Question

Two coherent point sources S₁ and S₂ vibrating in phase emit light of wavelength l. The separation between the sources is 2l. The smallest distance from S₂ on a line passing through S₂ and perpendicular to S₁S₂ where a minimum intensity occurs is –

• A. $\frac{7\lambda}{12}$
• B. $\frac{15\lambda}{4}$
• C. $\frac{\lambda}{2}$
• D. $\frac{3\lambda}{4}$

Answer 1

Answer: (A)

$\frac{7\lambda}{12}$

Answer 2

S₁P – S₂P = $\frac{3\lambda}{2}$

or $\sqrt{4\lambda^{2} + x^{2}}$ – x = $\frac{3\lambda}{2}$

On solving

x =$\frac{7\lambda}{12}$