Eyes and Vision

The ultimate source of light on earth is the sun, and both plants and animals evolved physical and biochemical mechanisms to capture light and make responses to its presence or absence. Light causes phototropic movements of leaves and stems and timing of flowering in many plants, and wavelengths in the 600–700 nm range promote photosynthesis. Light often influences sexual reproductive cycles, biological and seasonal rhythms, color changes in the skin, hormone secretion, and various chemical reactions (for example, synthesis of vitamin D in the skin of humans) in animals. Even the simplest plant and animal forms have pigments that enable them to respond to light. The electromagnetic radiation from the sun encompasses a wide spectrum from gamma rays and x-rays (<0.1 nm) to ultraviolet (UV), visible, infrared (IR), radio waves, and other longer wavelengths. Electromagnetic radiation from the sun that reaches the earth spans the range of about 300–900 nm, but the peak in intensity is near...