# X-rays Reflection

1. Nov 2, 2011

### roam

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

X-rays of wavelength 0.09 nm are incident on a crystal. A reflected beam is observed at 36 degrees from the incident beam. What is the spacing of the planes in the crystal?

2. Relevant equations

Bragg's law:

$2 d \ sin \theta = m \lambda$

3. The attempt at a solution

I thought this is a simple application of Bragg's law but I get

$2 d \ sin \ 36 = 0.09 \times 10^{-9} \implies d=7.65 \times 10^{-11}$

But the answer should be 4.73 x 10-11 m. What's wrong with my calculation?

2. Nov 2, 2011

### Simon Bridge

Draw a picture of the situation.

You have been given the angle between the incident and reflected beams. That is not the angle indicated by the theta in Bragg's law though is it?
http://en.wikipedia.org/wiki/Bragg's_law

3. Nov 4, 2011

### roam

I see, thank you. It makes perfect sense now. Thanks!

4. Nov 4, 2011

### Simon Bridge

Well done :) No worries.