Question

# Part 4: The second thing that can happen is that a wave can be inverted when...

Part 4: The second thing that can happen is that a wave can be inverted when it reflects from a surface. This turns peaks into valleys and valleys into peaks. Since this has the equivalent effect of shifting the phase of the wave by 180° this is called a phase reversal.
If this happens once, either when the light reflects from the top surface or when it reflects from the bottom surface, then the net effect would be to turn what would normally be constructive interference (if just looking at path length difference) into destructive interference and vice versa.
If this happens at both surfaces, the effect cancels out and you ignore it.
Determine whether the reflected light will interfere constructively with itself (bright) or destructively with itself (dark).
Constructive Destructive  Thickness= ¼?film, a phase reversal at the bottom surface and at the top surface.
Constructive Destructive  Thickness= ¼?film, a phase reversal at the bottom surface but not the top surface.
Constructive Destructive  Thickness= ½?film, a phase reversal at the top surface but not the bottom surface.

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