Photon Interactions: 4 Cases & Conditions

[metrictensor]

I read that photons can interact with electrons, atoms or simply disintegrate. The four cases I came across are.

1. Compton scattering.
2. Photoelectric effect
3. Knock an electron to a higher energy state in an atom.
4. electron-positron pair production.

While I understand each process what determines when one would occur as opposed to another. For example, under what conditions does Compton scattering occur vs. the photoelectric effect?

 

[jtbell]

In general, when a photon interacts with matter, any of the processes you name can occur, so long as it doesn't violate any conservation laws. For example, pair production can occur only if the photon has at least enough energy to create the masses of the electron and positron, namely 1.022 MeV.

Otherwise, a photon with a given energy, interacting with a given kind of material, can interact via the photoelectric effect with a certain probability, via the Compton effect with some other probability, etc. These probabilities are usually expressed via a quantity called the "interaction cross-section" for each process. They can be calculated using quantum electrodynamics.

 

[T.Roc]

5. photon/photon interaction. in the visible light band, responsible for color & "whiteness".


TRoc

 

[dextercioby]

What...?Please give details.What do you understand by "photon-photon interaction"?

Daniel.

 

[T.Roc]

Dex,

According to "additive color" theory in "the textbooks" they are:

red+green+blue=white
magenta+yellow+cyan=white
red+cyan=white
blue+yellow=white
magenta+green=unknown
red+green=yellow
blue+green=cyan
etc., etc.


TRoc

 

[dextercioby]

And what theory is that...?It's definitely not the one that explains color combinations in painting (red +green is never yellow)...And what's the connection to "photon-photon interactions",if any?

Daniel.

 

[jtbell]

red+green+blue=white

That stuff has nothing to do with photon-photon interactions and everything to do with the physiology of vision. In particular, it's connected with how light of different frequencies stimulates the cone cells in the retina of the eye, and how the brain processes the signals. Or is it the rod cells? I can never remember which is which.

 

[Hans de Vries]

Dex,

According to "additive color" theory in "the textbooks" they are:

red+green+blue=white
magenta+yellow+cyan=white
red+cyan=white
blue+yellow=white
magenta+green=unknown
red+green=yellow
blue+green=cyan
etc., etc.


TRoc

There is nothing what can be called photon-photon interaction here.

These are the additive rules for color combinations. These perceptions
take place in our brain's neurons only, by combining the inputs from the
red, green and blue sensors.


Regards, Hans

BTW, magenta + green = white and not "unknown"

 

[Hans de Vries]

And what theory is that...?It's definitely not the one that explains color combinations in painting (red +green is never yellow)
Daniel.

Painting is subtractive (white - red - green = blue)
not additive (red + green = yellow).

Regards, Hans.

 

[T.Roc]

Dex.

As Hans pointed out, the additive theory that I referred to is for light (photons). Why would I bring up paint (subtractive)?

Hans

I'm aware of the physiological aspects of light and color, but not talking about them.
However, the aspects of frequency interaction that regulate the stimulation of cones are there because the eye has evolved around making use of EM frequencies known as "visible light". These aspects are inherent in all frequencies. If photon/photon interaction were impossible, how would you produce the coherent resonance known as L.A.S.E.R.? When I said "the textbooks", I am referring to Physics, and not Biology.

My other reason for additive in quotes was because it is flawed. One of the big ones is magenta + green. Magenta is a color to which no frequency can be assigned. It does not exist in the currently used E-M spectrum chart, and therefore, cannot be added to anything.

Newton postulated that "white light contains all colours". This has not been rejected by modern science, and the color wheel for light mixture uses magenta because the subtractive theory uses it flawlessly (color tv, printers, etc.) and must be logically inverse to the additive theory. But again, there is no evidence of a frequency of light that can be described as magenta in any scientific text.

TRoc

 

[EL]

Maybe this should be moved to the "art-forum"...

 

[metrictensor]

Maybe this should be moved to the "art-forum"...

Really, this is a bunch of BS.

