History of light

As long humans were thinking about the nature of light, as many different theories of light there were and still are. The way of these attempts establishing a universal theory should be kept track abridged and partially.

Greek optics

Long before there is ever a possibility of transcription of thoughts and ideas, humans admire a sun god. The greeks take a try in ancient times, explaining nature not in religious, but in a rational way.

To begin with Anaximander (610-546 B.C.), who regards day light from the sky and sun light as two different kinds of light, being independent form each other. To him darkness has got an own physical and material existence and is not just the absence of light.

Pythagoras (536-497 B.C.) continues egyptian tradition, in which lights stands for the good and darkness representing the evil.

Anaxagoras (500-428 B.C.) and Empedocles (490-424 B.C.) are the first ones regarding darkness as the absence of light. By means of the phases of the moon, he explains that the moon reflects sun light, instead of emitting light by itself. Even for him it is the other way round, day light and sun light is connected. He is the first one trying to explain vision as the unity of fire in the eyes with the fire of the objects. From this point of time the theory of light is closely linked to the theory of vision.

While Democrit's (460-370 B.C.) intromission theory - based on his theory of atomic particles - assumes light rays emitting from the objects and received by the eyes, Plato (427-347 B.C.) establishes an extramission theory a little later, according to which the eyes emitting light rays to see the objects. But both theories are not able to explain certain phenomenons. The intromission theory e.g. can not satisfactory explain, how surfaces of large objects get into the eye without losing their shape. The extramission theory on the other hand has got no answer to the question, how rays from the humans' eyes can make it to far distant stars.

Aristotle (384-322 B.C.) constructs an intromission theory with one significant difference. To him the key to the nature of light lies in transparent bodies, which work as a medium, transferring not light, but colour. This medium contains a substance, that becomes transparent, e.g. by a luminous body like the sun. Aristotle's theory of light and vision is part of a comprehensive and unifying picture of the world and seems strange and hard to understand in nowadays. However it shows early approaches of a wave theory of light. Even his thoughts involve an intromissiontheory, there's a fundamental difference to Democrit's theory, in which atomic particles move from the regarded object to the eye. From this point of time on the intromission theory splits further into a particle and a wavetheory. This separation proceeds till modern times.

In his book Optica Euclid (330-370 B.C.) tries to put the nature of light into an exact mathematical formula. The result is geometric optics, which assumes a radial straight-line propagation of light in space. Extramission theory as a basis enables Euclid to forecast a variety of laws of perspective and geometric optics.

The last to be mentioned in a row of famous greek optics protagonists is Ptolemy (90-168 A.D.), who continues Euclid's tradition. Even he also favors an extramission theory, his deliberations are more physically shaded, than the Euclid's pure mathematical theory. Ptolemy adds experiments of refraction and diffraction to his theory and concludes that rays of vision from the eyes are of the same nature as light rays being emitted from a luminous body.

With that finally theory of light and theory of vision converge at the end of greek science.

Optic in the middle ages

After the fall of the roman empire science - incl. optics - falls into a long and deep sleep. In this period of time the arabic world becomes a locus of new scientific discoveries and preservation of classic writings from greek-roman times.

Al-Kindi (801/813-866; sources vary) is the first arabian contributor to the field of optics. As a supporter of Plato's extramission theory he develops a technique to analyse light rays on a point-to-point basis. He regards every single point of a luminous body as an emitter of rays, which propagates in all directions, independent of other nearby illuminating points. This technique is still used in geometric optics today.

As probably one of the greatest arabian scientists of the middle ages, Alhazen (965-1040) succeeds in combining Euclid's geometric optics and Ptolemy's intromission theory of vision - on the basis of Al-Kindi's point-to-point analysis. Besides his theory of vision - as maybe one of his greatest triumphs - he does further important discoveries on the field of theoretical and experimental optics.

Roger Bacon (1214-1292) adapts Alhazen's theory of vision almost in its entirety. To construct a consistent theory he takes a piece of almost every other author who writes about optics before.

The greatest advance in understanding about the nature of light brings Renaissance jarring and pulling down Aristotle's concept of the universe.

Renaissance of optics

In the 16th and early 17th century Nicolaus Copernicus (1473-1543), Johannes Kepler (1571-1630) and Galileo Galilei (1564-1642) dismantle the aristotelian concepts of astronomy and mechanics and lay the foundations of modern science. Kepler is initially successful in giving a comprehensive description of the mechanics of the eye, considering every light ray falling into it. He completes attempts of a unification of geometric optics with the help of intromission theory of vision, that Alhazen starts 300 years before.

French philosopher René Descartes (1596-1650) is considered to be one of the most influential scientific thinkers of his time. Like Aristotle 2.000 years prior, he tries to establish an uniform system of the world, that is able to explain every possible events in nature. Therefore he assumes a matter, which fills all space - the ether. In his theory light has got infinite speed and is not motion, but a tendency of motion. The impression of light in the eye is caused by little bodies crashing into each other.

Corpuscular theory

In 1705 Isaac Newton (1642-1727) publishes a book called Optics, in which he accepts Descartes' idea of the ether, but not the fact, that light is a result of disturbances - crashing particles - in the ether. This contradicts his assumption of straight-line propagation of light as rays. He concludes this from his observations of shadows cats by a candle lit object. This shadow is an exact image of the object, without distortion. He thinks of light as little particles - corpuscles - emitted by a luminous body and moving like bullets through the ether. He thinks that bullets moving in straight lines can best explain sharp zoned shadows of objects. If corpuscles bump onto an object, they recoil an change direction.

Newton does not change his opinion about that, even Francesco Grimaldi's (1613-1663) observation, which is published two years after his death, contradicts his theory. Grimaldi observes not exact defines shadows of the objects, cause by refraction and diffraction. This observation contradicts a particle theory of light and is to be considered as an evidence of the wave theory of light.

Wave theory

In his book Traité de la lumière (1690) Christian Huygens (1629-1695) establishes a theory in which light - like sound and water - propagates as a wave. With this he neglects geometric optics and ascribes the appearance of light to a universal principle - which is valid for every wave phenomena in nature. This principle assumes waves propagating from an epicenter in all directions.

Before Huygens' wave theory can assert against Isaac Newton's authority, one more century has to pass. Optical phenomena refraction and diffraction help wave theory to get its recognition.

At this point the history of light does not end yet even after 2.300 years. A variety of electromagnetic and quantum mechanic theories still try to disclose the secret of light. But of these theories should not be told any further. With the wave theory the history of light has come to a chapter, which is satisfactory as a basis for the understanding of mechanics of holography.