A YDSE is illuminated by a parallel beam coming from a medium n₁=1.2. A film of thickness 1.2 μm and refractive index 1.5 is in front of S₁. The wavelength measured in n₁ is 400 nm, and the medium between slits and screen has n₂=1.4. If the two contributions at the geometrical centre have equal intensity I, find the resultant intensity there.

Question

G1 Physics · Accepted Answer

At O the geometrical post-slit paths are equal. The film introduces an additional optical phase determined by its optical thickness relative to the incident medium. Using the given n₁, film index and thickness gives a phase difference π/2 modulo 2π. For equal individual intensities I, resultant intensity I=I+I+2Icos(π/2)=2I. Option B.