See also: List of light sources
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), 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". The blue color is most commonly seen in a gas
flame or a welder's torch.
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
Acceleration 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
. This is used in fluorescent lights
. 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
Certain other mechanisms can produce light:Theories about light
In ancient India
, the philosophical schools of Samkhya
, from around the 6th
-5th century BC
, developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra
) out of which emerge the gross elements. The atomicity
of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.
On the other hand, the Vaisheshika school gives an atomic theory
of the physical world on the non-atomic ground of ether
, space and time. (See Indian atomism
.) The basic atoms
are those of earth (prthivı
), water (apas
), fire (tejas
), and air (vayu
), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. Light rays are taken to be a stream of high velocity of tejas
(fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas
Later in 499 AD
, who proposed a heliocentric solar system
in his Aryabhatiya
, wrote that the planets and the Moon
do not have their own light but reflect the light of the Sun
The Indian Buddhists
, such as Dignāga
in the 5th century
in the 7th century
, developed a type of atomism
that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy, similar to the modern concept of photons
, though they also viewed all matter as being composed of these light/energy particles.
Greek and Hellenistic theories
In the fifth century BC, Empedocles
postulated that everything was composed of four elements
; fire, air, earth and water. He believed that Aphrodite
made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.
In about 300 BC, Euclid
, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes ones eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.
In 55 BC
, a Roman who carried on the ideas of earlier Greek atomists
"The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.
" - On the nature of the Universe
Despite being remarkably similar to how we think of light today, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.
(c. 2nd century CE
) wrote about the refraction
of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes.
scientist Alhazen Abu Ali al-Hasan ibn al-Haytham
), also known as Alhazen
, developed a broad theory that explained vision, using geometry
, 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. This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes. 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, and went on to discover the laws of refraction.
He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Kitab-at-Manazir
was translated into Latin
in the Middle Ages
, as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon. Through these extensive researches on optics, is considered as the father of modern optics
Al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny particles travelling in straight lines, are reflected from objects into our eyes. He understood that light must travel at a large but finite velocity, and that refraction is caused by the velocity being different in different substances. He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.
(1596-1650) 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 that 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.
(1592-1655), 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
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
. His reputation helped the particle theory of light to dominate physics during the 18th century
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
. 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 color 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
) that diffraction
could more easily be explained by a wave theory.
independently worked out his own wave theory of light, and presented it to the
. 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
, in 1850
. His result supported the wave theory, and the classical particle theory was finally abandoned.
discovered that the angle of polarization of a beam of light as it passed through a polarizing material could be altered by a magnetic
field, an effect now known as Faraday rotation
. 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 ether.
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
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 in vain to resolve this contradiction.
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
. A photon has an energy, E, proportional to its frequency, f, by
is Planck's constant
, λ is the wavelength and c
is the speed of light
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
for his part in the founding of quantum theory.
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.
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 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.
- M. Muller. Rig-Veda-Samhita together with the Commentary of Sayana, Oxford University Press, London, 1890.
- B. K. Matilal. Nyaya-Vaisesika, Otto Harrassowitz, Wiesbaden, 1977.
- K. H. Potter, Indian Metaphysics and Epistemology, Princeton University Press, Princeton, 1977.
- G. J. Larson and R. S. Bhattacharya. Samkhya: A Dualist Tradition in Indian Philosophy, Princeton University Press, Princeton, 1987.
- S. S. De. In Issues in Vedic Astronomy and Astrology, Motilal Banarsidass, 1992.
- P. V. Vartak. Scientific Knowledge in the Vedas, Nag Publishers, 1995.
- S. Kak. "The Speed of Light and Purāṇic Cosmology". In T. R. N. Rao and S. Kak, Computing Science in Ancient India, pages 80-90. USL Press, Lafayette, 1998. Available as e-print physics/9804020 on the arXiv.
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