What is light? Well, since ages, different kind of concepts have been developing to understand and explain this phenomenon but fundamentally, light is a tool which help us to see the things. Basically, we see the world when the reflected rays of light from a object reaches to our ratina. So what does define it? Gradual understanding in nature of light brought different definition with revision. Years before, light was seen as continuous entity generated from the source. The first postulation about the light was that it travels in straight line. Many years later, first time light was seen in the form of wave by Descartes in seventieth century. He established light as an mechanical entity and explained reflection and refraction, latter was not predicted correctly. Later P. Gassendi brought the concept of particles as corpuscular model of light. The theory presumed light as tiny particles of energy called corpuscles. This massless elastic particles helped Newton to explain the simple model of reflection and refraction. Although the theory explained reflection and refraction ( incorrectly) of light, it was not well versed in explaining the diffraction of light. However, Newton explained the phenomenon of diffraction by assuming localised behavior of particle as wave.
Well, later another theory was proposed by the Dutch physicist C. Huygens which was not very accepted theory at that time. He proposed light as a wave and he explained the phenomenon of reflection and refraction and derived laws. He presumed that light was emitted from a source as wave in all direction. He explained the phenomenon of interference of light wave. Well, in 19th century, a famous experiment by Thomas Young established the concept of light wave very firm. It was the two silt experiment. It consisted one light source placed equally from two slit and detector. When light passed through only one slit, it produced a smooth distribution of light on the detector. But when light was allowed to pass through both slit, light's wave properties created an interference pattern on detector which was called as interference fringes ( alternate dark and bright region). Well, even though wave nature of light had explained the slit experiment, the major problem was its propagation. Up to that time, it was thought that wave required medium for propagation. Wave character of light was not fit for its propagation as it could propagate through vacuum. A self propagating wave was required to explain wave characteristics of light.
Later Maxwell's discovery of electromagnetic wave helped to prove light as an electromagnetic wave which could propagate through vacuum.
Well, till 19th century, electromagnetic nature of light was well established and accepted. At the same time, many experiments were done and electron was discovered. In one of such experiments, phenomenon of photoelectric emissions was discovered by H. Hertz. Photoelectric emissions mean emissions of electron from a metal surface when light of suitable frequency illuminates it. Wave nature of light was unable to explain the instantaneous emissions of electron.
In beginning of twentieth century, a new concept of light was proposed. And it was by none other than the great physicist Albert Einstein. He proposed the electromagnetic radiation in new form. He proposed the concept of discrete unit of energy that was photon. He explained that light is consist of photon which possess a certain energy of value hv. Where h is Plank's constant and v is frequency of light. In photoelectric effect, electron at metal surfaces receives the energy in value of nhv where n is number of photons. This concept explained the photoelectric effect of light in best way.
Well, a particle nature of radiation was observed and a dual characteristics of light was proposed. Later, discovery of X- rays again confirmed the same. Now, photon is categorised according to frequency. Higher frequency means higher energy. Photon with highest frequencies called gamma rays. In lowering order, the rest of light's spectrum are x-rays, ultraviolet rays, visible light, infrared, microwave, and radio wave.
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