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Question 20243
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Electromagnetic Waves and Wave Optics: Newton's Rings — Dark Central Spot MCQ

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Newton's Rings — Dark Central SpotHardQuestion 20243

Question

Newton's rings show a dark spot at the centre in reflected light because:
  1. A
    No light reaches the centre
  2. B
    Phase change on reflection from the flat glass gives π phase shift at zero thickness → destructive interference
    Correct
  3. C
    The lens absorbs light at its centre
  4. D
    The curvature focuses light away from centre

Correct answer

Phase change on reflection from the flat glass gives π phase shift at zero thickness → destructive interference

Explanation

At the point of contact (t = 0): ray from top of air film (no phase change) and ray from bottom plate (π phase change at air-to-glass). Path difference = 0 but net phase difference = π → dark central spot in reflected light.

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