In order to study the structure of a crystalline solid, you want to illuminate it with EM

Question:

In order to study the structure of a crystalline solid, you want to illuminate it with EM radiation whose wavelength is the same as the spacing of the atoms in the crystal (0.20 nm).
(a) What is the frequency of the EM radiation?
(b) In what part of the EM spectrum (radio, visible, etc.) does it lie?