Inclined Glass Wedge Diffraction - Help

[Zaknife]

Homework Statement

Monochromatic beam of incident light on the surface of the glass wedge,
whose upper edge is inclined at an angle of γ = 0.05 ° from the base. In reflected light
observe a number of interference fringes, the distance between adjacent
dark streaks is △X = 0.21 mm. Calculate the wavelength λ of the incident light.
Refractive index of glass n = 1.5.

Homework Equations

$$d \sin \alpha = n \lambda$$

The Attempt at a Solution

I need a guidance ! Is it a thin-film problem ?

 

[rude man]

Need a picture of this setup.