Light not only bounces off surface it goes through some of them often slowing down and changing direction in the process called refraction. It occurs at the point where light travels from one medium to another of different density. Refraction produces mirages and rainbows.
When a ray of light (i.e., the incident ray) goes from rarer to denser medium, the ray (refracted ray) bends towards the normal in the denser medium.
If ray of light travels from denser to rarer medium the refracted ray bends away from the normal.
or
The above expression is an important result of the principle of reversibility of light which states that when a ray of light after suffering a large number of reflections and refractions has its final path reversed, it travels back along the same path in the opposite direction as shown below.
