Refraction of waves - Higher

Different materials have different densities. Light waves may change direction at the boundary between two transparent materials. is the change in direction of a wave at such a boundary.

It is important to be able to draw to show the refraction of a wave at a boundary.

A ray diagram showing refraction at the boundary between air and glass

Refraction can cause optical illusions as the light waves appear to come from a different position to their actual source.

Explaining refraction

The of a material affects the speed that a wave will be through it. In general, the denser the transparent material, the more slowly light travels through it.

Glass is denser than air, so a light ray passing from air into glass slows down. If the ray meets the boundary at an angle to the , it bends towards the normal.

The reverse is also true. A light ray speeds up as it passes from glass into air, and bends away from the normal by the same angle.

A useful way of remembering the speed and direction changes of light during refraction is 'FAST': Faster - Away, Slower - Towards.

Wave speed, frequency and wavelength in refraction

For a given of light, the is to the wave speed:

wave speed = frequency × wavelength

So if a wave slows down, its wavelength will decrease. The effect of this can be shown using wave front diagrams like the one below. The diagram shows that as wave travels into a denser medium, such as water, it slows down and wavelength decreases. Although the wave slows down, its frequency remains the same, due to the fact that its wavelength is shorter.

In this diagram, the nearest to the water side of the incoming wave slows down before the furthest side does. This causes the wave to change direction.

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