Question

What is Huygens principle?And conditions for constructive and destructive interference?

Answer 1

Huygen’s Principle

(i) Every point on given wavefront (called primary wavefront) acts as a fresh source of new disturbance called secondary wavelets.

(ii) The secondary wavelets travels in all the directions with the speed of light in the medium.

(iii) A surface touching these secondary wavelets tangentially in the forward direction at any instant gives the new (secondary) wave front of that instant.



Conditions for Constructive and Destructive Interference

For Constructive Interference

Phase difference, φ = 2nπ

Path difference, Δx = nλ

where, n = 0, 1, 2, 3,…

For Destructive Interference

Phase difference, φ = (2n – 1)π

Path difference, Δx = (2n – 1)π / 2

where, n = 1, 2, 3, …

If two waves of exactly same frequency and of amplitude a and b interfere, then amplitude of resultant wave is given by

R = √a2 + b2 + 2ab cos φ

where φ is the phase difference between two waves.

Rmax = (a + b)

Rmin = (a – b)


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