# Bragg's Law question

1. Feb 8, 2013

### steve233

I'm preparing for a midterm exam and this is one of the practise questions I'm having a bit of trouble with.

1. The problem statement, all variables and given/known data

$\theta$ is the inner angle between the sample and the incident beam.

Spacing in sample = d =10^-10 m
$\lambda$ = 10^-10 m

a) What values of $\theta$ will we measure X-rays at a detector, that is, when will there be constructive interference (assume elastic collision)?

b) If we use electrons instead of X-rays, at which energy should they be to detect the electrons at the same angle as the X-rays (assume elastic collision)?

2. Relevant equations

Bragg's Law:
$2 d sin(\theta) = m \lambda$
E = hf
p = h / $\lambda$
Compton effect

3. The attempt at a solution
For part (a) what I did was basically use:

$sin(\theta) = (1 \lambda) / (2d)$
$sin(\theta) = (2 \lambda) / (2d)$

Since $sin(\theta) = (2 \lambda) / (2d)$ gives 90 degrees, my answer is that the only angle is 30 degrees, that is, m = 1.

For part (b) I'm not really sure how to approach this question. Is this just the classical model of an elastic collision?

Thanks.