$($a$)$ A He$$Ne laser has a cavity length of $15$ cm$.$ If light is emitted by the gas atoms

over a frequency range of $1\mathrm{.}7$ GHz$,$ how many modes can lase simultaneously?

Take n $=1.($b$)$ A GaAs edge$-$emitting laser diode has a cavity length of $1$ mm and

a center operating wavelength of $850$ nm$.$ If light is emitted over a wavelength

range of $5$ nm$,$ how many modes can lase simultaneously?