Question

Consider the optical system shown in Fig. The point source of light $S$ is having wavelength equal to $\lambda$. The light is reaching screen

onlyafter reflection. For point $P$ to besecond maxima, the value of $\lambda$ would be $( D >>d$ and $d>>\lambda)$

• A. $\frac{12 d^{2}}{ D }$
• B. $\frac{6 d^{2}}{ D }$
• C. $\frac{3 d^{2}}{ D }$
• D. $\frac{24 d^{2}}{ D }$

Answer 1

Answer: (A)

At $P, \Delta x=\frac{(8 d) \times 3 d}{ D }$
For $2^{nd}$ maxima, $\Delta x=2 \lambda$
$\Rightarrow \frac{24 d^{2}}{ D }=2 \lambda $
$\Rightarrow \lambda=\frac{12 d^{2}}{ D }$