Find an approximate expression for the ratio of the power densities at the principal maximum to that at the first secondary maximum on either side, in the Fraunhofer diffraction pattern of an N-slit multiple aperture. Assume that the slits are much narrower than their separation, and that the peak of the secondary maximum occurs halfway between the first and second zeros of the pattern (not exactly true).
The width of the central diffraction peak
 Diffraction minima: nλ=b sinθ
 Central interference fringe: pλ=a sinθ
a: width of slit
b: width of portion between slits
θ: angle of diffraction
λ: wavelength of light
p & n: both integers
The Attempt at a Solution
Dividing  by 
pλ / mλ = p/n
a sinθ / b sinθ = a/b
so a/b = p/n
∴ 2(a/b) = ratio of widths of the central diffraction peak to the central interference fringe
Can this also be extended to the ratio of power densities of the Fraunhofer diffraction pattern? I can't really think of any other way to answer the question. Any help would be much appreciated.