Electrons are incident on a crystal

[aidantof]

Q: Electrons are incident on a crystal which can be thought of as a series of reflecting planes with spacing d=0.074nm. What Kinetic Energy should the electrons be given in order that at least four diffracton maxima are observable?

Attempt:
The equations i think i have to use are nλ=2dsinθ, P=h/λ and then K=P^2/2m (m being mass of the electron) and form the question n=4.

i'm supposed to get an answer of 1100.0eV.

my problem is that i can't work out λ as I'm not given the angle θ, is there another set of equations i should be using? how do i get about this?

thanks aidan.

 

[mgb_phys]

The wavelength of the electron ( or anything else ) depends on it's momentum, wavelength = h/momentum.

If you look at wikipedia for electron diffraction the equations are derived for momentum in terms of accelerating voltage.