Classical Greece and Hellenism In the fifth century BC,
Empedocles postulated that everything was composed of
four elements; fire, air, earth and water. He believed that goddess
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 one's eyes, then opens them at night. 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 that "The light & 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." (from
On the nature of the Universe). Despite being similar to later particle theories, Lucretius's views were not generally accepted.
Ptolemy (c. second century) wrote about the
refraction of light in his book
Optics.
Classical India In
ancient India, the
Hindu schools of
Samkhya and
Vaisheshika, from around the early centuries AD 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. The
Vishnu Purana refers to sunlight as "the seven rays of the sun". In 1637 he published a theory of the
refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves. Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media. Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes's theory of light is regarded as the start of modern physical optics. Another supporter of the wave theory was
Leonhard Euler. He argued in
Nova Theoria lucis et colorum (1746) that
diffraction could more easily be explained by a wave theory. 's sketch of water waves showing
diffraction The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by
Thomas Young). Young showed by means of numerous
diffraction experiments that light behaved as waves. He first publicly stated his "general law" of interference in January 1802, in his book
A Syllabus of a Course of Lectures on Natural and Experimental Philosophy:But the general law, by which all these appearances are governed, may be very easily deduced from the interference of two coincident undulations, which either cooperate, or destroy each other, in the same manner as two musical notes produce an alternate intension and remission, in the beating of an imperfect unison.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. In 1816
André-Marie Ampère gave
Augustin-Jean Fresnel an idea that the polarization of light can be explained by the wave theory if light were a
transverse wave. Later, Fresnel independently worked out his own wave theory of light and presented it to the
Académie des Sciences in 1817.
Siméon Denis Poisson challenged Fresnel's model, claiming that it predicted a bright spot in the shadow behind a circular obstacle contrary to common sense.
Dominique-François-Jean Arago created an experiment that showed the
bright spot: Poisson's challenge became new evidence for the wave theory. His result supported the wave theory, and the classical particle theory was finally abandoned (only to partly re-emerge in the twentieth century as
photons in
quantum theory).
Electromagnetic theory electromagnetic wave traveling along the z-axis, with E denoting the
electric field and perpendicular B denoting
magnetic field|400x400px In 1845,
Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the
magnetic field direction in the presence of a transparent
dielectric, an effect now known as
Faraday rotation. This was the first evidence that light was related to
electromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines. 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 behavior of electric and magnetic fields, still known as
Maxwell's equations. Soon after,
Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction and
interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging and wireless communications. In the quantum theory, photons are seen as
wave packets of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as
spectral lines).
Quantum theory In 1900
Max Planck, attempting to explain
black-body radiation, suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "
quanta" (from a Latin word for "how much"). In 1905, Albert Einstein used the idea of light quanta to explain the
photoelectric effect and suggested that these light quanta had a "real" existence. In 1923
Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called
Compton scattering) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926
Gilbert N. Lewis named these light quanta particles
photons. Eventually
quantum mechanics came to picture light as (in some sense)
both a particle and a wave, and (in another sense) as a phenomenon which is
neither a particle nor a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, under some approximations light can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles) and sometimes another macroscopic metaphor (waves). As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both. In 1924–1925,
Satyendra Nath Bose showed that light followed different statistics from that of classical particles. With Einstein, they generalized this result for a whole set of integer spin particles called
bosons (after Bose) that follow
Bose–Einstein statistics. The photon is a massless boson of
spin 1. In 1927,
Paul Dirac quantized the
electromagnetic field.
Pascual Jordan and
Vladimir Fock generalized this process to treat many-body systems as excitations of quantum fields, a process with the misnomer of
second quantization. And at the end of the 1940s a full theory of
quantum electrodynamics was developed using quantum fields based on the works of
Julian Schwinger,
Richard Feynman,
Freeman Dyson, and
Shinichiro Tomonaga.
Quantum optics John R. Klauder,
George Sudarshan,
Roy J. Glauber, and
Leonard Mandel applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the
statistics of light (see
degree of coherence). This led to the introduction of the
coherent state as a concept which addressed variations between laser light, thermal light, exotic
squeezed states, etc. as it became understood that light cannot be fully described just referring to the
electromagnetic fields describing the waves in the classical picture. In 1977,
H. Jeff Kimble et al. demonstrated a single atom emitting one photon at a time, further compelling evidence that light consists of photons. Previously unknown quantum states of light with characteristics unlike classical states, such as
squeezed light were subsequently discovered. Development of short and
ultrashort laser pulses—created by
Q switching and
modelocking techniques—opened the way to the study of what became known as ultrafast processes. Applications for solid state research (e.g.
Raman spectroscopy) were found, and mechanical forces of light on matter were studied. The latter led to levitating and positioning clouds of atoms or even small biological samples in an
optical trap or
optical tweezers by laser beam. This, along with
Doppler cooling and
Sisyphus cooling, was the crucial technology needed to achieve the celebrated
Bose–Einstein condensation. Other remarkable results are the
demonstration of quantum entanglement,
quantum teleportation, and
quantum logic gates. The latter are of much interest in
quantum information theory, a subject which partly emerged from quantum optics, partly from theoretical
computer science. ==Use for light on Earth==