When a coin is placed at the bottom of a glass containing water, it appears to rise due to the phenomenon of refraction of light. 

When an object is placed in the denser medium and it is viewed from rarer medium then it seems to be at less depth than the actual depth.

In this case, the water is the denser medium and air is the rarer medium. This refraction makes the coin appear to be at a higher position than its actual location at the bottom of the glass.