Two narrow slits S1 and S2 separated by a distance d are illuminated by monochromatic light of wavelength λ.
(a) Define the term "path difference" for the light reaching a distant point P on the screen from the two slits.
(b) State the general conditions under which the point P will appear bright or dark to an observer.
(c) The two slits are now fed by the same source but an adjustable phase plate is placed in front of S2 so that the initial phase of the light emerging from S2 can be changed by an amount ϕ (0≤ϕ≤2π). Explain qualitatively how the conditions for brightness and darkness at P are modified when the phase plate introduces (i) ϕ=π and (ii) ϕ=π/2. (Assume the point P is kept fixed while ϕ is varied.)