4.1.6Waves
Refraction — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 10
Part 1 of 5What refraction is
Definition
- Definition Refraction is the change in direction of a wave when it crosses a boundary between two materials in which it travels at different speeds.
Angles
- Angles The normal is a line at right angles to the boundary.
- Angles of incidence and refraction are always measured from the normal.
Straight in
- Straight in A ray travelling along the normal (angle of incidence 0°) is not deviated, but it still changes speed.
Part 2 of 5Direction of bending for light
| Passing from | Speed | Bends | Example |
|---|---|---|---|
| Less dense to more dense | Slows down | Towards the normal | Air into glass or water |
| More dense to less dense | Speeds up | Away from the normal | Glass or water into air |
| Along the normal | Changes | No change in direction | Ray hits the boundary at 90° |
Part 3 of 5Light through a glass block
1
A ray in air hits the glass surface at an angle to the normal.
2
On entering the glass the light slows down and bends towards the normal.
3
It travels in a straight line through the block.
4
On leaving the glass it speeds up and bends away from the normal.
5
With parallel sides, the emerging ray is parallel to the incident ray, just shifted sideways.
Part 4 of 5What changes and what does not
Refraction of a wave
Speed changes
- Speed changes Different media allow the wave to travel at different speeds.
Wavelength changes
- Wavelength changes Because v = f × λ and f is fixed, a slower wave has a shorter wavelength.
Frequency stays the same
- Frequency stays the same The source sets the frequency, so it is unchanged.
Colour stays the same
- Colour stays the same The colour of light depends on frequency, so light keeps its colour as it refracts.
Part 5 of 5Everyday effects
Pencil in water
- Pencil in water The pencil looks bent where it meets the water surface because rays from the submerged part change direction on leaving.
Water waves
- Water waves Ripples slow down and shorten their wavelength when they move from deep to shallow water, and bend at the boundary.
Lenses
- Lenses Lenses use refraction to focus light in cameras, glasses and the eye.
Key terms
| Term | Meaning |
|---|---|
| Refraction | A change in direction of a wave as it passes between media of different density. |
| Normal | An imaginary line at 90° to a boundary. |
| Angle of incidence | The angle between the incoming ray and the normal. |
| Angle of refraction | The angle between the refracted ray and the normal. |
| Medium | The material a wave travels through. |
Avoid these mistakes
1
Measuring angles from the surface instead of from the normal.
2
Saying frequency changes on refractionspeed and wavelength change, the frequency does not.
3
Drawing a ray bending away from the normal on entering glass: it bends towards it.
4
Saying light changes colour in water or glass: colour depends on frequency, which is constant.
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