Light sources
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
Indian theories
In
ancient India, the philosophical schools of
Samkhya and
Vaisheshika, 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 atoms.
Later in
499 AD,
Aryabhata, who proposed a
heliocentric solar system of
gravitation 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 and
Dharmakirti 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 wrote
Optica, 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,
Lucretius, a Roman who carried on the ideas of earlier Greek
atomists, wrote:
"
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.
Ptolemy (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.
Optical theory
The
Persian scientist
Alhazen Abu Ali al-Hasan ibn al-Haytham (c.
965-
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. 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.
The 'plenum'
(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.
Particle theory
Pierre Gassendi (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 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.
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 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 (
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
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
, in
1850. His result supported the wave theory, and the classical particle theory was finally abandoned.
Electromagnetic theory
In
1845,
Faraday 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.
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. A photon has an energy, E, proportional to its frequency, f, by
where
h 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 in
1918 for his part in the founding of quantum theory.
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.
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.
References
- 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.
See also
Wikimedia Commons has media related to:
LightThis article is licensed under the
GNU Free Documentation License. It uses material from the
Wikipedia article "Light".
Stumble It!
Tweet This
Bookmark on Delicious
Share on Facebook
Share on MySpace