1. The problem statement, all variables and given/known data 2. Relevant equations slit width = a , slit seperation = b = d (at photo), tanQ=h/L m.λ=b.sinQ λ.b=sinQ B=π.d.sinQ/λ α=π.a.sinQ/λ Iq=Imx(cosB)^2 x (sinα/α)^2// 3. The attempt at a solution My first move was to find intensity but I have no idea about the probability of finding one photon on the detector since It is not a point and It has own length.Should I think the detector as a point ? Location can be relevant with fringes but I'm not sure.