Light as a Wave:
In our daily life, we see many phenomena exhibited by light. These help us to form different pictures of the nature of light. The formation of sharp shadows makes us believe that light travels in a straight line as a ray. Newton tried to explain straight-line propagation, reflection, and refraction by assuming that light is made up of invisible minute particles, called corpuscles. However, some new phenomena like interference, diffraction, polarization, etc. required light to be considered as waves. Later in the 19th century, the stadium of electricity and magnetism led to Maxwell’s idea of electromagnetic waves. Hertz verified them experimentally. Even electromagnetic waves could not explain all phenomena exhibited by light. First, we consider the wave nature of light given by the scientist Huygen.
Let us consider identical waves originating from a point source. In an isotropic medium, they will all travel equal distances in all directions in a given time. The points at equal distances from the source will be on the surface of a sphere, and therefore the disturbances (state of motion of particles for mechanical wave and electric vector for e.m. wave) at this sphere will be all in the same phase. Such a surface of the constant phase is called a wavefront. It can be defined as the locus of points in space having the same phase of a given periodic variation. Thus, a point source gives a spherical wavefront. The speed with which the wavefront moves outward from the source is called the phase velocity of the wave. The energy of the wave moves in a direction perpendicular to the wavefront which is also the direction of the ray.
The figure shows the wavefront given out by a point source, slit or line source, and an extended source. From a point source, we get spherical wavefronts with ray direction along the radius at that point. A slit gives a cylindrical wavefront with a ray direction along the radius of the cylinder at that point. While from an extended source, the wavefront is plane. Further, for any wavefront at a large distance from the source., the wavefront can be taken as a plane. For example, the wavefront from the sun reaches the earth. Experimentally, a plane wavefront can be obtained by putting a point source at the focal point of a convex lens. The wave emerging from the lens is a plane wavefront.