 

[T.Roc]

el, m.tens

Faced with such overwhelming evidence..

proof 1. "Maybe this should be moved to the "art-forum"...
proof 2 "Really, this is a bunch of BS."

..I should abandon logic.

A laser is possible because of photon/photon resonance. An interaction capable of resonance, must also be capable of dissonance. This is why some colors reflect certain colors, and absorb others.

If you wish to eliminate evidence base on the neurons "perception", then you must also eliminate all symbols, numbers, and words, as they are just combinations of light and dark that trigger the cones in the eye, and then "perceived" by the brain to mean something. In other words, it is a hollow argument; please try again to convince me that this is not an interaction that takes place outside of the brain.

TRoc

 

[EL]

If photon/photon interaction were impossible, how would you produce the coherent resonance known as L.A.S.E.R.?

A laser is possible because of photon/photon resonance

Photon-photon scattering takes place (or more precise, should according to QED take place) through intermediate virtual electron-positron pairs, so photon-photon interactions does indeed exist. However, what does that have to do with lasers? And what does it have to do with colors?

 

[T.Roc]

EL,

By "colors", I just mean a specific frequency. I don't see why it bothers some folks. If science had a precise definition of each color (by frequency), this wouldn't be so difficult. I could then say, for example, a wave of 730nm, and everyone would know that I was referring to a NIST standard "red" photon.

My first post was simply that there was also photon/photon interactions. It was after being questioned that I eleborated on what I consider to be "elementary" physics.

Lasers are monochromatic, which by definition means "1 color". They started with a red laser, then were able to produce the more refined (expanded data - cd's) green. Blue is next...
is that enough of a reason to mention "laser" and "color" in the same post? A laser is not 1 photon, but a beam of many "interacting" coherent (resonant) photons.

Another example? A quote from Encyclopedia Brittanica:

"In addition to saturation spectroscopy, there are a number of other techniques that are capable of obtaining Doppler-free spectra. An important example is two-photon spectroscopy, another form of spectroscopy that was made possible by the high intensities available with lasers. All these techniques rely on the relative Doppler shift of counterpropagating beams to identify the correct resonance frequency and have been used to measure spectra with extremely high accuracy. These techniques, however, cannot eliminate another type of Doppler shift. "

Thank you for not getting "personal", after all this is a Forum. I'm here to expand, not contract!

TRoc

 

[EL]

By "colors", I just mean a specific frequency.

Yes, I understood that.

A laser is not 1 photon, but a beam of many "interacting" coherent (resonant) photons.

What do you mean by interacting? Where does an interaction between photons take place in a laser?
Usually we differ between "interaction" and "interference".

 

[Claude Bile]

A laser is possible because of photon/photon resonance. An interaction capable of resonance, must also be capable of dissonance.

Your assertion that photons interact with one another in a laser is incorrect. You cannot get a laser without a lasing medium, you need atoms to facilitate the interactions that would lead to lasing. It is the atoms that posess resonances, not photons.

This is why some colors reflect certain colors, and absorb others.
TRoc

What do you mean by 'colour'. Do you mean the frequency of a photon, or the absorption spectra of a particluar substance? Objects appear to have colour because they absorb certain frequencies and reflect others, this property is entirely dependant on the characteristics of the atoms/molecules and not the photons.

Lasers are monochromatic

This is incorrect, all lasers posess a finite linewidth, some lasers (supercontinuum lasers) have linewidths that can cover a large portion of the frequency spectrum.

Another example? A quote from Encyclopedia Brittanica:

"In addition to saturation spectroscopy, there are a number of other techniques that are capable of obtaining Doppler-free spectra. An important example is two-photon spectroscopy, another form of spectroscopy that was made possible by the high intensities available with lasers. All these techniques rely on the relative Doppler shift of counterpropagating beams to identify the correct resonance frequency and have been used to measure spectra with extremely high accuracy. These techniques, however, cannot eliminate another type of Doppler shift. "

This does not support any of your arguments. Two-photon spectroscopy works essentially because of what happens to an atom when two counterpropagating photons are incident upon it at the same time. Again, this has nothing to do with photons interacting with one another, it is photons interacting with atoms.

