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.