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When water waves travel from a deep region

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When water waves travel from a deep region to a shallower region, its wavelength decreases, explain fully why this happens.

This problem has appeared in my recent examination and I'm not sure if the examiner will accept my explanation:

In the shallow region, there is more "ground", so when waves enter from a deep region to the shallow region, it "hits the extra ground", and therefore its speed decreases. Since the frequency of the wave stays constant, its wavelength is decreased.

I know the first part of my explanation is probably not the examiners are looking for, the explanation they are likely to accept is:

The shallow region are higher than the deep region, so when the waves enter the shallow region, it loses kinetic energy in terms to gain potential energy.

The marking schedule for this question is likely to ranges from Not Achieve, Achieve, Merit to Excellence (this is New Zealand's new education system). What kind of grades would I be likely to get?
 
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HallsofIvy

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"The shallow region are higher than the deep region"

This doesn't really make sense to me. I assume you MEANT the bottom is higher. The water itself if NOT going up (sea level is sea level!) so it is not gaining potential energy.

Honestly, you first answer seemed better to me. You might phrase it more precisely: the waves are losing energy (and so speed) because the water is "rubbing" against the bottom more and losing energy to friction.
 

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