The story behind this piece
A plane wave of red He–Ne light, λ = 633 nm, meets two slits of width a = 2λ, set d = 8λ apart. Beyond them the plate is a true calculation: twenty-four coherent Huygens wavelets are summed at every point, and the crests glow where they reinforce and fade to black along the nodal lines. On the screen at the right is the resulting intensity, computed from the same sum, with the bright maxima m = 0, ±1, ±2 found where d sin θ = mλ. The single-slit envelope dims the outer fringes and wipes out the fourth order entirely, because d is exactly four times a.
