Wedge interference - solving for 'separation of fringes' without lambda

[iffybom]

Homework Statement

plane plates of glass are in contact along one side and held apart by a wire 0.05mm in diameter, parallel to the edge in contact and 20cm distant. using filtered green mercury light, directed normally on the air film between plates, interference fringes are seen. calculate the separation of dark fringes. how many fringes appear between the edge and the wire?

Homework Equations

t=x*theta=m*(lambda/2)
delta x = x*(lambda/(2*t))

The Attempt at a Solution

i've been using ratio and proportion here since theta will remain constant. however, i cannot get to cancel out lambda nor m. is it necessary to solve for the wavelength of 'filtered green mercury light' first, so everything else will be plug and chug?

 

[rl.bhat]

Yes. Without the wave length you cannot find the fring width.