Radiant Energy

Radiant energy exists in a range of wavelengths that extends from radio waves that many be thousands of meters long to gamma rays with wavelengths as short as a million-millionth (10^-12) of a meter.

Of particular importance to living things are

• light rays (~400–700 nm),
• the somewhat longer infrared or heat rays, and
• the somewhat shorter ultraviolet rays [Example].

• UV-A, 400–320 nm — the most abundant in sunlight;
• UV-B, 320–280 nm — responsible for tanning and the synthesis of vitamin D;
• UV-C, < 280 nm — a powerful germicidal agent.

The shorter wavelengths of ultraviolet 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).