Question

At what angles for the first-order diffraction, spacing between two planes respectively is $\lambda$ and $\frac{\lambda }{2}$?

• A. $0^{\circ },90^{\circ }$
• B. $90^{\circ },0^{\circ }$
• C. $30^{\circ },90^{\circ }$
• D. $90^{\circ },30^{\circ }$

Answer 1

Answer: (C)

$2dsin\theta =n\lambda$. (Bragg’s equation)
For $n=1$
$2dsin\theta =\lambda$.
(i) If $\theta =30^{\circ }$
$\Rightarrow 2dsin30^{\circ }=\lambda$
$\therefore d=\lambda$
(ii) If $\theta =90^{\circ }=2dsin90^{\circ }=\lambda$
$\Rightarrow d=\lambda /2$