(a) How many fringes appear between the first diffraction-envelope minima to either side of the central maximum in a double-slit pattern if = 566 nm,...

(a) How many fringes appear between the first diffraction-envelope minima to either side of the central maximum in a double-slit pattern if λ = 566 nm, d = 0.135 mm, and a = 27.6 µm?

(b) What is the ratio of the intensity of the third bright fringe to the intensity of the central fringe?