Characterstics of light:
Light is nothing but sensation to our vision that makes us to know things before us. One thing to remember that light itself is not visible to our naked eye. This light follows three laws under geometrical optics. They are
i)Rectilinear propogation of light
ii)Laws of reflection iii)Laws of refraction
Rectilinear propagation of light:
In homogeneous medium, light rays travel in straight line.
Reflection and refraction of light:
It is the property of light that when light rays travel from one medium to another medium, part of it gets reflected in the same medium and another part refracts through another medium. Travelling the light ray in the same medium is called reflection. When light travels through second medium, then that part is called refracted ray. That phenomenon is called refraction.
To explain the nature of the light, four theories were proposed by physicists. They are
i)Newton's Corpuscular Theory:
According to theory, light contains particles. They are called corpuscles. In this theory, source of light gives or emits particles in all directions. According Newton, different colours of light have different sizes of corpuscles. These corpuscles travel with great speed even there is no external force on them. This explains law of rectilinear propagation of light. These corpuscles cause mechanical action of sensation light when they reach the retina. When these corpuscles reach to the near of the material of the medium, then their path is modified. According to this theory, reflection of light is defined as the repulsion of the particles of denser medium on the corpuscles. This theory defines refraction is as the force of attraction by the particles of denser medium.
Huygens' theory of light:
According to this theory, it was assumed that the waves of light travel through dilute and very high elastic medium. This medium was named as ether. When this medium is assumed then velocity of light passing through this medium will become high. But, light travels from the sun to the earth without any medium. This theory could not explain polarisation. At the same time, this theory does not explain black body radiation, photoelectric effect and Compton effect.
Electromagnetic theory of light:
This theory is proposed by Maxwell that light rays are propagated as electromagnetic waves. These electromagnetic waves are generated by accelerated charges. An accelerated charge always produces time varying electric field associated with time varying magnetic field. Electromagnetic waves are always tranverse in nature. Electromagnetic waves travel in the medium with the speed of light. Using electromagnetic spectrum, we can determine the different rays according to its wavelength. When we move from left to right in electromagnetic spectrum, then frequency increases and wavelength decreases. Electromagnetic waves do not require any material medium for its propagation.
This principle states that each point on the wavefront is the source of new disturbance called secondary source that emits wavelets. This disturbance travels in the medium with the velocity of light. The envelope of all secondary wavelets drawn in the direction in which the wave travels is the new wavefront.
"Short and to the point" characteristic of this article is appreciable but it would have been much better if the writer has elaborated it a bit more with suitable ray diagrams.
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