Light Visible Light Speed Of Light Refraction Optics

Visible light is that portion of the electromagnetic spectrum between the frequencies of 7.5×10¹⁴ hertz (abbreviated 'Hz') and 3.8×10¹⁴ Hz. Since the speed (v), frequency (f or ν), and wavelength (λ) of a wave obey the relation:

and the speed of lightCherenkov effect in a "swimming pool" nuclear reactor. The effect is due to electrons moving faster than the speed at which light moves in water. The speed of light (denoted as c reputedly from the Latin celeritas "speed", and also known as Einstein's con in a vacuum c is fixed, visible light can also be characterised by its wavelength of between 400 nanometreA nanometre ( American spelling: nanometer is 10−9 metres—or one billionth of a metre. Its symbol is nm . It is an SI measure of length, commonly used in measuring the wavelengths of visible light, ultraviolet radiation and gamma rays; amongst others (abbreviated 'nm') and 800 nm (in a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also).

Visible light excites the rod cellRod cells are photoreceptor cells in the retina that function in less intense light. Rods are named for their cylindrical shape. They are concentrated at the outer edges of the retina (see peripheral vision). There are about 120 million rod cells in the hs and cone cellCone cells or cones are cells in the retina which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea and gradually becoming sparser towards the outside of the retina. Cones are less sensitive ths in the retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu of the human eye, creating electrical nerve impulses that travel up the optic nerve to the brain, producing vision.

2 Speed of light

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, it has 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.

3 Refraction

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 enter in a medium from vacuum or another medium, it keep the same frequency and changes its wavelenth. 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.

4 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 offers many clues as to the nature of light as well as much enjoyment.

5 Color and wavelengths

The different wavelengths are 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, we do perceive the near IR (shorter wavelength, higher frequency, higher energey) as heat through receptors in the skin. Cameras that can detect IR and convert it to visible light are called night-vision cameras.

UV radiation is not perceived by humans at all except in a very delayed fashion, as overexposure of the skin to UV light causes sunburn, or skin cancer. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR using pits in their heads.

6 Measurement of light

The following quantities and units are used to measure light.

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).

6.1 SI light units

SI light units [ }\|action=edit}} Edit }]
Quantity	SI unit	Symbol	Notes
Luminous intensity	candela	cd
Luminous flux	lumen or ( candela · steradian )	lm
Luminance	candela / square metre	cd/ m²
Illuminance	lux or (lumen / square metre)	lx
Luminous efficacy	lumens per watt	lm/ w

7 Light sources

There are many sources of visible 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 visible 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 signs, mercury 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 light 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 visible light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in strip light s.

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

Certain other mechanisms can produce visible light:

8 Theories about light

8.1 Early Greek ideas

In 55 BC Lucretius, continuing the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles.

Ptolemy also wrote about the refraction of light.

8.2 10th century optical theory

The scientist Abu Ali al-Hasan ibn al-Haytham (965-c.1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light. Alhazen's work did not become known in Europe until the late 16th century.

8.3 The 'plenum'

René Descartes held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light travelled faster in a denser medium, by analogy with the behaviour of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.

8.4 Particle theory

Pierre Gassendi, an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.

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.

8.5 Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

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 colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured 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. 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 later disproved.

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.

8.6 Electromagnetic theory

In 1845, Faraday discovered that the angle of polarisation of a beam of light as it passed through a polarising material could be altered by a magnetic field. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether.

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.

8.7 Particle theory revisited

The wave theory was accepted until the late 19th century, when Einstein described the photoelectric effect, by which light striking a surface caused electrons to change their momentum, which indicated a particle-like nature of light. This clearly contradicted the wave theory, and for years physicists tried to rectify this contradiction without success.

8.8 Quantum theory

In 1900, Max Planck described quantum theory, in which light is considered to be as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. 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.

8.9 Wave-particle duality

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature, and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

8.10 A light wave

This is a light wave frozen in time and shows the two components of light; an electric field and a magnetic field that oscillate perpendicular to each other and to the direction of motion (a transverse wave).

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 the above statements about the 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.

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1 Visible light