Wave theory of Light

Wave theory of light

1. The wave theory was proposed by Dutch scientist Christian Huygens in 1678.
2. Light travels in form of waves(longitudinal) which travels with uniform velocity in homogenous medium.
3. This light wave was due to vibration of the particles of the medium.
4. As propagation of longitudinal waves required medium to travel ,so Huygen suggested there exist a hypothetical medium called 'luminiferous ether' which is available in vaccum medium.

Wave front and Wave Normal

A locus of all point of a medium to which waves reaches simultaneously so that all the point are in the same phase is called wavefront.

A perpendicular drawn to the surface of a wavefront at any point of a wavefront in direction of propagation of light is called wave normal.

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Huygen' s Principle

1. Every point on a wavefront act as a secondary waves in all direction wherever possible.
2. Every point on new secondary waveform creates new wavelets in forward direction.
3. The resultant wavefront is given by the tangent to all the secondary wavelets at that instant.
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Construction of spherical wavefront

Let AB is a cross section of a spherical wavefront from a point source (S)

Let X,Y,Z be the point on wavefront AB. Now they behave as a secondary source and start sending secondary wavelets in forward direction according to Huygen's Principle.Let this system is in air medium hence speed of light is 'c' and thus this wave cover a distance of ct in time 't'. Now common tangent to the wave with centre X,Y,Z gives the new wavefront A' B' after time 't'.

Construction of Plain wavefront

Let  AB be the plain wavefront  due to point source (S)  called as primary wavefront.Let  M,N,O,P on wavefront AB which act as a secondary source and start sending secondary wavelets in forward direction according to Huygen's Principle.Let this system is in air medium hence speed of light is 'c' and thus this wave cover a distance of ct in time 't'. Now common tangent to the wave with centre M,N,O,P gives the new wavefront A' B' after time 't'.

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