Thread: when light meets light

What happens when a laser beam hits another laser beam of the same frequency?,

what probability is there of a photon colliding head-on with another photon, and, if this happens, is there any interference or loss of energy?

It's an interesting question. Laser beams are generally considered to be electromagnetic waves and if two beams intersect and they are coherent then one would expect inteference to occur depending on the phase difference at the point of intersection. If you choose to think about two such beams in terms of photons, then interference would be expected to happen as before. However, it doesn't seem to be case of one photon interfering with another. This has been demonstrated by allowing single photons to pass through two slits as in Young's experiment. Even when photons pass through one at a time, interference patterns still build up. This suggests that the interference arises from the wavelike characteristics of a single photon rather than by two such photons interacting. Exactly how that can be is a bit mysterious, to put it mildly, but it seems to be the case.

(By referring to "head on" collisions between photons you might be pushing the particle "picture" too far. We don't know what photons look like! :-)

Thanks.
By "head-on' I meant only 180°
So there can never be reduction of energy-frequency, they go through one another, like waves in a pond, right?

There would be no overall loss in energy. An interesting aspect of interference is that constructive interference at one place is always attended by destructive interference somewhere else. This ensures that there is no net loss or gain in energy. If you just had constructive or destructive interference alone, energy wouldn't be conserved.

I doubt if there will be no gain in energy.If you shine one beam of light on a paper,passing through a lense,then you expect the paper to get burnt soon.Now if two different beams meet and then strike the paper,it means there could be a destructive interferance or constructive. Whatever one it is,the number of photons hitting the paper should imcrease or decrease.Then the rate at which the paper burns should decrease.

I doubt if there will be no gain in energy

I take that to mean that there will be a gain in energy. But where does this energy come from? If two waves of amplitude a interfere, the resultant wave will have an amplitude of 2a. However, the intensity of light ( energy per second per unit area) is proportional to the amplitude squared. So the incoming intensity would be 2 a^2 and the resultant intensity would be (2a)^2 or 4a^2.. This clearly contravenes conservation of energy.

In interference, this apparent anomaly doesn't happen because constructive and destructive interference occur together but at different places. Exactly how energy is partitioned between regions of constructive and destructive interference is not obvious - that is, although one can easily calculate how much energy is in each region from the intensity of light in that region, explaining how it came to be there is by no means transparent.

Actually, nothing would happen. Interference is a true happening. But in vacuum waves do not interect. For the simple reason; how could they?
There are no forces that would make 2 different beams of light interact. These forces only exist in a medium. And even a small simple medium is often enough. So many times just certain air molecules can be enough if their broadening allows reasonable interference. But no, nothing happens with two different beams of light. (except when they are indistinguishable, but since they have distinctly no overlap in propagation direction this is not the case in your example)

Constructive interferance and destructive interferance,cannot happen at sametime. When the crest of a wave and the crest of another meet at a point,another crest with a greater amplitude is forme.This is same for destructive.When crest meets with a tough,they cancel each other out and there is no viberation or movement. If the above happens to two beams of light,then we can expect energy gain or loss,or no energy.

Originally Posted by merumario

Constructive interferance and destructive interferance,cannot happen at sametime. When the crest of a wave and the crest of another meet at a point,another crest with a greater amplitude is forme.This is same for destructive.When crest meets with a tough,they cancel each other out and there is no viberation or movement. If the above happens to two beams of light,then we can expect energy gain or loss,or no energy.

Actually no. There is no energy gain or loss. After all that would imply a flow of energy and this not occur. Not even if a superposition would form. If energy were to flow then energy would go from one wave to the other. this doesn't happen as colours don't change. And even with the forming of beats or stuff of the alike, there is no energy gain or loss.

wave is after all the flow of energy,we are looking at the beams as waves and not only as particles. if color does'nt change then how do you explain the mixture of primary colors forming a secondary color? the is a pigment in any colorful thing that makes the material absorb the other frequencies of light and reflect a particular one.if that is the case,after the mixture,another frequency of light will be reflected,then color must have changed.meaning energy has changed as well.

Originally Posted by merumario

If color does'nt change then how do you explain the mixture of primary colors forming a secondary color? the is a pigment in any colorful thing that makes the material absorb the other frequencies of light and reflect a particular one.if that is the case,after the mixture,another frequency of light will be reflected,then color must have changed.meaning energy has changed as well.

This isn't the case at all. Reflectance of light is first of all a medium of interaction. And blaming the reflectance of a subtance on the energy of light is a bit like saying that Cars all drive in the same direction, because they come from a garage.

Even in combining light, a medium is always required through which light can interact. Without a medium, light doesn't interact. The argument is a little bit more subtle in the case of optical cavities.

Originally Posted by merumario

wave is after all the flow of energy,we are looking at the beams as waves and not only as particles. if color does'nt change then how do you explain the mixture of primary colors forming a secondary color? the is a pigment in any colorful thing that makes the material absorb the other frequencies of light and reflect a particular one.if that is the case,after the mixture,another frequency of light will be reflected,then color must have changed.meaning energy has changed as well.

You are a bit mixed up here, merumario. When you present two differently-colored beams to your eye, your visual system may interpret the result as a different color. No new wavelength has been generated. The two beams do not interact. Photons are bosons, not fermions; there is no exclusion principle that forbids any number of photons possessing the same quantum state. They therefore act very differently from, say, electrons. That's why photonics and electronics are quite different technologies.

Tk421 raises an indeed crucial point in this all. Photons are force carriers.

what happens when light from two projectors intercept before falling on a screen? surely nothing changes.


i mnow get it. thanks tk4 and kerling.

Originally Posted by JonG

There would be no overall loss in energy. An interesting aspect of interference is that constructive interference at one place is always attended by destructive interference somewhere else. This ensures that there is no net loss or gain in energy. If you just had constructive or destructive interference alone, energy wouldn't be conserved.

er..Energy would still be conserved because the net gain or net loss would be zero: -1 + 1 = 0 (assuming full phase cancellation)

er..Energy would still be conserved because the net gain or net loss would be zero: -1 + 1 = 0 (assuming full phase cancellation)

-1 + 1 is completely irrelevant. The intensity of a wave is proportional to its ampltude squared. If two waves of amplitude a interfere contructively, the amplitude of the resulting wave would be 2a so the intensity would be four times ( 2 squared is 4 ) as large as that of each of the component waves. That in itself would obviously contravene conservation of energy. Energy is conserved in practice because constructive interference is always accompanied by destructive interference.