The history of optics as a scientific field begins in Alexandria, around 300 BC. At that time, science was flourishing in Greece¹, and geometry was the hottest scientific topic (like nuclear physics was in the 1970s). One of Alexandria's biggest geometry hot-shots was a fellow by the name of Euclid. He was the man that came up with most of the geometry stuff we're taught in school, but he also observed that light travels in straight lines. The first descriptions on the laws of reflection can be found in his work.

Introduction

The works of Euclid, Heron and Claudius Ptolemy in Alexandria between 300 BC and 140 AD have been the standard for nearly 1000 years. Most of the reflection laws, and even considerations on optical aberrations, can be found in there.

At the end of the 13th Century, some smart people from from western Europe started reviewing optics and started to think about the colourful side of the subject. It was not enough for them merely to explain the path of light beams after reflections; other phenomena, like vision, light colour, the rainbow, and total reflection on transparent surfaces (like water) needed to be explained as well. The first attempts to explain these phenomena seem silly these days. For example, some people thought that the colour of light was a combination between darkness and 'pure' white light. In addition, people didn't understand that the eyes worked passively like light detectors. In fact, until Kepler's Ad Vitellionem Paralipomena from 1604, which contains a more or less correct description of vision, it was thought that the eyes emanated something which was used to palpate the surroundings.

It was not until the middle of the 17th Century, with the development of telescopes, microscopes and other optical paraphernalia, that the works of Jensen, Kepler, Galileo, Snelius, Cavalieri, Fermat, Newton and others provided a more profound understanding of optics. In 1665, Francesco Grimaldi was experimenting with light and small pinholes (also called apertures). He noticed a certain diffraction pattern behind the aperture, which led him to interpret a fluid-like behaviour for light. This is the first reference to the wave-theory of light. Newton believed in the corpuscular interpretation of light. He suggested that the light particles interacted with the medium - a very volatile all-pervading substance (the so-called ether), which would cause waves.

From the mid-1600s three fundamental problems concerning optics emerged: the discussion around the wave/particle problem, the attempts to detect the ether, and the measuring of light-speed. These discussions are still not completely resolved today - even with more powerful scientific methods, preciser equipment, smarter computers and more people working on these problems. These debates turned out to fuel science and technology. As a result, interesting by-products of these discussions, like several types of spectroscopy, the quantum theory, and lasers, have been developed.

Speed of Light

Here's a short history of the study of the speed of light:

• In 1676 a Danish astronomer by the name of Olaf Römer calculated a finite speed for light of about 200,000 km/s² from detailed astronomic observations, but nobody really believed him.

• In 1849 a Frenchman, Armand Fizeau, used a rotating toothed wheel to break up a light beam into a series of pulses, which were split and sent different lengths to a retroreflector. After being reflected back they were recombined and, from the time lapse between the pulses, Fizeau calculated the light speed in air, obtaining a value of 313,300 km/s³.

• Another method was suggested by another famous Frenchman named Jean Foucault⁴. He used rapidly rotating mirrors synchronized with the toothed wheel, and from the angle difference between the arriving pulses he calculated a speed of 298,000 km/s⁵. The rotating mirror method was then used to determine the speed of light in diverse media.

• Finally, in 1941 the speed of light was determined by more precise methods, yielding a value of 299,776 km/s.

An intriguing, modern light-speed related topic was introduced by Albert Einstein: nothing can exceed the speed of light. Einstein was a very intelligent man, and few dare to doubt him; nevertheless, it's merely a postulate, and some mischiefs in the scientific community are trying to find the loophole. The discussion is not over yet.

Another weird thing some scientists came up with recently is the following: How slow can light become? A phenomenon called self-induced transparency was discovered in the 1970s. Certain super-cooled metals become transparent after irradiating lots of laser light on them. In the 1990s people found that the refractive index of such a transparent metal is so enormous that the light passing inside of such a medium is as slow as 60 km/h (a velocity easily achieved by common racing bikes). The experimental proof followed shortly after.

The Ether

In 1678, Dutchman Huygens brewed up his wave theory of light (which he eventually published in 1690). According to him, light is transmitted through an all-filling ether that is made up of small elastic particles, which in their turn can act as a secondary source of wavelets. With this model many of the known propagation characteristics of light could be explained. Centuries later, Albert Michelson and Edward Morley described their unsuccessful attempts to detect the motion of the Earth relatively to the 'Luminiferous Aether' by investigating whether the speed of light depends upon the direction in which the light beam moves.

Today it is known that the 'ether' as a material medium in which light moves is a misconception. The discussion around the existence of an 'ether' has taken a more fundamental aspect. For instance: if there is no ether, what are we in then? Space? Time? What is the nature of space and time? Is space-time the ether? What is nothing then?

The Wave-particle Nature of Light

A short history:

• By 1704, Newton, in his work entitled Opticks, had put forward his view that light itself is corpuscular, but that the corpuscles are able to excite waves in the ether. Newton preferred to see light as a corpuscular phenomenon because light obviously travels in straight lines, whereas waves can bend into the region of shadow. Newton was a very intelligent man, and few dared to seriously doubt him by then.

