Nov 21, 2012

Light

Light is a form of energy visible to the human eye that is radiated by moving charged particles. Light from the Sun provides the energy needed for plant growth. Plants convert the energy in sunlight into storable chemical form through a process called photosynthesis. Petroleum, coal, and natural gas are the remains of plants that lived millions of years ago, and the energy these fuels release when they burn is the chemical energy converted from sunlight. When animals digest the plants and animals they eat, they also release energy stored by photosynthesis.

Scientists have learned through experimentation that light behaves like a particle at times and like a wave at other times. The particle-like features are called photons. Photons are different from particles of matter in that they have no mass and always move at the constant speed of about 300,000 km/sec (186,000 mi/sec) when they are in a vacuum. When light diffracts, or bends slightly as it passes around a corner, it shows wavelike behavior. The waves associated with light are called electromagnetic waves because they consist of changing electric and magnetic fields.

Light as an electromagnetic radiation that can be detected by the human eye. In terms of wavelength, electromagnetic radiation occurs over an extremely wide range, from gamma rays with a wavelength of 3x10−14 centimeter to long radio waves measured in millions of kilometers. In that spectrum the wavelengths visible to humans occupy a very narrow band, from about 7x10−5 centimeter (red light) down to about 4x10−5 centimeter (violet). The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. (Adapted from Britannica Encyclopedia and Encarta Encyclopedia)