# Homework Help: Bragg's law

1. Sep 4, 2010

### atomqwerty

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

An electron beam (energy 119,9 eV) contacts perpendicularly on a Ni plate, with interatomic spacing d= 0,215 nm. At which angle Theta will be obtained an electron beam with the lower wavelenght possible?

2. Relevant equations

$$n\lambda=d sin(\theta)$$ (Bragg's for perpendicular incident beam)

3. The attempt at a solution

I've tried to calculate the wavelenght of the electron beam, by using the fact that E= hf = h c/lamba. Then, with the wavelenght, the Bragg law give us the angle Theta (with n=1), that will be refracted the electron beam, buy Bragg's Law says that the lower wavelenght implies the highest order n, that implies in itself, the major abgle, Theta = 90º.

I dont get it...

thanks