White light is incident on a diffraction grating with $439$ line$($s$)/($m$)$m$.$

HINT

$($a$)$

Calculate the angle $\backslash$theta r$2($in degrees$)$ to the second$-$order maximum for a wavelength of $690$ nm$.$

$\backslash$deg

$($b$)$

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

nm