(a) Show that the positions of the interference minima on a screen a large distance L away from four equally spaced sources (spacing d, with d >> λ) are given approximately by

y = nλL/4d

Where n = 1, 2, 3, 5, 6, 7, 9, 10, . . .

That is, n is not a multiple of 4.

(b) For L = 2 m, λ = 6 × 10–7 m, and d = 0.1 mm, calculate the width of the principal interference maxima (the distance between successive minima) for four sources. Compare this width with that for two sources with the same spacing.