Question

Light is incident normally on a diffraction grating through which the first order diffraction is seen at ${32}^{\circ }$. The second order diffraction will be seen at

• A. ${48}^{\circ }$
• B. ${64}^{\circ }$
• C. ${80}^{\circ }$
• D. there is no second order diffraction in this case.

Answer 1

The correct option is D there is no second order diffraction in this case.

We know, that if ${\theta }_1$ and ${\theta }_2$ be the angles formed in the two orders of diffraction, then
$\frac{sin{\theta }_2}{sin{\theta }_1}=\frac{n_2}{n_1}$
Hence, $sin{\theta }_2=2sin32>1$
Hence, a second order diffraction pattern doesnt exist in this case.