Refraction

The path of light can be changed by reflection, but there is also another way it can bend. Swimming pools look shallower than they really are because of the way the light from the bottom of the pool is refracted (bent) as it moves from the water into the air.

Light travels fastest through space, and moves more slowly through air. Water slows light down by about 25 percent, and glass slows it down even more (approximately 35% slower). When light travels through glass, the light waves that hit the glass first slow down, but the light waves behind it continue to move at the same speed. This causes the light to bend or change direction (refract). The light waves slow down, one by one, bending the beam toward the glass. When the light waves pass back out of the glass they speed back up, while those behind it continue to move slowly. This type of bending light ( passing from water to air) is called refraction.

Another example of refraction is reaching into dishwater or picking up a stone in a stream.  In both cases, the things you see above water are not in the location you think they are.  When you reach into the dishwater to scoop up a fork, or into a stream to pick up a rock, you may very well be reaching for nothing at all! The location of the fork and the rock would seem to be obvious however, light is playing a trick on you through the process of refraction. Both the fork and the rock appear to be further away than they really are. Try testing it out for yourself!