• In 1801 Thomas Young provided support for the wave theory by demonstrating the interference of light in a typical wave-phenomenon.

• In France, ten years later, a very serious scientist named Fresnel presented a rigorous treatment of diffraction and interference phenomena, and showed that they can be explained in terms of a wave theory of light. As a result of the investigations by Fresnel and Dominique Francois Arago on the interference of polarized light and their subsequent interpretation by Thomas Young, scientists concluded that light, indeed, behaves like waves. Furthermore, they showed that these waves are transverse like the waves on the sea and not, as had been previously thought, longitudinal like sound waves.

• In 1845, Michael Faraday described the rotation of the plane of polarized light that passes through glass in a magnetic field (another wave-behaviour phenomenon). By then it seemed clear that light behaved like a wave, and the few people defending the corpuscular interpretation were eventually scoffed at.

• 20 years later James Clerk Maxwell convinced even the most die-hard corpuscularists by demonstrating that light is a electromagnetic wave. He also found that the speed of an electromagnetic wave should, within experimental error, be the same as the speed of light.

• It took another genius to revive the corpuscular theory: Einstein. In 1905 he elucidated the photoelectric effect, which was discovered by chance in 1887 by another German named Heinrich Hertz. Besides winning the Nobel prize for this, Einstein showed that light has momentum, a typical particle thing. For that reason, electrons can be scattered off a metal by irradiating light on it.

• In the same year another German called Gustav Mie published an account on light scattering from particles that are big compared to the wavelength of light, taking into account the particle shape and the difference in refractive index between the particles and the supporting medium - this is perhaps a little hard to understand, but it's another proof that light behaves like particles.

Today we know that light is neither wave nor particle, but it behaves like both. In fact it is known that there is a strange dualism inherent to all matter, especially on the sub-atomic scale called the wave-particle duality.

Modern By-products of Optics

On a sunny day in the summer of 1802, William Wollaston decided to take a closer look at the sunlight's spectrum by analysing it with a prism. He found out that the spectrum is crossed by a number of dark lines, which, ironically, were named after Joseph Fraunhofer, who rediscovered the dark lines in 1814 on a sunny day in Munich.

The correct interpretation came decades later by Kirchhoff and Bunsen who lived in Heidelberg and made a living by observing the emission spectra of alkali metals in flames⁶. They also noted the presence of dark lines arising from absorption when observing the spectrum of a bright light source through the flame.

The origin of these dark lines was similar to that of the dark lines in the solar spectrum observed by Wollaston. They figured out that the lines are due to the absorption of light by gases in the solar atmosphere. For more details consult Absorption and Emission Lines. The investigation of absorption and emission lines is a technique called spectroscopy. Nowadays spectroscopy is probably the most common tool used in science. The story does not stop here. Nobody really understood spectroscopy in the 1850s. For that people had to find out what was going on inside of the atoms and why they would emit in such characteristic ways.

Optics and Technical Development

In 1885 Johann Jakob Balmer presented an empirical formula describing the position of the emission lines in the visible part of the spectrum of hydrogen, which were subconsequently named the Balmer-lines. In 1900 Max Planck developed the quantum theory⁷ against his own convictions, because otherwise the Balmer-lines could not be explained. This revolutionized science for a good while, and scientists suddenly realised that there was a lot they didn't know. After the revolution was over, science had discovered the atom model, the quantum theory and many effects, such as the stimulated emission process, which eventually made the development of lasers - the ultimate light source - possible.

Before having this light source, however, people experimenting with light, spectroscopy and optics felt the need for better light sources than flames or sunlight. In 1879 Joseph Swan demonstrated an electric lamp with a carbon filament in England. Edison later developed the electric lamp using cotton as the filament and mass-produced it as a practical device. As a consequence, electricity had to be made available for the masses. With the sudden availability of light and electricity many applications involving optics were developed and/or improved (eg better microscopes, filming-cameras, flash-lights, smoke detectors, signaling devices, measuring equipment, tungsten-filament lamps etc).

By 1920 measurement and optical devices had become so precise that even spookier effects could be observed. In 1919 Sir Arthur Eddington observed the eclipse of the Sun on 29th May from Principe Island with the intention of determining the apparent position of stars that appeared close to the Sun's disk. He was the first person to see the path of light being bent by the Sun's gravitational field, one of the weirdest effects which had been predicted by Einstein. In 1928 Chandrasekhar Raman and Kariamanikkam Krishnan observed weak inelastic scattering of light from liquids, an effect arisng from the scattering of light by vibrating molecules, predicted in 1926 by Adolf Smekal and now known as the 'Raman effect'. In 1948 Dennis Gabor conceived the principles of wavefront reconstruction, later to become known as holography. In 1961 a group of scientists demonstrated first effects of non-linear optics by generating harmonic 'overtones' of light by passing it through a quartz crystal - that is, they converted red light into blue light. On 25 April, 1990, the most powerful optical telescope ever built, the Hubble Space Telescope, was positioned in orbit. The Hubble Telescope can record extremely well-resolved pictures because it is outside of the earth's atmosphere, which perturbates light paths⁸. With the development of Adaptive Optics eventually even this problem was solved.