# Inteference of incoherent light waves

1. Aug 24, 2014

### desmond iking

1. The problem statement, all variables and given/known data

i know that for intefernece of light waves to occur , the light waves must be coherent. How if two light waves are incoherent?
according to my notes, inteference can still occur if waves are not coherent, but the position of pattern will shift constantly with time to give a uniform illumination.

2. Relevant equations

3. The attempt at a solution
In my opinion , the constructive intefrence and destructive inteference occur too fast , namely countless times in a second . So our eyes cant detect it. Thus , only 'bright region' will be obersved all the time. please correct me if i am wrong.

2. Aug 24, 2014

### Maharshi Roy

Yes, actually interference is a phase independent phenomena. If you have read YDSE, then you must know of the equation:-
Intensity(central maxima)=4Icos^2(δ/2)
where, δ and I are phase difference b/w the slits and intensity of the slit respectively.

Now, for incoherent sources, δ varies constantly,
So we have to take the average intensity, the average value of cos^2 function is (1/2)
Hence resultant intensity is constant=2I

Your conclusion is correct that our eyes can't detect that rapid change.

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