GCSE Physics - Water Waves - Shallow to Deep Water

This tutorial is about how waves can speed up or slow down when then enter a material with a different optical density, or when water waves enter regions of different depths.

This change of velocity can cause the waves to change direction - this is called REFRACTION.

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Water waves will refract when they move from shallow to deep water causing them to speed up. As a result, their wavelength will increase and the refracted ray will 'SPEED AWAY' from the normal line.

Remember that the wavefronts are always at 90 degrees to the ray.

Use 'RNAR' to work through the steps:
1. Ray (incident ray)
2. Normal (line perpendicular to surface where the ray enters)
3. Angles (label the angle of incident and angle of refraction)
4. Use the refraction rule "SPEED AWAY" to determine which direction the refracted ray will bend.

Quick question: During refraction, the wavelength and the speed of the wave changes. What does NOT change about the wave?

(Answer... the frequency of the wave does not change)

So why do waves get faster in deeper water? The answer is a bit complex, but here is an explanation posted at the Illinois Department of Physics:
1. For a shallow fluid, the motion of the fluid is mostly side-to-side.
2. In order to accumulate more fluid in one place (to make the crest of the wave), each little bit of fluid must travel a little farther than it would have to in deeper water.
3. When a wave passes, the bits of fluid (if you could watch one at a time) travel in ellipses. For shallow water, the ellipses are stretched out horizontally, and in very deep water, they are very nearly circular.
4. So for a wave of the same height (top to bottom of the ellipse), the bits of water must travel farther in the shallow tray than the deep tray.
5. Because the waves of the same height in shallow and deep water exert the same pressure differences due to gravity to get the water moving (although the motion is different due to the fact that the bottom is there), similar forces push and pull on the water.
6. To get the water moving farther and faster with the same force takes a longer time for each push, and hence a slower speed for the wave which travels in the shallow water. "
(From https://van.physics.illinois.edu/qa/l...)

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