AQA GCSE Using wave front diagrams to explain refraction(Physics)
Building Ray diagrams for Refraction
Refraction is the change in direction of a wave when it crosses a boundary from one medium into another. As the wave enters the new medium its speed can change, this can cause a change in direction.
Wave front diagrams
The diagrams below will show what happens when water waves move from deep to shallow water crossing a boundary at an angle other than 90 degrees.
Lets draw the normal line on, this is at 90 degree to the boundary.
To keep the explanation simple most of the other waves have been removed from the diagram. This will allow you to see more clearly.
Notice how the direction of wave arrow at the boundary no longer stays horizontal. It refracts downwards towards the normal line. This means that the water waves have refracted towards the normal.
Speed of the water waves
As the water waves move from the deep to the shallow water, do you notice that they appear to be dragging backwards or lagging behind slightly. This is because as the waves move from the deep water to the shallow water, they slow down slightly.
Wavelength of the waves in water.
The distance between the wave fronts is the wavelength. Lets compare the size of the wavelength in the deep and shallow water.
The wavelength is greater in the deep water compared to the shallow water.
Putting all of this together!
The wave speed equation is
Wave speed, v = Frequency,f x wavelength,λ
From the deep water to the shallow water the frequency will stay constant
From deep water to the shallow water, both wave speed and wavelength will decrease.
Practice Question
1. Complete the following diagram to show refraction
2. From shallow to deep water the water wave refracts towards or away from the normal?
3. Using the wave fronts, how can we find the wavelength?
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