Homework Help: Inteferenxce pattern of air wedge

1. Sep 5, 2014

somecelxis

1. The problem statement, all variables and given/known data
please refer to the question, since at the positon Q , the lighr ray is blocked by the substance , (as in the diagram) , why the bright fringe is seen at Q , In my opinion , it it should be a dark fringe.
whereas at P , the lighr ray is not blocked , so it should be bright fringe is seen at P.. But the ans is the dark fringe is seen at P . why is it so?
the ans is B .
Correct me if i am wrong. thanks!

2. Relevant equations

3. The attempt at a solution

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2. Sep 5, 2014

Staff: Mentor

I'm quite sure that point Q is not intended to be under the shadow of the opaque material!!

Q is in the air wedge but towards one end of the air wedge, P is near the other end. The photo purports to be of the interference pattern, so such shadows would not be included here anyway because a shadow is unrelated to interference pattern.

3. Sep 5, 2014

4. Sep 5, 2014

Simon Bridge

He means the shadow of the opaque material is not part of the interference pattern shown in the photo.

5. Sep 6, 2014

somecelxis

well , can you explain why at P , the fringes is dark? This means destructive intefrence occur at P ? how do you know that destructive intefrence occur at P ?

6. Sep 6, 2014

Staff: Mentor

In the region around P there is almost no path difference for the two rays, yet they still cancel. How can this be? What is it that is happening to one ray but not happening to the other?

I'm sure there was another thread on the air wedge just recently.

How do I know the fringe is dark at P? Because I remember that from my high school physics!

7. Sep 6, 2014

Simon Bridge

Well that's will be why the fringe is dark at P: destructive interference.

You know there is destructive interference at P for the same reason you know there is destructive interference anywhere. Work through the reasoning - show us how you are doing that - then we can help.