Q. A plane wavefront $(\lambda = 6 \times 10^{- 7}m)$ falls on a slit $0.4\ mm$ wide. A convex lens of focal length $0.8m$ placed behind the slit focusses the light on a screen. What is the linear diameter of second maximum
A) $6mm$
B) $12mm$
C) $3mm$
D) $9mm$
Explanation
(a) For secondary maxima $d\sin\theta = \frac{5\lambda}{2}$
$\Rightarrow d\theta = d.\frac{x}{D( \approx f)} = \frac{5\lambda}{2}$
$\Rightarrow 2x = \frac{5\lambda f}{d} = \frac{5 \times 0.8 \times 6 \times 10^{- 7}}{4 \times 10^{- 4}} = 6 \times 10^{- 3}m = 6mm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.