Thin Film Interference: Can You Have a Phase Difference>2pi

[Riley Yakel]

If the equation for phase difference, 2(pi)(path difference/wavelength) gave you a value like 12.25pi
would that be the phase difference or would it be .25pi?

 

[blue_leaf77]

Both are equivalent.

 

[lychette]

think about what phase difference means. If you start with a phase difference of 0.25π then complete 1 whole circle you will be back where you start, which is now 2π + 0.25π = 2.25π, one more rotation brings you to 4.25π, one more to 6.25π...you see all the possibilities that are 'equal'

 

[sophiecentaur]

Both are equivalent.

Mathematically yes but, in real life the vector addition can go wrong because of the Coherence length of the source. Another way of looking at it is the bandwidth of the source and the phase will not be stable. This means that a signal with a large path difference will not necessarily be identical to the undelayed signal and cancellation can fail. Interference fringes get fuzzier and fuzzier for off-axis parts of the pattern.

 

[blue_leaf77]

Mathematically yes but, in real life the vector addition can go wrong because of the Coherence length of the source. Another way of looking at it is the bandwidth of the source and the phase will not be stable. This means that a signal with a large path difference will not necessarily be identical to the undelayed signal and cancellation can fail. Interference fringes get fuzzier and fuzzier for off-axis parts of the pattern.

Ah, that's a good point. I completely forget about the possibility for the source to have limited coherence length.