Problem $2[15$ points$]$

The first$-$order line of $489-nm$ light falling on a diffraction grating is observed at a $13\mathrm{.}5$ angle. $1nm={10}^{-9}m.$

A$)[5$ points$]$ How far apart are the slits?

B$)[5$ points$]$ At what angle will the third order be

observed? Express your answer in degrees.

C$)$ at what angle will the $4$th be observed