Prism splitting light

Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. The three basic dimensions of light (and of all electromagnetic radiation) are:

• intensity (or brilliance or amplitude, perceived by humans as the brightness of the light),
• frequency (or wavelength, perceived by humans as the color of the light), and
• polarization (or angle of vibration and not perceivable by humans under ordinary circumstances)

Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Light is the visible portion of the electromagnetic spectrum, between the frequencies of 380 THz (3.8×10¹⁴ hertz) and 750 THz (7.5×10¹⁴ hertz). Since the speed (v), frequency (f or ν), and wavelength (λ) of a wave obey the relation:

Because the speed of light in a vacuum is fixed, visible light can also be characterised by its wavelength of between 400 nanometres (abbreviated 'nm') and 800 nm (in a vacuum).

Light excites the rod cells and cone cells in the retina of the human eye, creating electrical nerve impulses that travel up the optic nerve to the brain, producing vision.

Main article: Speed of light

Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term.

The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (a Danish physicist), in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000 kilometres per second (approximately 141,050 miles per second).

The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second.

Albert A. Michelson improved on Rømer's work in 1926 used rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 miles/second (299,796 kilometres/second). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.

Main article: Refraction

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). When light passes through a transparent substance, such as air, water or glass, its speed is reduced, and it suffers refraction. The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as:

Thus, n=1 in a vacuum and n>1 in matter.

When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. If the incident beam is not orthogonal to the edge between the media, the direction of the beam will change. Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.

Main article: Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

The different wavelengths are detected by the human eye and then interpreted by the human brain as colors, ranging from red at the longest wavelengths (lowest frequencies) to violet at the shortest wavelengths (highest frequencies). The intervening frequencies are seen as orange, yellow, green, blue, and, conventionally, indigo.

The wavelengths of the electromagnetic spectrum immediately outside the range that the human eye is able to perceive are called ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength (low frequency) end. Although humans cannot see IR, they do perceive the near IR (shorter wavelength, higher frequency, higher energy) as heat through receptors in the skin. Cameras that can detect IR and convert it to light are called, depending on their application, night-vision cameras or infrared cameras (not to be confused with an image intensifier that only amplifies available visible light).

UV radiation is not directly perceived by humans at all except in a very delayed fashion, as overexposure of the skin to UV light can cause sunburn, or skin cancer, and underexposure can cause depression due to vitamin D deficiency. However, because UV is a higher frequency radation than visible light, it very easily can cause materials to fluoresce visible light.

Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR using pits in their heads.

• brightness (or temperature)
• illuminance or illumination (SI unit: lux)
• luminous flux (SI unit: lumen)
• luminous intensity (SI unit: candela)

Light can also be characterised by:

• brilliance (or amplitude),
• color (or frequency), and
• polarization (or angle of vibration).

There are many sources of light. A body at a given temperature will emit a characteristic spectrum of black body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which are generally very inefficient, emitting only around 10% of their energy as light and the remainder as "heat", i.e. infrared) and glowing solid particles in flames (see fire, red hot, white hot).

Atoms emit and absorb light at characteristic energies. Emission lines can either be stimulated, such as visible lasers and microwave maser emission, 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) or spontaneous.

Acceleration of a free charged particle, such as an electron, can produce visible radiation: Cyclotron radiation, Synchrotron radiation, and Bremsstrahlung radiation. 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. For example, fireflies produce chemicals that produce light by these mechanisms, and boats moving through water can disturb phosphorescent plankton.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in strip lights.

Particles striking certain chemicals can produce light by phosphorescence, for example, cathodoluminescence. This mechanism is used in oscilloscopes and televisions, and cathode ray tube.

Certain other mechanisms can produce light:

• scintillation
  • scintillator
• electroluminescence
• bioluminescence
• sonoluminescence
• triboluminescence
• radioactive decay
• particle-antiparticle annihilation