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Real and apparent depth

The refraction of light at the surface of water makes ponds and swimming pools appear shallower than they really are. A 1m deep pond would only appear to be 0.75 m deep when viewed from directly above.



When light emerges from glass or water into air it speeds up again. If it meets the glass-air boundary at any angle other than 0o it will refract away from the normal. This is true for small angles something else happens when the angles get larger.

If you look at a stick that is poking into some water at an angle the stick looks bent because of refraction. The bottom of the stick seems to be nearer the surface of the water than it really is. It also explains why flat-based swimming pools appear to get shallower as you look towards the end furthest from you.

There is a connection between the real and apparent depths of the water. It can be proved that:

 
Refractive index = real depth/apparent depth


 
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© Keith Gibbs 2013