When light rays reach an angulated surface of a different material, it causes the light rays to bend. This is called refraction. When light reaches a convex lens, the light rays bend toward the center:
When light rays reach a concave lens, the light rays bend away from the center:
The eye has multiple angulated surfaces that cause light to bend. These are:
- The interface between the air and the front of the cornea
- The interface between the back of the cornea and the aqueous humor
- The interface between the aqueous humor and the front of the lens
- The interface between the back of the lens and the vitreous humor
When everything is working correctly, light makes it through these four interfaces and arrives at the retina in perfect focus.