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How to calculate path difference between the light from two slits?

The question is attached with the answer being C. If anybody can explain why it is C, I would be thankful!
https://imageshack.com/i/pn63fXlcj
Reply 1
You need to imagine drawing straight lines from each of the slits to the point on the screen you are interested in.

If you draw straight lines from each slit to O, then clearly, the lines are the same length. The lengths of the paths from the slits to O are the same, so the difference in the lengths of the paths is zero. In this case, the path difference is zero. There is a bright fringe here becasue if the light waves coming out of each slit are in phase, they will still be in phase at O, as they have each travelled the same distance and therefore there are the same number of wavelengths along each path.

Now think about what has happened the next time that there is a bright fringe, the nearest one to O (so this is not the point P yet). Let's think about the first bright fringe above O. The straight line from the bottom slit to this point is longer than the straight line from the top slit to this point, but if they started out in phase and ended up in phase, the longer path must have an entire extra wavelength along its path, so it is one wavelength longer than the shorter path.

I expect you can carry on with this and see the answer for yourself!

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