Electromagnetic Waves

$$X-$$ray falling on a material 
The wave emitted by any atom or molecule must have some finite total length which is known as coherence length. For sodium light, this length is $$2\ cm$$.
A wave has wavelength $$50A^o$$.
Instantaneous displacement current $$1A$$ in the space between the parallel plates of $$1\mu F$$ capacitor can be established by changing the potential difference at the rate of:
The waves which can not travel in vacuum are :