Optics Question 171
Question: Light is incident normally on a diffraction grating through which the first order diffraction is seen at 32o. The second order diffraction will be seen at
Options:
A) 48o
B) 64o
C) 80o
D) There is no second order diffraction in this case
Show Answer
Answer:
Correct Answer: D
Solution:
For a grating $ (e+d)\sin {\theta _{n}}=n\lambda $
where $ (e+d)= $ grating element $ \sin {\theta _{n}}=\frac{n\lambda }{(e+d)} $ For $ n=1, $
$ \sin {\theta _{1}}=\frac{\lambda }{(e+d)}=\sin 32{}^\circ $
This is more than 0.5. Now $ \sin {\theta _{2}} $ will be more than $ 2\times 0.5, $ which is not possible.
