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The book I'm reading about optics says at some point that "each color (wavelength) contained in the white light interferes only with itself". But why is this so?

Edit: I moved the rest of the question elsewhere.

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up vote 10 down vote accepted

Add any instant in time, light of different wavelengths can be said to interfere. However, because of the extreme frequencies of optical light, any cross interference will get time-averaged away very quickly unless the two waves are very close in frequency.

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Very simple and clear. Just one thing: did you mean "At any instant"? Also, I was wondering if I should strip down the question and retain only the first sentence; the rest might be a bit messy or slightly off-topic. What do you think? – waldyrious Jul 12 '11 at 17:09
I think it might be better if I moved the rest of it to another question. I'll do so now. – waldyrious Jul 12 '11 at 17:19

Interference is not an instant phenomenon but a time-lasting one. The time should be sufficient to speak of certain frequencies. Two different frequencies can be easily distnguished with the corresponding resonators but this process takes time. It is in a long term condition that two frequencies are separated (resonators pumped). Instantly we cannot even speak of a frequency.

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Imagine two light beam each of its own wavelength $L_1=720\text{ nm}$ and $L_2=719\text{ nm}$. One can show I suppose that every $720$ cycles of the $L_2$ beam the total constructive interference shall repeat.

In air the $L_1$ beam light frequency is $$F_1=\frac{C}{L_1}=417\times 10^{14}\text{ Hz}$$

and therefore the frequency of the interference effect is $$F_\text{int}=\frac{F_1}{720}=57.9*10^{12}\text{ Hz}$$

This frequency is rather high to be detectable. When two light beams are of identical wavelength however, the interference effect is time independent and therefore observed as an interference pattern. It is obvious that there is the interference between light beams of a different "color" albeit difficult to observe. Please read about the white light interferometry.

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