White light is incident on a diffraction grating with $487$ lines$/$mm$.$

HINT

$($a$)$

Calculate the angle $$r$2($in degrees$)$ to the second$-$order maximum for a wavelength of $630$ nm$.$

$$

$($b$)$

Calculate the wavelength in nm of light with a third$-$order maximum at the same angle $$r$2.$

nm