dark slit question?

Question

A pair of narrow slits, separated by 1.8 mm is illuminated by a monochromatic light source. Light waves arrive at the two slits in phase. A fringe pattern is observed on a screen 4.8 m from the slits
monochromatic light of 450 nm wavelength is used. The angular separation between adjacent dark fringes on the screen, measured at the slits, in m rad, is closest to:


a.0.20
b. 0.36
c. 0.15
d. 0.30
e. 0.25

I think it is about 0.125 mrad, but I am not sure (if I take m=0)

Answer 1

Spacing of the fringes is given by:

dy = s/a*wl where s =distance to screen, a = slit separation, and wl = wavelength

angular separation dq = dy/s = wl/a = 450 nm/1.8 mm = 0.25 mrad

Answer 2

It's a trick question....the answer is actually "bleen"