′ ′ A B will be a spherical wavefront from a point object which is at a finite distance as shown in Figure . (a) and it is a plane wavefront if the source of light is at a large distance (infinity) as shown in Figure . (b). A' A' P P R R B' B' Q Q (a) Spherical wavefront (b) Plane wavefront Figure .
Huygens’ Principle When a wave propagates it is treated as the propagation of wavefront. The wavefront is always perpendicular to the direction of the propagation of the wave. As the direction of ray is in the direction of propagation of the wave, the wavefront is always perpendicular to the ray as shown in Figure . (b).
(a) (b) Figure . (a) Ripples on water surface (b) Wavefront and ray The shape of a wavefront observed at a point depends on the shape of the source and also the distance at which the source is located. A point source located at a finite distance gives spherical wavefronts. An extended (or) line source at finite distance gives cylindrical wavefronts.
Any source that is located at infinity gives plane wavefront as shown in Figure . . Point source O Line source Source at infinity Figure . Wavefronts - - - - Unit wave optics the reflection.
The time taken for the light to travel from B to ′ B and A to ′ A are the same. Thus, the distance BB ′ is equal to the distance AA ′ ; (AA = BB ) ′ ′ . (i) The incident rays, the reflected rays and the normal are in the same plane. (ii) Angle of incidence, ∠ i = ∠ NAL = o – ∠ NAB = ∠ BAB ′ Angle of reflection, ∠ r = ∠ ′ ′ ′ N B M = o – ∠ ′ ′ ′ N B A = ∠ ′ ′ A B A For the two right angle triangles, ∆ ABB ′ and ∆ ′ ′ B A A , the two right angles, ∠ B