A coin in a glass beaker appears to rise as the beaker is slowly filled with water, why?

It happens due to the, phenomenon of refraction of light. When the rays of light from the coin, in the denser medium which is water, fall on the interface separating the two media, the rays of light move away from the normal after refraction. The point from which the refracted rays appear to come gives the apparent position of the coin. As the rays appear to come from a point above the coin, therefore, the coin appears some distance above the base of beaker.