GREPhysics.NET: GR8677 #13

GR8677 #13

Problem

GREPhysics.NET Official Solution

Optics $\Rightarrow$ }Interference

Fact: The human eye can only see things in motion up to about 25 Hz. (One can approximate this knowing that the human eye blinks once every three seconds on average.)

Now, the problem mentions that the relative phase is varied, at a constant frequency of 500 Hz, which is much greater than the maximum frequency of the human eye. Interference is produced as long as the sources are coherent, and the sources are coherent as long as there's a constant relation between relative phase in time.

(A) The frequency of the phase change has nothing to do with the color of light.

(B) Interference pattern is different for $\pi$ and $2\pi$ phase changes...

(C) Interference can exist for other phase differences.

(D) One can have interference even with polychromatic sources.

(E) The interference pattern shifts position (since the source remains coherent from the constant relation with relative phase to time) at a rate too fast for the human eye, as explained above.

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Comments

bingsong
2010-02-26 04:09:37

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glkjap
2008-05-04 14:23:09

You said: "Interference is produced as long as the sources are coherent, and the sources are coherent as long as there's a constant relation between relative phase in time."

But the sources don't have a constant phase. It changes from 0 to 2 $\pi$ , right? So how are they coherent?

dean
2008-10-09 20:11:29

There are degrees of coherence, and the difference between frequencies of 10^2 and 10^14 is great enough that we can say the beams are still coherent. The light sources are at pretty much the same phase for many wavelengths.

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You said: "Interference is produced as long as the sources are coherent, and the sources are coherent as long as there's a constant relation between relative phase in time." But the sources don't have a constant phase. It changes from 0 to 2 $\pi$ , right? So how are they coherent?

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