Question

Two monochromatic light waves of amplitudes 3 A and 2 A interfering at a point have a phase difference of 60⁰. The intensity at that point will be proportional to :

• A. 5A²
• B. 13A²
• C. 7A²
• D. 19A²

Answer 1

Answer: (D)

19A²

Answer 2

: Here, $A_{1} = 3A,A_{2} = 2A,\varphi = 60{^\circ}$

The resultant amplitude at a point is

$R = \sqrt{A_{1}^{2} + A_{2}^{2} + 2A_{1}A_{2}\cos\varphi}$

$\sqrt{(3A)^{2} + (2A)^{2} + 2 \times 3A \times 2A \times \cos 60{^\circ}}$

$= \sqrt{9A^{2} + 4A^{2} + 6A^{2}} = A\sqrt{19}$