Optics Question 206
Question: A plane wavefront $ (\lambda =6\times {{10}^{-7}}m) $ falls on a slit $ 0.4mm $ 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
[RPMT 2001]
Options:
A) $ 6mm $
B) $ 12mm $
C) $ 3mm $
D) $ 9mm $
Show Answer
Answer:
Correct Answer: A
Solution:
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 {{10}^{-7}}}{4\times {{10}^{-4}}}=6\times {{10}^{-3}}m=6mm $
