Light Wave Nature
The rays’ optics uses the geometry of straight lines to account for the macroscopic phenomena like rectilinear propagation of light and refraction of light and reflection of light etc. however, the macroscopic phenomena like interference, diffraction and polarization could not be accounted for by ray optics. To explain these phenomena, concept of waves was introduced. The new branch of Physics based on the wave concept of light was called Wave Optics or Physical optics. The wave theory of light was put forward first of all by Huygens and later on modified by Fresnel. According to wave theory of light; the light is a form of energy which travels through a medium in the form of transverse wave motion. The speed of light in a medium depends upon the nature of medium. Huygens supposed the existence of a hypothetical medium called luminiferous ether which filled the entire space. This medium was supposed to be massless with extremely high elasticity and very low density.
In this aspect of Physics, we will discuss the various phenomena related to wave nature of light.
Huygens gave a hypothesis for geometrical construction of the position of a common was front at any instant, during the propagation of waves in a medium. This principle tells the way in which the wave front is propagated further in a medium. According to him,
1. Every point on the given wave front (called disturbance, called secondary wavelets, which travel in all directions with the velocity of light in the medium.
2. A surface touching these secondary wavelets, tangentially in the forward direction at any instant gives the new wavefront at that instant. This is called secondary wave front.
To illustrate Huygens principle, let AB be a section of the given wavefront (called primary wavefront) at any instant. To find the position of a secondary wavefront after a time t, take points 1, 2, 3, 4,……… on the wavefront AB. The distance travelled by light in the medium c × t, where c is the velocity of light in the medium taking each point as centre, draw spheres of radius = c × t, these spherical surfaces represent the positions of secondary wavelets at time t. draw a surface A1B1 touching tangentially, all the secondary wavelets in the forward direction. The surface A1B1 is the secondary wavefront. A surface A2B2 touching tangentially, all the secondary wavelets in the backward direction can also be drawn in the backward secondary wavefront, but it is contrary to observation. Therefore, we conclude that there is no backward flow of energy during propagation of a wave.
From the above discussion, it is noted that in a homogeneous medium, during the wave propagation, the radius of the spherical divergent propagation, the radius of the spherical divergent increases uniformly with time and a wavefront is propagated parallel to itself.
Huygens principle can be used to explain the phenomena of reflection and refraction of light on the basis of wave theory.
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