The wave nature of electrons suggests that they will produce diffraction effects. Davisson and Germer experimentally proved this by diffracting electrons from a crystal. The law of diffraction from a crystal is obtained from the constructive interference of electron waves reflected from the planes of atoms in the crystal. When electrons are incident on the crystal plane at an angle $i$ with the normal to the plane,and the distance between the planes is $d$,which of the following equations can be used to calculate the de Broglie wavelength of the electrons if a sharp diffraction peak is obtained?

• A
  $d\sin i = n\lambda$
• B
  $2d\cos i = n\lambda$
• C
  $2d\sin i = n\lambda$
• D
  $d\cos i = n\lambda$