Light serves two important functions for life on earth: source of energy that sustains all life, provides organisms with information about the physical world. Reason for life on earth has to do with distance from earth and sun. Sun converts 4 million tonnes of matter into energy every second, which is given off as electromagnetic radiation which travels at the speed of light and reaches earth. Light is defined as the portion of the electromagnetic spectrum that is visible which spans wavelengths from 400nm to 700nm. Light behaves as a stream of energy particles whose packets of energy are called photons. Each photon contains a specific amount of energy, the amount which is inversely related to its wavelength. Light is able to interact with and change matter which allow light to be used by living things. When photons hit and object, three possibilities: can be reflected off object, transmitted through object, absorbed by object.