Two coherent sources send light of wavelength 500 nm to three points P, Q and R on a screen…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Two coherent sources send light of wavelength nm to three points , and on a screen. The paths to the two sources differ by at , at and at . Find the phase difference at each point, in units of rad, and say which points are bright and which are dark.
Need a reminder? The key ideaShow
Two coherent waves of phase difference give . A path difference gives . Bright: . Dark: .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. We count the wavelengths in each path difference. Then times that number.
Step 1. Point P. nm, which is wavelengths. So rad.
Step 2. Point Q. nm, which is wavelengths. So rad.
Step 3. Point R. nm, which is wavelengths. So rad.
Answer: , and rad. Points and have an odd multiple of , so they are dark. Point has an even multiple of , so it is bright.
Check. A bright point has a whole number of wavelengths ( at ). A dark point has a half-integer number ( at and at ).
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