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Why Newtons Rings are circular and centre of interference pattern is dark?
1 Answer
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  • The path difference between the reflected ray and incident ray depends upon the thickness of the air gap between lens and the base.

  • As the lens is symmetric along its axis, the thickness is constant along the circumference of a ring of a given radius.

  • Hence, Newton’s rings are circular.

  • At the centre, the air gap thickness is zero.

  • Hence the path difference is zero.

  • However, as the ray reflects from a denser medium, it undergoes a phase change of $π$

  • Thus incident ray and reflected ray interfere destructively.

  • Hence, centre of interference pattern is dark.

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