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I got an interesting theoretical question about light particle emission rate (if that even makes sense). The setting:

In a perfect vacuum, we have a perfect laser pointer (the beam is perfectly focused, so the "dot" is the size of aperture at any distance) with circular aperture (say 1 mm if that is relevant) and infinite plane parallel to laser pointer at some finite distance. Laser pointer is being rotated at 1 rad/s (axis of rotation coincides with aperture).

The further the plane is the faster the dot moves across the plane. "Normally" we would observe a line painted by the laser pointer. If light is emitted as distinct particles, there must be certain emission rate. Then if the plane is far enough, linear speed of "dot" will be so high, that distinct particles would hit the plane at distances larger than Planck's distance and the line would become dotted/dashed instead of solid.

The questions. Is it appropriate to say that light is emitted in distinct particles or is light emission continuous no matter what? What is light particle emission rate (wrt intensity/power and possibly other properties)? What should the linear speed of the "dot" (distance from pointer axis of rotation to plane) be that we could observe dotted line?

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I can give you a rough answer to this, but a really thorough answer would need a deeper grasp of quantum optics than I possess. It might be worth waiting a while to see if anyone else posts an answer.

Anyhow, a light beam is not simply a hail of little balls called photons. The photons in a light beam are delocalised and the light ray is a coherent state of many photons. That means you can't look at a light ray and point to any individual photon.

However when the light ray exchanges energy with anything it does so in quanta of energy $E=h\nu$, and those are photons. So if your plane was a CCD it would be registering separate events as the light wave exchanged photons with it. If you swept the light ray across the CCD then you would indeed see a chain of separate detections separated by some distance that depended on the speed you swept the light across the surface.

Calculating the detection separation is straightforward. If the power of the light ray is $P$ joules per second then the number of photons per second exchanged with the plane is just $P/(h\nu)$. If the speed with which you swept the light ray over the surface is $v$ then the spacing between detections is just $P/(h\nu v)$.

The same applies to the emission of the light ray. The ray is created when something transfers energy to it. In the case of a laser this is when excited atoms decay and emit a photon. But be cautious about a mental image of an atom handing over a ball of light to the light ray. It is better thought of as an exchange of energy between the atom and the electromagnetic field of the light ray.

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