If I had two coherent point sources of visible light, above a piece of paper maybe, would you expect to see an interference pattern?

Then, if one had a slight shift in phase, would you then expect to see a different pattern, but still an interference pattern?

Then, say you had a strip of point sources with constant spacing and the ability to control the phase of each independently, could you sustainably create a point where each wave superimpose to create a bright spot?

Since the wavelength of visible light is so small I would expect the interference pattern to be practically invisible, but theoretically would there be one?

Would it be possible with enough point sources to create a point so bright that it would be visible to the naked eye?


That is a lot of questions, but they are all strung together sensibly, so I will try to answer all of them.

Yes, two point sources if they are mutually coherent and are stationary will produce an interference pattern where their beams overlap.

If you shift the phase of one of the sources (e.g., by interposing a thin sheet of glass to retard one of the beams), the interference fringes will shift laterally by an amount that corresponds to the phase shift.

The strip of point sources (again, they must be mutually coherent and stationary) you describe is what's called a "phased array". If they are close enough together, and if their relative phase varies along the strip in the right way, the result would indeed be a point focus. And, there is no reason that the point focus can't be extremely bright. It can contain essentially all of the light emitted by all the point sources together.

By the way, a phased array can do a lot more than form a point focus. It is essentially a low-resolution hologram, and can form a 3D image. You might be interested in learning about "phase-only spatial light modulators", which accomplish essentially what you're talking about.

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