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# Coherent waves watch

1. From my understanding, for two waves to produce a noticeable inteference pattern they must be coherent. I know this means a constant phases difference and therefore the same frequency. What if the two waves have different wavelengths? Are they still coherent?
2. Yep, provided that the two waves still have the same phase difference and frequency. (incorrect, see reply)
3. (Original post by McCannKay)
For 2 waves to produce a noticeable inteference pattern, must be coherent. Means constant phase difference, thus same freq. What if waves have different λs? Still coherent?
In nearly all cases, the interfering waves have equal speed too e.g) 2 EM or 2 sound waves. Thus same freq => same λ. Strictly, only need same freq for a fixed path difference, hence coherence.

Experiments have been done where a beam has 2 branches; 1 travels in medium of higher refractive index (slower => smaller λ, same freq). This still produces good interference.
4. (Original post by McCannKay)
From my understanding, for two waves to produce a noticeable inteference pattern they must be coherent. I know this means a constant phases difference and therefore the same frequency. What if the two waves have different wavelengths? Are they still coherent?
If the waves have different wavelengths (so different frequency), it is not possible they have the same phase difference, thus they would not be coherent anylonger.
5. (Original post by ivser)
Yep, provided that the two waves still have the same phase difference and frequency.
My apologies, I misread. Their wavelength does have to be the same, as it is inversely proportional to frequency. Their amplitude, however, does not.

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