Properties of the Light

• Vision (sight) is the perception of objects in the environ­ment by means of the light that they emit or reflect.
• Light is visible electromagnetic radiation.
• Human vision is lim­ited to wavelengths ranging from about 400 to 750 nm. The ultraviolet (UV) radiation just below 400 nm and the infrared (IR) radiation just above 700 nm are invisible to us, although some animals can see a little farther into those ranges than we can. Most solar radiation that reaches the surface of the earth falls within this range; radiation of shorter and longer wavelengths is generally filtered out by ozone, carbon dioxide, and water vapor in the atmosphere. Vision is thus adapted to take advantage of the radiation that is most available to us.Yet there is further reason for vision to be limited to this range of wavelengths.
• To produce a physiological response, light must cause a photochemical reaction—a change in chemical structure caused by light energy. When an electron absorbs a photon of light, it is boosted to a higher energy level (orbit) around its nucleus and may transfer to another atom. The transfer of an electron from one atom to another is the essence of a chemical reaction. Ultraviolet radiation has so much energy that it ionizes organic molecules and kills cells. It is useful for sterilizing food and instruments, but it has too much energy for the biochemical processes of vision. Infrared radiation has too little energy to activate the visual process. It warms the tis­sues (heat lamps are based on this principle) but does not usually cause chemical reactions.

Image from wikipedia;www.hf.faa.gov/webtraining/VisualDisplays/VisDispImages/fireworksPNG/Elspectrum.png