Light of wavelength 540 nm illuminates two slits of width 0.015 mm and separation 0.105 mm.
(a) How many interference maxima fall within the full width of the central diffraction maximum?


(b) What is the ratio of the intensity of the third interference maximum to the side of the centerline (not counting the center interference maximum) to the intensity of the center interference maximum?

Need help please, I solved question a, but am having problems solving b.

I'm feeling generous today.

I=I0*diffraction term * interference term note : I don't want to check out the formula, so I'm being lazy and writing this instead.
At the center, you should have I0.
So the ratio would be : I0*diffraction term * interference term / I0 = diffraction term * interference term

If I remember right, the interference term is a sinc(b*sin(theta)/lambda). Sinc is maximum when the argument is 0. You then want b*sin(theta)/lambda =0 => sin(theta)=0 => theta= n*pi. so you take n = 3 (which implies that theta=3*pi), because you want the third interference maximum.

Now you have all your values needed and you can just plug them in the ration equation above.
Is it clear now?