A plane light wave falls on Fresnel mirrors with an angle `alpha = 2.0` between them. Determine the wavelength of light if the width of the fringe on the screen `Delta x = 0.55 mm`.

Explanation

To get this case we must let `r rarr oo` in the formula for `Delta x` of the lest example.
So `Deltax = ((b + r)lambda)/(2 alpha r) rarr (lambda)/(2 alpha)`.
(A plane wave is like light emitted from a point source at `oo`).
Then `lambda= 2 alpha Delta x = 0.64 mu m`.