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# 6.3.1: Refraction

A wave that changes speed as it crosses the boundary of between two materials will also change direction if it crosses the boundary at an angle other than perpendicular. This is because the part of the wavefront that gets to the boundary first, slows down first. The bending of a wave due to changes in speed as it crosses a boundary is called refraction. As mentioned in the last chapter, light in air or a vacuum travels at $$\text{c} = 3.0\times 10^{8}\text{ m/s}$$ but slows down when passing through glass. As shown in the diagram below, this will cause light to change direction a little. For a piece of glass with flat surfaces this isn't very noticeable unless the glass is very thick. But for a curved surface the light ends up leaving the glass going in a different direction and this is how lenses for glasses, telescopes, microscopes, binoculars, etc. are made.

Figure $$\PageIndex{1}$$

What about sound? Sound also undergoes refraction. Recall from the last chapter that wind can change the speed of air. In the following picture notice that Jill can hear Jack because the wind speeds up the upper edges of the sound, bending it back towards the ground. Jill can't hear Dana because the wind bends the sound upward.

Figure $$\PageIndex{2}$$

Likewise we know that the speed of sound depends on density which changes with temperature and humidity. In the following picture notice that Jill can hear Jack because the warmer temperature speeds up the upper edges of the sound, bending it back towards the ground. In the second picture there is a temperature inversion with warmer air trapped underneath cooler air. Jill sees but does not hear the lightning (this is sometimes called heat lightning, as shown in the second figure below).

Figure $$\PageIndex{3}$$

Figure $$\PageIndex{4}$$