Refraction of coin in glass

When there is no water in the glass, the light from the coin experiences very little refraction and we can see the light from the penny. When there is water in the glass, the light from the coin experiences refraction (bending) and reflection (total internal reflection, TIR) such the light it is not able to reach the observer’s eyes. A small amount of refraction occurs as the light leaves the coin and travels through the bottom of the glass, then refraction occurs again as it enters the water. When refraction occurs at the side of the glass there is Total Internal Reflection because of the angle of incidence. The light ends up traveling at such an angle that we are unable to see the coin.