In the diagram shown, S is a source of light emitting plane polarized light with plane of polarization parallel to the plane of the figure having wavelength λ. M is a perfectly reflecting plane mirror. Intensity of light emitted by the source S is I0. If SM = MP = SP = Nλ where N = 106, then intensity of light at P is

Due to reflection at the mirror M, the reflected ray suffers an additional phase difference of 180o which is equivalent to a path difference of λ2. So total path difference at P=SM+MP−SP+λ2=106λ+λ2. Hence the waves interfering at P has a phase difference of 180o. Therefore the angle between the electric fields at P is 120o as shown in the figure. ∴ ER=E0cos60o+E0cos60o=E0∴ IRI0=ERE02=1⇒IR=I0