Light is necessary for the survival of life on Earth, as almost all energy used by life is derived from the sun. Plants and algae use light energy in photosynthesis, which provides usable chemical energy for heterotrophic organisms. In addition, without light, the Earth would be completely cold and dark, as even the moon gets its glow from reflecting the sun.
Light is also used by many organisms in the sense of sight. While the most significant source of light is the sun, other sources of light, such as fire and electric lighting, are useful to human civilization. Some organisms are capable of producing their own light, or bioluminescence, through chemical means. Examples include the firefly, the railroad worm and the vampire squid. Remarkably, however, bioluminescence produces little heat.
All light is electromagnetic radiation. Visible light is usually between 400 and 700 nanometers in wavelength. Infrared light is characterized by wavelengths longer than those of visible light, while ultraviolet light is typified by wavelengths that are shorter. Wavelength is determined by frequency, where higher frequencies result in shorter wavelengths and lower frequencies in longer ones. The speed of light is considered a constant of the universe at approximately 186,000 miles per second. Photons are the individual unit of light and have properties similar to both waves and particles.