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)
