A brief history of light

Basic laws of geometric optics were known and exploited in ancient times in the oriental as well as in the occidental parts of the world. There is evidence (Sines and Sakellarakis, 1987) that glass was used as a magnifying lens or burning glass in ancient times already. The most common theories of light assumed that light was associated with particles or corpuscles that were either emitted by objects and perceived by the eyes (intromission theory) or, reversely, cast by the eyes towards the objects (emission theory). Euclid already formulated basic laws regarding the nature of reflection and the propagation of light in straight lines, whereas Ptolemy even detected the effect of atmospheric light refraction based on accurate astronomic observations. A remarkably detailed and accurate model of light was introduced by Ibn al-Haytham, an Arabic physicist and mathematician. AlHaytham supported the intromission theory and identified the sun as the original source of light. He analysed and described the refraction and reflection of spherical and parabolic surfaces.

While Isaac Newton was convinced to have found evidence that light consists of minute particles, Christiaan Huygens established a new wave theory, which was able to describe observed reflection and refraction phenomena based on the assumption that light would propagate with a lower speed in dense media. Thomas Young added supporting evidence to the wave theory by conducting interference experiments using pin-holes and double-slits. The observed interference patterns caused by
Light, sun and optics –
applied principles, models and methods
the diffraction of light could finally be explained by Auguste Fresnel, who extended and refined Huygens’s wave theory, even though it is unclear if he was aware of Huygens’s work. James Clerk Maxwell established a new model which coupled electric and magnetic fields. His famous theory consists of four partial differential equations which allow a sinusoidal wave-like solution. Calculating the propagation velocity of these newly discovered electromagnetic waves, he found that it was almost equal to the speed of light. Remarkably, the speed of light had already been measured at that time. This led Maxwell to the significant conclusion stated in his publication in the year 1865:
The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws. (Maxwell, 1865, p.499)
The discovery of the electromagnetic nature of light did not yet set an end to the famous misconception that light would require a medium to propagate, which was called the ether (or luminiferous aether). In fact, Maxwell inspired the famous Michelson and Morley experiment in the year 1881, which was designed to determine the speed of the earth relative to a stationary ether. The seemingly paradoxical null result of the experiment, showing that light emitted on the moving earth propagates into all directions with the same constant speed, left physicists baffled. It took another 24 years and the genius of Albert Einstein to resolve this supposed paradox in his Special Theory of Relativity. However, in the very same year, Einstein also revived the particle theory in his explanation of the photoelectric effect by introducing the concept of photons. Einstein’s theoretical concept was based on the experimental results of Hertz, Lenard and others but also on the theoretical work of Max Planck. In 1900, Planck presented a theory that could predict the spectral distribution of black-body radiation by introducing discrete (quantized) energy states. The resulting new theory of quantum mechanics by Schrödinger, Heisenberg and others put an end to the particle/wave discussion, however, leaving us with a big paradox beyond our everyday perception of the world, as A. Einstein noted:
But what is light really? Is it a wave or a shower of photons? There seems no likelihood for forming a consistent description of the phenomena of light by a choice of only one of the two languages. It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do. (Einstein and Infeld, 1938, p.278)
Finally, the comprehensive field theory, called quantum electrodynamics and founded by Dirac, Bethe, Feynman and others, was able to combine special relativity, quantum mechanics and electrodynamics to provide a unified model, which is able to describe most light-matter interactions in a very accurate way. Richard Feynman’s book QED-The Strange Theory of Light and Matter (Feynman, 1985) provides an introduction to the essentials of quantum electrodynamics in an accessible language.