In a Young's double-slit experiment, a thin transparent sheet of thickness t and refractive index n is placed over one slit. The fringe pattern
A{'text': 'Remains unchanged in its position and its overall structure', 'label': 'A'}
B{'text': 'Shifts by (n−1) t D/d toward the slit carrying the sheet', 'label': 'B'}
C{'text': 'Disappears completely because coherence is destroyed', 'label': 'C'}
D{'text': 'Doubles its fringe width without any lateral shift on screen', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Shifts by (n−1) t D/d toward the slit carrying the sheet', 'label': 'B'}
1. Introducing a thin sheet changes the optical path in one slit by (n − 1) t.
2. This extra optical path shifts the central bright fringe toward the slit with the sheet.
3. Fringe shift = (n − 1) t × D / d.
4. Fringe width remains λD/d (unchanged); only the pattern shifts as a whole.
_Source: NCERT Class 12 Physics, Ch 10 "Wave Optics", §10.4_
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