Thin Film — Wedge-ShapedHardQuestion 20242
Question
A thin wedge-shaped air film forms between two glass plates meeting at one edge. Observed in reflected monochromatic light, the fringe nearest the apex (thin edge) is:
- ABright
- BDark due to phase change on reflectionCorrect
- CRed coloured
- DMissing
Correct answer
Dark due to phase change on reflection
Explanation
At the apex (zero thickness), ray 1 reflects from the top surface of the air film (glass to air → no phase change) and ray 2 from the bottom (air to glass → π phase change). At zero thickness, path difference = 0 but phase difference = π → dark fringe at the edge. Each subsequent dark fringe forms where 2t = nλ.