A spacecraft flying away from earth at a speed of $0\mathrm{.}95$ c shines a communications laser back towards earth.

a$)$ How fast does an observer on earth measure the laser light to be travelling?

 

 b$)$ Earth$$s atmosphere has minimum absorption for electromagnetic waves around a wavelength of $3500$ nm$.$ What wavelength and colour should the ships communication 

laser be to maximize the chances of it being received by a ground station on earth?

$\mathrm{}$