5 The Eye
The human eye is the organ that evolved to convert visible wavelengths of electromagnetic radiation into electrical impulses that our brains can interpret.
The iris (coloured portion of the eye) acts as a diaphragm to expand or contract the pupil (the dark center of the eye). This determines the amount of light that is allowed to enter the eye through the transparent outer membrane, called the cornea, before passing through the focusing lens to strike the retina.
The retina is the light-sensitive collection of specialized cells at the back of the eye. A delicate film of nerve fibres branch out from these light-sensitive cells to the optic nerve, and then on to the brain.
The retina is made of two different types of optic nerve receptors which permit vision: rods which provide sensitivity to brightness, allowing us to see at low levels of light, and the cones which allow us to perceive finer details and to distinguish colours.
The ability and function of the human eye is often compared to an analog camera, in that both use a lens to focus images at various distances onto a light-sensitive surface.