Question

Two beams of light having intensities I and 4I interfere to produce a fringe pattern on a screen. (The phase difference between beam is (π / 2) at the point A and π at point B.)

(A) I (B) 4I (C) 2I (D) 6I

Answer 1

The correct option is (B) 4I.

Explanation:

IA = I1 + I2 + 2√(I1 I2) cos ɸ

At point A, IA = I + 4I + 2√(I × 4I) cos (π / 2) = 5I + 0 = 5I

At point B, IB = I + 4I + 2√(I × 4I) cos π

= 5I + (2 × 2I × – 1)

= 5I – 4I = I

IA – IB = 5I – I = 4I

i.e. difference between intensities = 4I