Question

23) Light of wavelength λ strikes a pane of glass of thickness T and refractive index n, as shown in the figure below. Part of the beam is reflected off the upper surface of the glass, and part is transmitted and then reflected off the lower surface of the glass. Constructive interference between these two beams will occur if

Answer 1

the answer is T = / 2 n

because the normal reflection on the top produces a phase reversal

on the basis of n_air < n_glass.

and also vice versa of above is bottom reflection does not produce a phase reversal because of n_glass> n_air.

so the extra path length 2 T will be provide a wavelength,

phase shift for destructive interference with the half wavelength shift of the top reflection

_glass = _air / n

2 T = _glass =_air / n

2 T = / n

T = / 2 n

here = wavelength.