= Solution
For a nonrelativistic <electron>,
$$
\lambda=\frac hp=\frac h{\sqrt{2m_eE}},\qquad
E=20(1.6\times10^{-19})=3.2\times10^{-18}\ {\rm J}.
$$
Using the supplied constants gives
$$
\boxed{\lambda\simeq2.75\times10^{-10}\ {\rm m}=0.275\ {\rm nm}.}
$$
This is comparable with atomic lattice spacings. A suitable crystalline surface or sufficiently thin crystal can therefore diffract the <electron> beam; the reciprocal-lattice geometry and scattering conditions determine the actual angles. A grating or slit can also diffract matter waves if its scale and coherence conditions are suitable. The <kinetic energy> is tiny compared with the <electron> rest <energy>, so relativistic corrections are negligible at the stated precision.
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