Uncover the Mysterious Veil of Light

Study of light （optics） is one of the oldest branches in physics. It always a fantastic object having many thoughts and ideas. The mysterious veil of light has been uncovered gradually after many centuries of research.

That was an unusual gathering. The reason why it raises concerns of later generations is because that a few celebrities in British literary circle intensively and frankly has talked about their views on science， or in other words， hostility on science. Time stands still on December 28， 1817， it was in Benjamin Haydn， a British painter and commenter's study in London. On that day， after introducing poet John Keats to another poet William Wordsworth， essayist and commenter Charles Lamb， the owner took out and showed his new works to the guests. On the picture， Jesus is waiting to enter Jerusalem， and his believers， Isaac Newton， a scientist and Voltaire， a sceptic are standing next to him.

"You should not draw Newton"， slightly tipsy Lamb blamed Haydn， "the guy believes nothing except prisms". Keats agreed with Lamb， and said that Newton destroyed all the poetry of rainbow， and in Newton's eyes， rainbow was nothing but arrangement of spectrum， so he proposed "let Newton go to hell" while clinking glasses.

"It is impossible to resist Newton！" Haydn replied， "We have to toast for the health of Newton， let mathematics go to hell".

Let us return to the UK in 1666. In that year， 23-year-old Newton made the first important experiment exploring the nature of light. According to his own narrative， he bought a glass prism （made a triangular prism with pure glass） "to test color phenomenon". To this end， "I darken my room， open a small hole on my shutter so that the right amount of sunlight can enter， I put my prism on the position sunlight enters， therefore， the sunlight will be refracted to the opposite wall".

At that time， people did not believe that color was one of the basic characteristics of light， but believed that color was changes occurred when light got through another substance， such as water， clouds， or glass. It was roughly realized that rainbow was a unique phenomenon caused by sunlight refracting and reflecting in water droplets. Newton observed in the experiment that the beam of light refracted on the wall was more than a wider light spot， also a color light band with the colors arranged in the same sequence with that of a rainbow， i.e.， red， orange， yellow， green， blue， purple， and the length many times larger than its width. Was the color spectrum produced by the prism？ Newton made this "rainbow band" going through the second reversely placed prism （substitute it with a lens） to recombine these colored light， as a result， they really became white light （spot）.

After conducting more experiments， Newton held the viewpoint that colors certainly were the basic element of light. Ordinary white light was a mixture of several light rays of different colors， however， when these light rays acted on eyes individually， there would be different color visions. Each color had a unique and different refractive index. A light band forming as various components disperse was called "spectrum". Elongation of spectrum was because glass had different reflectivity to these different light rays. Actually Newton also uncovered the mystery of colors of materials. In fact， different stuff presents different colors because these colors have different reflectivity and refractive indexes on the stuff.

Initial Wrestle

between Two Theories

Dispersion experiment made Newton become famous overnight， and at the same time， made him seriously think about the nature of light. After elected to the Royal Society in 1672， Newton immediately submitted his research findings， a paper titled On the Theory of Light and Color， his first public published paper which was published on Philosophical Transactions of the Royal Society.

In this paper， Newton put forward "corpuscular theory" of light， and believed that light was composed of small particles moving at high speed. These particles are emitted from a light source， like a bullet shot from the barrel of a gun. Or， light is a beam of particle flow moving from a luminophor to the eyes， which may explain why light advances in straight line， and can cast a clear shadow. He also elaborated the color theory of light in corpuscular theory， and believed that composition and decomposition of light was like particles of different colors mixed up and separated.

However， this theory cannot explain some obvious facts. For example， why green light refracts more seriously than yellow light？ Why two beams of light can cross without interfering with each other， namely why particles composing of light do not collide with each other？ It was difficult for Newton's contemporary distinguished academics to accept his theory that light had several different and independent colors which could not change to each other and was characterized by a certain refractability because it seemed that the theory could be refuted by such an experiment that a pigment of a third color could be produced by mixing pigments of two colors.

For this reason， corpuscular theory of light was fiercely opposed as soon as it came out. Basically， Board of Regents of the Royal Society formed by Robert Hooke， a famous physicist， Robert Boyle， a famous chemist and others held a negative attitude towards Newton's submitted paper on optics. Robert Boyle pointed out that colors of an object were not the nature of the object itself， but the effect produced when the light irradiated on the object. He also recorded the colored stripes on the soap bubbles and glass ball surface for the first time.

