Light with wavelength `lambda = 0.55 mu m` from a distant point source falls normally on the surface of a glass wedge. A fringe pattern whose neighbouring maxima on the surface of the wedge are separated by a distance `Delta x = 0.21 mm` is observed in reflected light. Find:
(a) the angle between the wedge faces,
(b) the degree of light monochromatism `(Delta lambda//lambda)` if the fringes disappear at a distance `l ~~ 1.5 cm` from the wedge's edge.

Explanation

(a) For normal incidence we have using the above formula
`Deltax = (lambda)/(2n alpha)`
so `alpha = (lambda)/(2n Deltax) = 3'` on putting the values
(b) In a distance `l` on the wedge there are `N = (l)/(Deltax)` fringes.
If the fringes disappear there, it must be due to the fact that the maxima deu to the component of wavelength `lambda` coincide with the minnima due to the component of wavelength `lambda + Delta lambda`. Thus
`N lambda = (N - (1)/(2)) (lambda + Deltax)` or `Delta lambda = (lambda)/(2N)`
so `(Delta lambda)/(lambda) = (1)/(2N) = (Delta x)/(2l) = (0.21)/(30) = 0.007`.
The answer given in the book is off byu a factor `2`.