Sunlight is used in a double$-$slit interference experiment. The fourth$-$order maximum for a wavelength of $450$ nm occurs at an angle of $=90.$ Thus, it is on the verge of being eliminated from the pattern because $$ cannot exceed $90$ in Eq$.35-14.($a$)$ What least wavelength in the visible range $(400$ nm to $700$ nm $)$ are not present in the third$-$order maxima? To eliminate all of the visible light in the fourth$-$order maximum, $($b$)$ should the slit separation be increased or decreased and $($c$)$ what least change in separation is needed?

$($a$)$ Number $$ Units $$

$($b$)$

$($c$)$ Number $$ Units $$