Newton's Rings and Central Fringe
Why the central fringe appears to be light rather than
dark in formations of Newton's Rings?
Dark only happens if waves cancel.
Waves only cancel if they have different phases such as 0 and 180 degrees,
or 0, 120, and 240 degrees, or a similar balanced set.
Newton's rings are diffraction around a circular object.
It is circularly symmetric at the center-point, so all paths to that point will
have equal distance and phase.
All will be 0 degrees relative to each other.
So they must add constructively instead of canceling.
Hence a bright spot.
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Update: June 2012