One way to produce an interference pattern is to use two mirrors (Fresnel's double mirror) which form between them an angle close to 180°.

A linear source of light of wavelength λ gives rise to two close-up coerent images, which produce an interference pattern on a screen away.

Denote by 180° - α the angle between the two mirrors, with r the distance of the light source from the line of contact between the mirrors

and with L the distance of this line from the screen on which they form the interference pattern.

a. Prove that the distance between the two images of the source is d = 2·r·sinθ ≃2r·α (α in radians)

b. Uses the analogy with the figure produced by the two slits and shows that the distance between the fringes on the screen is: $\mathrm{\Delta y}=\frac{L+r}{2r·\alpha }·\lambda$