# What happens when two light beams collide ?

• phanidee
In summary: Welcome to the forums, first off :-)The photon-photon scattering cross-section (i.e. the probability that one photon will collide with another and both scatter) is vanishingly small (but not zero). Interference effects between two beams can occur if the beams are related to each other somehow: perhaps the originate from the same source, as occurs in an interferometer. So, the short answer is, unless you carefully construct the beams, nothing will happen and the beams will continue on their merry way, blissfully unaware of each other.Hey Andy,I didn't even know that photons have a scattering c.s. for other photons. What will such a scattering look like, if
phanidee
Iam very new to this forum. I just had this doubt for the last week.
pardon me if this is a silly one. the doubt is :

what happens when two light beams traveling in perpendicular directions
collide ? collide in the sense i mean what happens at their intersection point ?
what i have thought is that there would the effect of superposition or something else.

Uhm, stupid question in return: Why do you anticipate anything at all? Do you have two flashlights at your disposal? Take them and carry out the experiment. Maybe a black hole will emerge from the intersection, who knows. Just kidding - as regards the last sentence. Will you please specify why you'd expect an "effect on superposition"?

€: Rude me. Welcome to the forums, first off :-)

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The photon-photon scattering cross-section (i.e. the probability that one photon will collide with another and both scatter) is vanishingly small (but not zero).

Interference effects between two beams can occur if the beams are related to each other somehow: perhaps the originate from the same source, as occurs in an interferometer.

So, the short answer is, unless you carefully construct the beams, nothing will happen and the beams will continue on their merry way, blissfully unaware of each other.

Hey Andy,

I didn't even know that photons have a scattering c.s. for other photons. What will such a scattering look like, if it occurst at all? Will one of the photons really change trajectory if it's scattered? Or will this have other effects so the scattered photon will not really stay alive?

Do you have something to read on this?

I think superposition would occur...

i remember in my childhood physics classes, when waves are generated
at two opposite ends of a water beaker, we can observe the points at which
something happens( i don't know what exactly those points are called ? ).
good example for this is http://www.kleinbottle.com/SpoutingBowls.htm" .
Even I have seen in geographic channel ancient chinese kings used these
to wash their faces :) (its really true).

so, will the same happen in case of light ?
if so what will we see at those points ?

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Phanidee, that's nothing else but plain interference. And yes: That also happes to light. You may have heard of the double-slit experiment where to coherent beams, emmitted from one laser or lamp, are shot through the slits, interfere with each other, forming an interference picture on a screen behind the slits.

ManDay said:
What will such a scattering look like, if it occurst at all? Will one of the photons really change trajectory if it's scattered?

Something like this, I think, via virtual electrons and positrons:

#### Attachments

• photons.gif
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Virtual electrons? Tho I know a bit of Feynman diagrams I've never heard of virtual electrons. What is the probability of existence/emission for a virtual electron by a photon?

Thanks for the nice drawing, by the way :-) t goes horizontally, right?

Well at least, as electrons are far heavier than photons the moderating virtual electron would explain the very small c.s. because of its mass.

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It's not a stupid question at all. I would love to know.

I have a couple of lasers, and I shone a 5mW red laser (in line projection mode, like a barcode scanner) at itself using a mirror. It does interfere with itself, on the mirror itself (due to partial reflection) there is a series of concentric circles with the centre located exactly where light directly from the diode hits the mirror - like these:

http://www.chemie.de/content/images...Figure 2 (right) fine particle scattering.jpg

Of course, this is a head on "collision".

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jtbell said:
Something like this, I think, via virtual electrons and positrons:
That picture is the Feynman diagram for scattering of photons by photons, mediated by virtual electrons. It was first derived over 50 years ago by Max Delbreuk (my spelling might be wrong.), and consequently the process bears his name, although it seems unknown to forumers. Since no one took him seriously, he became a famous biologist.
As a virtual process, it is even a hot experimental process today and for the future.

The name is spelled Delbruck.
Put "scattering of photons by photons" into Google to get many references.

pam said:
The name is spelled Delbruck.

Delbrück. Don't forget the umlaut!

In high school, I once missed acing a German test by forgetting an umlaut, and ended up with a score of 99.75%. The teacher wrote, "Ah ha! Ich habe einen Fehler gefunden!"

ManDay said:
Well at least, as electrons are far heavier than photons the moderating virtual electron would explain the very small c.s. because of its mass.

No, small coupling constant of EM field to fermions is the reason that cross section for such process is very small. Moreover photon-photon scattering is the process of 4th order.

ChrisLM said:
I think superposition would occur...

to have this phenomenon you need energy> 1TeV so with a simple flash light, don't worry
no sparticles will appear

## 1. What is the result when two light beams collide?

When two light beams collide, they either pass through each other or reflect off each other, depending on their properties and the medium they are passing through.

## 2. Can two light beams cancel each other out upon collision?

No, two light beams cannot cancel each other out upon collision as they are both forms of energy that cannot be destroyed. However, they can interfere with each other and create patterns of light and dark areas.

## 3. Do light beams lose energy when they collide?

No, light beams do not lose energy when they collide. In fact, they conserve their energy during the collision process. However, the angle at which they reflect off each other may change depending on the properties of the light beams and the medium they are passing through.

## 4. What happens to the wavelength of light when two light beams collide?

The wavelength of light remains the same when two light beams collide. However, depending on the properties of the light beams, their wavelengths may interfere constructively or destructively, resulting in a change in the intensity of the light.

## 5. Can two light beams merge into one upon collision?

No, two light beams cannot merge into one upon collision. Light beams are made up of individual photons and cannot combine into one beam. However, they can overlap and interfere with each other, creating new patterns of light.

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