Light
Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps.
Generally, electromagnetic radiation (EMR) is classified by wavelength into radio waves, microwaves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays and gamma rays.
When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.
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.
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 313000000 m/s.
The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light.
The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images.
A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared and only a fraction in the visible spectrum.
The peak of the black-body spectrum is in the deep infrared, at about 10 micrometre wavelength, for relatively cool objects like human beings.
Blue-white thermal emission is not often seen, except in stars (the commonly seen pure-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 and is not seen in stars or pure thermal radiation).
The cone cells in the human eye are of three types which respond differently across the visible spectrum and the cumulative response peaks at a wavelength of around 555 nm.
The photometry units are designed to take this into account and therefore are a better representation of how "bright" a light appears to be than raw intensity.
They relate to raw power by a quantity called luminous efficacy and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings.
[22] However, in nanometre-scale applications such as nanoelectromechanical systems (NEMS), the effect of light pressure is more significant and exploiting light pressure to drive NEMS mechanisms and to flip nanometre-scale physical switches in integrated circuits is an active area of research.
[23] At larger scales, light pressure can cause asteroids to spin faster,[24] acting on their irregular shapes as on the vanes of a windmill.
[28] As a consequence of light pressure, Einstein in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter.
Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically.
[33] In ancient India, the Hindu schools of Samkhya and Vaisheshika, from around the early centuries AD developed theories on light.
[34] René Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Witelo as well as the "species" of Roger Bacon, Robert Grosseteste and Johannes Kepler.
Isaac Newton studied Gassendi's work at an early age and preferred his view to Descartes's theory of the plenum.
A translation of Newton's essay on light appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.
To explain the origin of colours, Robert Hooke (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work Micrographia ("Observation IX").
He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.
The existence of the hypothetical substance luminiferous aether proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the Michelson–Morley experiment.
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.
Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging and wireless communications.
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.
With Einstein, they generalized this result for a whole set of integer spin particles called bosons (after Bose) that follow Bose–Einstein statistics.
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).
Applications for solid state research (e.g. Raman spectroscopy) were found, and mechanical forces of light on matter were studied.
