How Is the Angle of Electron Reflection Determined in Quantum Experiments?

[roam]

Homework Statement

For this question, h= 6.63×10^–34 J.s, c = 3×10⁸ m/s, m_e= 9.11×10^–31 kg, e=1.6×10^–19 C. Electrons are accelerated from rest by a voltage of 78.0 V. After that acceleration their speed is 5230000 m/s, their linear momentum is 4.77×10^-24 and their De Broglie wavelength is 0.139 nm.

The accelerated electrons are then reflected by a crystal with crystal spacing d of 0.085 nm. Through which angle are they reflected?

(correct answer: 70.3°)

Homework Equations

The Attempt at a Solution

What formula can I use here? I tried using $$d sin \theta = m \lambda$$, where m is the order number. However I don't know what value to substitute for m...
Is there a better way of appraching this problem?

 

[rl.bhat]

Use 2dsin(θ) = mλ.