In a double-slit experiment, the fourth-order maximum for a wavelength of 450 nm occurs at an angle of = 90. Thus, it is on

In a double-slit experiment, the fourth-order maximum for a wavelength of 450 nm occurs at an angle of θ = 90°. Thus, it is on the verge of being eliminated from the pattern because θ cannot exceed 90° in Eq. 35-14.
d sin θ = mλ, for m = 0, 1, 2, . . .(maxima-bright fringes).
(a) What range of wavelengths 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?