Light had always been a guide and a source of inspiration to humanity throughout the human history. Light is bright which gives humans life. The light coming from the stars including the sun has always amazed human beings. If there was no light there was no life. The light from stars has guided the sailors for centuries, has provided the signs of astrology, helped in forming the calendars, time and knowing the seasonal patterns.
In early Greece a scholar used light to measure the size of earth. He used a stick at two different angular positions on earth and measured the length of the shadows formed and through trigonometry determined the size of the earth. Newton showed that light is-composed of a spectrum of different colors representing different wavelengths through his prism experiment. When light falls on prism it is-dispersed into different wavelengths of light. Galileo used light from the heavens to discover different moons of Jupiter through his reflecting telescope.
There has always been debate about the nature of light. It was Newton who propounded the theory that light consists of corpuscles or particles. Light was supposed-to-be a beam of particles travelling at very high velocity. His theory faced opposition from Christian Huygens who argued that light travels in waves forming the crests and the troughs. Thereafter the behavior of light was-understood very little and there was no major breakthrough in the theory of light.
It was not until the eighteenth century that Thomas young performed his interference experiments to suggest that light is a wave. Light when made to pass through two small slits an interference pattern is-formed on the screen behind the slits. The pattern consists of alternate bands of bright and dark fringes formed by constructive and destructive interference of waves. Had light had been particle in nature the interference would not have occurred with these patterns. So the wave nature of light was finally accepted as the nature of light.
Is velocity of light constant? Velocity depends on medium in which it is travelling? It is a variable? These were some of the questions prevailing in the eighteenth century. Maxwell proved light to-be consisting of alternate waves of electric and magnetic fields. The equations Maxwell framed also proved light to-be travelling at a certain speed. At that time the space was-supposed to-be filled with ether which is a medium through which the light travels so light does not has a fixed velocity. Propagation of light through the ether determines its velocity. So whether light travels a fixed speed was a puzzle not solved till now.
The question what would be the speed of light to the observer moving at the speed of light or moving at half the speed of light was-thought by Einstein. It would take special theory of relativity that would prove that in whatever frame of note the observer travels, the speed of light will appear the same. So if whether you are in a spaceship or in a boat on still water, velocity of light will be the same for each observer. This was a remarkable observation that would change everything known about light. It was this assumption of constant velocity of light that would lead Einstein to propose his special and general theory of relativity.
However it would take a special experiment by Michelson and Morley of the United States to show that light travels at a constant speed. They would measure the light in two directions one perpendicular to the earth's rotation and the other parallel to the earth's rotation. Rotation of earth affects movement of ether, the medium through which light travels thereby affecting the speed of the light. But to their surprise they would find out that the speed of light is same in both the directions. Once this was-proved it gave credibility to Einstein's work. Hence it was finally proved that light travels at a constant speed.
Other great work was the Hubble's discovery of the expansionary universe. The light that came from the distant stars showed a characteristic pattern of spectrum showing that it is-red shifted. From Doppler Effect it was clear that the galaxies are receding away from each other. This new discovery gave stimulus to the theory of expansionary universe thereby abandoning the old static theory of the universe.
The debate whether light is a wave or particle continued till when the photoelectric effect experiment proved that light is-made of particles. The light when impact the metal sheets the fast-moving electrons in light would knock out the electrons in metal sheet. This proved the corpuscular theory of newton to-be right. But how it is possible that light has a particle as well as a wave nature? It sounds strange but this was possible that gave rise to a new physics of Quantum mechanics. Under this new field of study the electrons travels at the speed of light can show wave patterns. It was finally agreed that light can-be wave as well as particle. Hence the debate which was going on for centuries was finally settled and the discovery of particle nature certainly laid a new path for the field of physics to follow.
Light journeyed from it being a harbinger of life to its heated discussion among scientists about its nature for centuries. It would take two monumental works of interference and the photoelectric effect that would put an end to the question on the nature of light. Ultimately light neither has a particle nor a wave nature but a dual nature of wave as well as a particle.
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