The speed of an electromagnetic wave in a material medium is given by $v=\frac{1}{\sqrt{\mu \epsilon }}, \mu$ being the permeability of the medium and $\epsilon$ is the permittivity. How does its frequency change?

Name the constituent radiation of electromagnetic spectrum which is used for studying crystal structure.

Name the constituent radiation of electromagnetic spectrum which is similar to the radiations emitted during decay of radioactive nuclei.