This is due to the phenomenon of refraction of light. The rays of light coming from the pool (or from the coil lying at the bottom of the can) travel from water to air. As the rays pass from denser (water) to rarer (air) medium they bend away from the normal. When the rays are produced back they form an image of the coin (bottom of the pool) at a point above the real position. hence the coin appears to be slightly raised and the pool papers to be less deep that it really is.
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