Radiant Energy

Over this long span, the only wavelengths that stimulate living things are

• light rays (~400 - 700 nm)
• the somewhat longer infrared or heat rays, and - in a few animals -
• the somewhat shorter ultraviolet rays.

However, x-rays and gamma rays can be absorbed by DNA and damage it - causing mutations.

Light makes possible a number of essential functions in living things:

• photosynthesis ("essential" is too mild a word here: photosynthesis provides the organic molecules upon which virtually all life depends)
• vision
• plant growth responses
  • photoperiodism
  • phototropism
• setting the biological "clocks" that adjust many metabolic functions in virtually all organisms (e.g., circadian rhythms).

Approximately one-half of the energy reaching the earth from the sun is light; that is, it is within the visible portion of the spectrum. The remainder arrives as heat and a small amount of ultraviolet light (which can, however, have a large effect on the aging of skin and the development of skin cancer).