TBR Physics Section 10, Problem 38. Help!!!

[4563728]

For this question we were given the setup: AIR | GLASS | PLASTIC (|= and interface)
We are also given the equation: delta(r)= (wavelength*phase difference)/ (2*pi)

delta (r) is described as the path difference, but I still don't understand what the mean by path difference...

The question asks, "If using this set up, the phase difference is measured to be pi radians, then which of the following is a possible value for the thickness of the glass, if the incident angle is assumed to be negligibly small?

The answer was 0.25 meters and the logic behind it was that if the light ray is perpendicular to the surface of the glass then delta r=2t (t=thickness). Why would that be true? I think I don't understand what delta (r) is, so if someone could explain that would be great.

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[QuinnTheEskimo]

For this question we were given the setup: AIR | GLASS | PLASTIC (|= and interface)
We are also given the equation: delta(r)= (wavelength*phase difference)/ (2*pi)

delta (r) is described as the path difference, but I still don't understand what the mean by path difference...

The question asks, "If using this set up, the phase difference is measured to be pi radians, then which of the following is a possible value for the thickness of the glass, if the incident angle is assumed to be negligibly small?

The answer was 0.25 meters and the logic behind it was that if the light ray is perpendicular to the surface of the glass then delta r=2t (t=thickness). Why would that be true? I think I don't understand what delta (r) is, so if someone could explain that would be great.

I think by "path difference" they are referring to one light ray bouncing off the glass, and another bouncing off the plastic surface, and then meeting in the air. Can you visualize this? If the glass is 0.25 meters thick, the path difference would be 0.5 meters (the ray that bounces off the plastic interface would travel an extra 0.5 meters, compared to the ray that bounces directly off the glass).

 

[4563728]

Everything you said makes sense for the most part but what do you mean by 'then meeting in the air?' I though the reflecting waves would be parallel to each other.