light or
visible light is the portion of
electromagnetic radiation that is
visible to the
human eye, responsible for the sense of
sight. Visible light has a
wavelength in a range from about 380 or 400
nanometres to about 760 or 780 nm,
[1] with a frequency range of about 405 THz to 790 THz. In
physics, the term
light often comprises the adjacent radiation regions of
infrared (at lower frequencies) and
ultraviolet (at higher), not visible to the human eye.
[2][3]
Primary properties of light are
intensity, propagation direction,
frequency or
wavelength spectrum, and
polarization, while its speed, about 300,000,000 meters per second (300,000 kilometers per second) in a vacuum, is one of the fundamental constants of nature.
Light, which is emitted and absorbed in tiny "packets" called
photons, exhibits properties of both
waves and
particles. This property is referred to as the
wave–particle duality. The study of light, known as
optics, is an important research area in modern physics.
There are
many sources of light. The most common light sources are thermal: a body at a given
temperature emits a characteristic spectrum of
black-body radiation. Examples include
sunlight (the radiation emitted by the
chromosphere of the
Sun at around 6,000
Kelvin peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units
[6] and roughly 40% of sunlight is visible),
incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in
flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is
heated to "red hot" or "white hot". Blue
thermal emission is not often seen. The commonly seen blue colour in a
gas flame or a
welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm).
Atoms emit and absorb light at characteristic energies. This produces "
emission lines" in the spectrum of each atom.
Emission can be
spontaneous, as in
light-emitting diodes,
gas discharge lamps (such as
neon lamps and
neon signs,
mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example,
sodium in a gas flame emits characteristic yellow light). Emission can also be
stimulated, as in a
laser or a microwave
maser.
Deceleration of a free charged particle, such as an
electron, can produce visible radiation:
cyclotron radiation,
synchrotron radiation, and
bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible
Cherenkov radiation.
Certain chemicals produce visible radiation by
chemoluminescence. In living things, this process is called
bioluminescence. For example,
fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as
fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as
phosphorescence.
Phosphorescent materials can also be excited by bombarding them with subatomic particles.
Cathodoluminescence is one example. This mechanism is used in
cathode ray tube television sets and
computer monitors.
