light

This i from me Dhafer for everyone in 4Victorious:].This is the information about heat.I copied it from this website.this is the shortcut **[]** **Light** is [|electromagnetic radiation], particularly radiation of a [|wavelength] that is [|visible] to the [|human eye] (about 400–700 [|nm], or perhaps 380–750 nm[|[1]]). In [|physics], the term //light// sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[|[2]][|[3]] Four primary properties of light are: Light, which exists 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]. The speed of light in a [|vacuum] is presently defined to be exactly 299,792,458 [|m/s] (approximately 186,282 miles per second). This definition of the speed of light means that the [|metre] is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited. Different physicists have attempted to measure the speed of light throughout history. [|Galileo] attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by [|Ole Rømer], a Danish physicist, in 1676. Using a telescope, Ole observed the motions of [|Jupiter] and one of its [|moons], [|Io]. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of [|Earth]'s orbit.[|[4]] Unfortunately, its size was not known at that time. If Ole had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by [|Hippolyte Fizeau] in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would 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 was able to calculate the speed of light as 313,000,000 m/s. [|Léon Foucault] used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. [|Albert A. Michelson] conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods 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 299,796,000 m/s. Two independent teams of physicists were able to bring light to a complete standstill by passing it through a [|Bose-Einstein Condensate] of the element rubidium, one led by Dr. Lene Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.[//[|citation needed]//]
 * [|Intensity]
 * [|Frequency] or [|wavelength]
 * [|Polarization]
 * [|Phase]

[[|edit]] Electromagnetic spectrum
Main article: [|Electromagnetic spectrum][|Electromagnetic spectrum] with light highlighted Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and [|near fields]) is classified by wavelength into [|radio], [|microwave], [|infrared], the [|visible region] we perceive as light, [|ultraviolet], [|X-rays] and [|gamma rays]. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

[[|edit]] Refraction
//Main article: [|Refraction]// Refraction is the bending of light rays when passing from one transparent material to another. It is described by [|Snell's Law]: where θ1  is the angle between the ray and the [|normal] in the first medium,  θ2  is the angle between the ray and the [|normal] in the second medium, and n1 and n2 are the [|indeces of refraction], //n// = 1 in a [|vacuum] and //n// > 1 in a [|transparent] [|substance]. When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not [|orthogonal] (or rather [|normal]) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as [|refraction]. The refractive quality of [|lenses] is frequently used to manipulate light in order to change the apparent size of images. [|Magnifying glasses], [|spectacles], [|contact lenses], [|microscopes] and [|refracting telescopes] are all examples of this manipulation. Light refraction is the main basis of measurement for [|gloss]. Gloss is measured using a [|glossmeter].

[[|edit]] Optics
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] and the [|aurora borealis] offer many clues as to the nature of light as well as much enjoyment.

[[|edit]] Light sources
See also: [|List of light sources]A [|cloud] illuminated by [|sunlight] 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 [|K] peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units [|[1]] 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 of this. This mechanism is used in [|cathode ray tube] [|televisions].