Physics of class-12 Newtons Corpuscular Theory Wave Optics Huygen's Wave Theory Wavefront Board & Competition
Newtons Corpuscular theory. 1-Light sources continuously emits small particles called corpuscles 2-Light wave propagate in the form of corpuscles. 3-The sensation of different colours is because of the different size of Corpuscles. 4-when these particles strike the retina of the eye,a sensation of vision is produced. Causes for Failure of the Newtons Corpuscular Theory 1-By this principle the phenomena of interference diffraction and 2-Infinite number of corpuscles come out of the light source. Hence 3-According to this theory, velocity of light must depend on temperature polarisation could not to be explained. the mass of source must decrease but practically it does not happen. of the source, but practically it has been found that velocity of light is Independent of the temperature of source.
Huygen's Wave Theory 1-light travels in the form of waves. 2-These waves travel in all directions with the velocity of 3. 10 m/s 3-The whole universe with all matter and space is filled with a luminiferous medium called ether, of very low density and very high elasticity. 4-The sensation of different colours is because of the different wavelengths. 5-The light waves were supposed longitudinal. Huygen's theory could explain reflection, refraction, diffraction,polarisation But could not explain the phenomenon of rectilinear propagation of light. Therefore wavefront is imagined to explain it.
Wavefront A wavefront is defined as the continuous locus of all such particles of the medium which are vibrating in the same phase at any instant. Thus a wavefront is a surface of constant phase. The speed with which the wavefront moves outwards from the source is called the phase speed Ray Ray Cylindrical- wavefront Ray Spherical wavefronts Plane wavefronts
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This principle is based on the following assumptions 1. Each point on a wavefront acts as a fresh source of 3. The new wavefront at any later time is given by the new disturbance, called secondary waves or wavelets.forward envelope (tangential surface in the forward direction) of the secondary wavelets at that time. 2. The secondary wavelets spread out in all directions with the speed of light in the given medium. A2 A A According to Huygens principle,each Point on the wavefront becomes a source of secondry disturbance, which Travels with the same speed B,BB,
Behaviour of wavefronts in prisms, lenses and mirrors Consider a plane wave passing through a thin prism. Clearly, the portion of the incoming wavefront which travels through the greatest thickness of glass has been delayed the most. Since light travels more slowly in glass, this explains the tilt in the emerging wavefront. Similarly, the central part of an incident plane wave traverses the thickest portion of a convex lens and is delayed the most. The emerging wavefront has a depression at the centre. It is spherical and converges to a focus. A concave mirror produces a similar effect. The centre of the wavefront has to travel a greater distance before and after getting reflected, when compared to the edge. This again produces a converging spherical wavefront. Concave lenses and convex mirrors can be understood from time delay arguments in a similar manner. One interesting property which is obvious from the pictures of wavefronts is that the total time taken from a point on the object to the corresponding point on the image is the same measured along any ray Pris m Lens Mirror