1. Theories of Light
(a) Newton’s corpuscular theory
(b) Huygens’ wave theory
(c) Maxwell’s electromagnetic theory
(d) Planck’s quantum theory.
At present, we assume that light has a dual nature.
2. A wavefront is a surface, all the points of which are in the same phase. A normal to the wavefront at any point is called the wave normal at that point.
Types of wave front:
(i) Spherical wave front: due to a point source of light.
(ii) Plane wave front: due to a point source of light, kept at infinity.
(iii) Cylindrical wave front: it is produced by a linear source, like illuminated slit.