Lenses and Reflections

If you wear glasses you are familiar with lenses.

A lens is actually a device that bends incoming light to form an image of the object you want to see.

Lenses are typically made of a transparent substance like glass that has at least one curved surface. The key to understanding how a lens works in knowing that light waves slow down and bend when they travel through different substances. 
For example, light will slow down as it travels from the air in the room through the material that your glasses are made of. The rays of light that pass through a lens are bent or refracted.