Question

For Bragg's diffraction by a crystal to occur, then which of the following is correct for the X-ray of wavelength $\lambda$ and interatomic distance d :

• A. $\lambda$ is greater than $2d$
• B. $\lambda$ equals $2d$
• C. $\lambda$ is smaller than or equal to $2d$
• D. $\lambda$ is smaller than $2d$

Answer 1

The correct option is C $\lambda$ is smaller than or equal to $2d$

The condition for Bragg's diffraction is

$2dsin\theta =n\lambda$

$⟹sin\theta =\frac{n\lambda }{2d}\le 1$

$⟹\lambda \le \frac{2d}{n}$

where $n$ is a positive integer

Hence the maximum value of $\lambda$ occurs for $n=1$.

Hence $\lambda \le 2d$