Life on earth receives the blessings of the light emitted by the sun. The energy that reaches the earth from the sun is about 2 calories per square centimeter per minute, the figure known as solar constant. Calculations based on this figure indicate that every second the sun emits as much energy into space as burning 10 quadrillion (10,000 trillion) tons of coal.

The greatest of the sun's gifts to earth is photosynthesis by plants. When a material absorbs the energy of light, the light is changed into heat, thereby raising the object's temperature. There are also cases where fluorescence or phosphorescence is emitted, but most of the time materials do not change. Sometimes, however, materials do chemically react to light. This is called a "photochemical reaction." Photochemical reactions do not occur with infrared light, but happen primarily when visible light and ultraviolet rays, which have shorter wavelengths, are absorbed.