Electrons in a vacuum are accelerated from rest through a potential difference (p.d.) 
to form a beam. The electrons each have mass 
and charge 
. The beam is incident on a graphite crystal that acts as a diffraction grating. After passing through the crystal, the beam reaches a fluorescent screen. An interference pattern is observed on this screen.
Explain what this observation shows about the nature of electrons.
Determine an expression, in terms of , , and , for the momentum 
of an electron in the beam.
The p.d. through which the electrons are accelerated is now increased to a greater value. Describe and explain the effect of this change on the interference pattern observed
