index of crystal? Refractive index = speed of light in air speed of light in crystal Refractive index = 300,000,000 150,000,000 Refractive index of crystal = 2.0

Refractive index = sin i Snell’s law Snell’s law can be used in experiments and calculations to find out the refractive index of a material. Refractive index = sin i sin r n = Refractive Index, i = Angle of incidence, r = Angle of refraction.

Snell’s law – Example 1 A light ray incident upon a glass block at 45º is refracted to 28º, calculate the refractive index of the glass. n = sin i  sin r n = sin (45)  sin (28) n = 0.71  0.47 n = 1.51

Snell’s law – Example 2 A light ray incident upon a glass block at 48º is refracted to 32º, calculate the refractive index of the glass. n = sin i  sin r n = sin (48)  sin (32) n = 0.74  0.53 n = 1.40

Snell’s law – Example 3 A light ray incident upon a glass block is refracted to 25º by a block of refractive index 1.55, calculate the angle of incidence. n = sin i  sin r sin i = n × sin r sin i = 1.55 × sin (25) sin i = 1.55 × 0.42 = 0.66 i = 41º

Snell’s law – Example 4 A light ray is incident at 60º to a material with refractive index of 2.4 Calculate the angle of refraction of the light ray. n = sin i  sin r sin r = sin i  n sin r = sin (60)  2.4 sin r = 0.87  2.4 = 0.36 r = 21º

You need to know this experiment for your IGCSE examination

Determining refractive index Aim: To determine the refractive index of a perspex block. Method: Use a slit and raybox to illuminate a glass block at a range of angles. Carefully record the angle of incidence and angle of refraction each time. Complete the following table:

Refractive index table Angle of incidence Sin (i) Angle of refraction Sin (r)

Graph and gradient Draw a graph of sin (i) against sin (r). Calculate the gradient of the graph. The refractive index of the glass is equal to the gradient of the graph. Remember n = sin (i)  sin (r)

Animated refraction

Summary – Refraction When light changes the material it is travelling through it is refracted. The change in material causes a change in speed. Light entering a glass block from air is refracted towards the normal line and light leaving glass is refracted away from the normal line. Snell’s law is given by n = sin i  sin r Where: n = refractive index, i = angle of incidence in the air and r = angle of refraction in the medium.