How we see
The images we see are made up of light reflected from the objects we look at. This light enters the eye through the cornea. Because this part of the eye is curved, it bends the light, creating an upside-down image on the retina (this is eventually put the right way up by the brain).
What happens when light reaches the retina?
The retina is a complex part of the eye, but only the very back of it is light-sensitive. This part of the retina has roughly the area of a quarter coin, and is packed with photosensitive cells called rods and cones. These allow us to see images in color and detail, and to see at night.
Cones are the cells responsible for daylight vision. There are three kinds - each responding to a different wavelength of light: red, green and blue. The cones allow us to see in color and detail.
Rods are responsible for night vision. They are sensitive to light but not to color. In darkness, the cones do not function at all.
Focusing the image
The lens focuses the image. It can do this because it is adjustable - using muscles to change shape and help us focus on objects at different distances. The automatic focusing of the lens is a reflex response and is not controlled by the brain.
Sending the image to the brain
Once the image is clearly focused on the sensitive part of the retina, energy in the light that makes up that image creates an electrical signal. Nerve impulses can then carry information about that image to the brain through the optic nerve.