A more powerful "Sunday punch" that the oppose side held was an experiment designed by Francisco Grim Aldi， an Italian physicist in 1665. In the experiment， he made a beam of light go through two slits and project it on a blank screen in a darkroom， and found that the light band appeared was wider than the light beam passing through the first slit. Thus， he believed that this beam of light was curved outwardly on the edge of the slit， which was obviously the result the light bypassed an obstacle. He called this phenomenon as "diffraction". Followed by further experiments， Grim Aldi got photograms of light and dark stripes. He thought that the phenomenon was very similar to the water waves， and inferred that light may be a fluid which was able to move in a wavelike form， and different colors of light may be produced by different vibration frequencies. Grim Aldi became the earliest advocator for wave theory of light. Through repeating the experiment of Grim Aldi and observing the colors on lathers， Hooke proposed the hypothesis that light was a kind of longitudinal waves of ether.

Initial Wrestle

between Two Theories

Views of Descartes and Hooke on the nature of light were soon referred and evolved further by Christian Huygens， famous Dutch mathematician， physicist and astronomer.

Huygens proposed that light was a kind of mechanical oscillating waves， and points on wave surface were wave source causing vibration of media. Media of load light wave （ether） should be suffused in the whole space including vacuum and be able to impregnate into general substances. In 1678， Huygens submitted his optical treatise， Treatise of Light to Academy Royale des Sciences. This book systematically elaborates the wave theory of light.

Exclusive Newton certainly would not easily admit "defeat". However， in order to avoid being haunted by Hooke who was always hard on him （afterwards， the two opponents hostile to each other fought for years for law of gravitation， etc.）， he did not make the development of his optical study to the public for a time. In 1704， the second year after Hooke's death， Newton published his works， Optics. In this book， he stated some deficiencies of wave theory. First， wave theory could not well explain rectilinear propagation of light and light's birefringence in calcite； second， wave theory relied on the presence of the media， but there was no evidence to indicate that there were such media in the sky.

There were some changes in the dispute between corpuscular theory and wave theory in the 19th century. Thomas Young， a British physicist and physician made an experiment in about 1800. In the experiment， light was emitted from a small but very bright light source， and finally fell on a screen through two narrow rectangular splits. If light was composed of particles， then， the light on the screen should be two small rectangular apertures.

However， the light on the screen found by Thomas Young was dark and bright， black and white stripes. These light bands were caused by the diffraction of the corners noticed by Grim Aldi about 140 years ago， while corpuscular theory could not explain such a phenomenon. After that， Thomas Young proposed some reasons to deny corpuscular theory in his paper， Experiments and Issues on Light and Sound. First， light irradiated from strong light source had the same speed with that from weak light source， which could not be well explained in corpuscular theory； second， when light rays entered another medium from one medium， some of which were reflected， while the rest were refracted， which was hardly explained in corpuscular theory. Corpuscular theory was forced to the blind side for a time. However， Louis Mauls， a French physicist discovered the phenomenon of polarization of light （light waves vibrate on a given plane） accidentally in 1808， while light of longitudinal waves would not have such polarization. This discovery immediately became powerful evidence opposing the wave theory.

Subsequently， after further study， Thomas Young proposed a view that light was a transverse wave and thereby successfully explained the phenomenon of polarization of light， at that time the wave theory was able to extricate itself from a difficult position.

In 1864， James Maxwell， a British physicist proposed the concept of electromagnetic waves and a new theory， "field theory"， and speculated that light itself （including radiant heat and other radiation） was a kind of electromagnetic interference， was the form of waves， and propagated via the electromagnetic field in accordance with electromagnetic laws.

With the establishment of electromagnetic theory of light， failure of Michelson-Morley experiment testing ether theory， and the advent of Einstein's special theory of relativity in early 20th century， ether quit the stage of science finally. People began to accept the concept that the electromagnetic field itself was a form of material existence， and the field could transmit in the form of waves in vacuum.

Especially when Einstein introduced the concept of "light quantum" （called "photon" later） in his paper published in 1905， Photoelectric Effect， put forward the hypothesis of light quantum， and believed that light had two characteristics， i.e.， waves and particles， the dispute between corpuscular theory and wave theory lasting for centuries was truly resolved. Einstein's theory became a milestone in the development of quantum mechanics， and won Nobel Prize in physics in 1921 for Einstein.