The chief natural sources of light are sunlight, moonlight, starlight and the light from light-producing or luminescent organisms. However, only sunlight has greatest ecological significance.
Light can be defined as the visible part of the spectrum of solar radiant energy. In subsection 11-4.1 A (1), it has already been discussed that visible light comprises radiations of wave-lengths ranging from 390 mµ to 700 mµ and constitutes about 48 per cent of the total energy received on the earth’s surface. Visible light is made of a series of colours such as violet, indigo, blue, green, yellow, orange and red, constituting the visible spectrum (Fig. 11.5). Each colour of visible spectrum has different wave-length.
Further, light is highly directional. It differs in this respect from the temperature factor s:nce heat often reaches the living organism from many different directions at the same time. Light is also extremely variable. Ecologically variations in the quality of light (wavelength or colour), the intensity of light (actual energy measured in gram-calories or foot candles) and the duration of light (length of day) are known to influence different ecosystems.