Physicists please

[Mr. Brightside]

I'm doing A2 physics this year and have got a bit of a problem with the explanation of Young's Double slits experiment we were given.

I understand and appreciate that fringe interference patterns are created.
I also understand that this only happens with coherent sources, which have been defined to us as sources that have constant phase difference.

The problem I have is that we've been told that for constructive interference to occur, the path difference must be a whole number of wavelengths, and for destructive interference, the path difference a whole number of wavelengths plus a half.

I appreciate that this is true if the sources are in phase, but fail to see how this is true if the sources are out of phase, even if they do have constant phase difference.

Diagram:

The point that I dispute is that we have been told the path difference is always a whole number of wavelengths for constructive interference to occur. I disagree with this and say it is only true for sources in phase, as in my above example the path difference would have to be 0.25 wavelengths (i.e distance from S2 should be 10.75 wavelengths) for constructive interference to occur at point P.

Hopefully I've been able to explain my point in a comprehensible way, now:

Have I been really stupid and missed something obvious or is my thinking correct?

Thanks for reading

 

[Coran]

I'd say you're right and your teachers are simplifying for the rest of the class. They're assuming the light is a point source equidistant from each slit so you don't have to deal with any complications.

 

[oddjob62]

That experiment (at least when i have seen it) is carried out with a laser, with the space between the slits being very small. As it is a laser the light going between neighboring slits is in phase, as it's the same source.

As you say, if the sources are out of phase then the calculations do not work.

 

[SpeedFreak]

Let me get this straight, you have two light sources, one out of phase to the other by 90 degrees passing through the double slits?

If so, it makes sense that compensation for this phase difference must be considered before total interference can occur. If they are offset by 1/4 wavelength, then the interference would indeed occur when the phase difference is (x+1/4) lambda where x is a whole number.