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.
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.
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.
In the 1660sRobert 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.
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 in 1862 in On Physical Lines of Force. In 1873 he published 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.
This theory combined the previous three theories, and proposed that light behaves as both particles and waves. It was pioneered at end of the nineteenth century by Max Planck, who proposed in 1900 that light waves are made of packets (quanta) of energy known as photons. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory, although he was not the first to propose the particle nature of light.
See speed of light. Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities (i.e. those associated with a direction). The speed of light is a scalar quantity (i.e. it has 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 Olaus Roemer's (a Danish physicist), in 1676. He had developed a method for measuring light. He observed and noted the motions of Jupiter and one of its moonss with a telescope. It was possible to time the revolution of the moon because it was eclipsed by Jupiter at regular intervalss. Roemer discovered that the moon revolved around Jupiter once every 42-1/2 hours when Earth was closest to Jupiter. The problem was that when Earth and Jupiter were not as close, the moon's revolution seemed to be more. It was clear that light took longer to reach Earth when it was farther away from Jupiter. The speed of light was calculated by analyzing the distance between the two planets at various times. Roemer reached a speed of 227,000 kilometers per second (approximately 141,050 miles per second).
Albert A. Michelson improved on Roemer's work in 1926. He 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 kilometers/second). In daily use, the figures are rounded off to 186,000 mi/s and 300,000 km/s.
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 frequencies of the spectrum immediately outside the range the human eye is able to perceive are called ultraviolet (UV) at the high frequency end and infrared (IR) at the low. Though humans cannot see IR, we do perceive it by receptors in the skin as heat. Cameras that can pick up 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.
