1. The locus of all particles of the medium vibrating in the same phase at a given instant is known as wavefront. Depending on the shape of sources of light, wavefront can be of three types.
2. Spherical wavefront: When the source of light is a point source, the wavefront is spherical.
3. Cylindrical wavefront: When the source of light is linear, the wavefront is cylindrical.
4. Plane wavefront: When the point source or linear source of light is at a very large distance, a small portion of the spherical or cylindrical wavefront appears to be plane. Such a wavefront is known as a plane wavefront.
5. Huygens principle: According to Huygens principle, (a) Every point on a given wavefront (primary wavefront) acts as a fresh source of new disturbance, called secondary wavelets. (b) The secondary wavelets spread out in all directions with the speed of light in the medium. A surface touching these secondary wavelets tangentially in the forward direction at any instant gives the new (secondary) wavefront at that instant.
6. Huygen's principle can be used to verify laws of reflection and refraction.
7. The sources of light, which emit continuously light waves of the same wavelength (monochromatic light), same frequency, and in the same phase or have a constant phase difference with time are known as coherent sources. Two sources of light that do not emit light waves with a constant phase difference are called incoherent sources.
