A thin plane-parallel glass plate is placed in front of one slit of a Young’s…
PHYSICS · Single correct · A Level · Physics · Waves · Waves
Problem
A thin plane-parallel glass plate is placed in front of one slit of a Young’s double-slit set-up. The central bright fringe on the screen shifts towards the slit having the plate. Which one of the following correctly explains this observation?
(a) It decreases the optical path length through that slit, producing a phase lead that drags the central fringe toward it.
(b) Because a portion of the light is reflected/absorbed by the plate, the intensity behind that slit falls, and the central bright fringe shifts there to balance brightness.
(c) It increases the optical path length of light through that slit, introducing an additional phase lag.
(d) Refraction in the plate bends the beam slightly, effectively tilting the emerging wavefronts and displacing the central fringe toward that slit.