[image] Light is Electromagnetic radiation with a Wavelength that is visible to the Eye (visible light) or, in a technical or scientific setting, electromagnetic radiation of any wavelength. The three basic dimensions of light (i.e., all electromagnetic radiation) are:
Visible light is the portion of the Electromagnetic spectrum between the frequencies of 380 THz (3.8×1014 Hertz) and 750 THz (7.5×1014 Hertz). The speed (c), Frequency (f or
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 entering the eye is absorbed by light-sensitive pigments within the rod cells and cone cells in the Retina, triggering a cascade of events that creates electrical nerve impulses that travel through the Optic nerve to the Brain, producing vision.
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.
Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved 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 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.
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 undergoes refraction. The reduction of the speed of light in a denser material can be indicated by the Refractive index, n, which is defined as:
| n = | c –– v | ! |
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.
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.
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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 (longer wavelength, lower 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 radiation 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.
Light can also be characterised by:
| SI photometry units | ||||
|---|---|---|---|---|
| Quantity | SI unit | Symbol | Notes | |
| Luminous energy | lumen seconds | lm · s | lumen seconds are sometimes called Talbots | |
| Luminous flux | lumen or (candela · Steradian) | lm | also called luminous power | |
| Luminous intensity | Candela or (lumen / steradian) | cd | ||
| Luminance | candela / Square metre | cd/m2 | also called luminosity | |
| Illuminance | Lux or (lumen / square metre) | lx | ||
| Luminous efficacy | lumens per Watt | lm/W | maximum possible (@ 555 nm) is 683 | |
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. In living things, this process is called Bioluminescence: for example, fireflies produce light by this means, and boats moving through water can disturb glowing 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:
Ptolemy also wrote about the refraction of light.
Newton's theory could be used to predict the Reflection of light, but could only explain Refraction by incorrectly assuming that light accelerated upon entering a denser Medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.
The wave theory predicted that light waves could interfere with each other like Sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye.
Another supporter of the wave theory was Euler. He argued in Nova theoria lucis et colorum (1746) that Diffraction could more easily be explained by a wave theory.
Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.
The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the Luminiferous aether was proposed, but its existence was cast into strong doubt by the Michelson-Morley experiment.
Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the Speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.
Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. The technology of Radio transmission was, and still is, based on this theory.
The constant speed of light predicted by Maxwell's equations contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. A solution to this contradiction would later be found by Albert Einstein.
| E f = hf = | hc ––– λ | ! |
where h is Planck's constant,
As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.
The electric and magnetic fields are perpendicular to the direction of travel and to each other. This picture depicts a very special case, linearly polarized light. See Polarization for a description of the general case and an explanation of linear polarization.
While these relations of the electric and magnetic fields are always true, the subtle difference in the general case is that the direction and amplitude of the magnetic (or electric) field can vary, in one place, with time, or, in one instant, can vary along the direction of propagation.
| Electromagnetic Spectrum Gamma ray | X-ray | Ultraviolet | Optical spectrum | Infrared | Terahertz radiation | Microwave | Radio waves Optical (visible) spectrum: Violet | Indigo | Blue | Cyan | Green | Yellow | Orange | Red |