There are certain interactions termed photon-photon interactions, however they all occur inside media, because they rely on the presence of a nonlinear polarisation. So called 'photon-photon' interactions also require the presence of an atom, so technically, it is a 'photon-photon-atom' interaction.

I am not aware of any interaction that occurs between two photons, and only two photons.

Claude.

 

[EL]

You've got my point Claude Bile...
However:

There are certain interactions termed photon-photon interactions, however they all occur inside media, because they rely on the presence of a nonlinear polarisation. So called 'photon-photon' interactions also require the presence of an atom, so technically, it is a 'photon-photon-atom' interaction.

I am not aware of any interaction that occurs between two photons, and only two photons.

Claude.

Photon-photon interaction is in fact possible without the presence of a medium. Or more correctly: Photon-photon scattering is predicted by QED, but has yet not been measured due to the small cross-section. To lowest order it is described by the Feynman diagram with two incoming real photons, a square of virtual fermions, and finally two outgoing real photons.
It is possible to via an effective-action approach rewrite this in terms of a polarization of the vacuum, giving rise to non-linear corrections to Maxwell's equations.

 

[T.Roc]

Claude,

Of course you are right, one can not talk about 2 photons without the 2 atoms they came from. I am no expert in lasers, and have only read the term "monochromatic" used to describe their frequency. Do you know by how many nanometers they can range and still function?

I'm sure that this has not been done, but what do you think would happen if 2 photons of the same phase and frequency were "fired" simultaneously through a single slit? Would you get an interference pattern?

Also, can photons experience superposition?

TRoc

 

[Claude Bile]

With regard to your first question:

Supercontiuum, short-pulse (femtosecond) lasers can have linewidths (bandwidths) in excess of 100 nm.

With regard to your second question:

If you fired two and only two photons you wouldn't get an intereference pattern, you would just get two localised events. If you sent many pairs through one pair at a time, you most certainly would get an interference pattern.

A neat experiment I did in my undergraduate coursework was a standard two slit diffraction experiment, except the interference pattern was measured using a photomultiplier, and the whole setup was shielded from any ambient light. Essentially what we did was attenuate the laser beam until the power was so low, only one photon (on average) existed in the box at anyone time, and showed that an interferance pattern is still obtained. In conclusion, an interefence pattern is obtained when you send one photon through at a time, much less two.

Claude.

 

[T.Roc]

Claude,

Thanks for the answers, do you have time for a couple more?

1.)I've read that the first 2 slit experiment was done circa 1801. If this was done with monochromatic light, how was it produced?

2.)Why isn't there yellow lasers?

TRoc

 

[Caper]

There are yellow lasers, here's a link to a website selling yellow laser pointers and how they're made

http://www.blueskymarketing.co.uk/laser_pointer/yellow_laser_pointer.htm

As for creating monochromatic light, not sure how they did it in the 1800's, but I guess one way would be to use a prism to separate the light into different wavelengths and exclude all but a narrow band.

 

[Claude Bile]

I believe the first two slit experiment was done using pinholes rather than slits and incoherent light was used.

There are some Yellow lasers, Cu vapor lasers lase at 578 nm, however they also lase strongly in the Green too, but it is possible to split the colours and obtain pure 578 nm from the laser.

Many yellow lasers being engineered these days use nonlinear optics to shift the frequency of a Nd:YAG or a Yb:YAG laser from the infrared spectrum into the visible using a combination of Raman shifting and frequency doubling.

Orange lasers have proven to be particularly difficult to engineer, because the spectrum we perceive as orange is actually quite narrow (580-595 nm), and there is no (known) convenient way of obtaining wavelengths in this region without a fancy optical setup.

Claude.

 

[BlackBaron]

1.)I've read that the first 2 slit experiment was done circa 1801. If this was done with monochromatic light, how was it produced?

I don't know how were those experiments done, but you can do the double slit experiment with light that isn't monochromatic, of course, it may not be as convenient (depending on what exactly you want), because different colors are affected in different ways, but you can still see the interference pattern.
In fact, I've actually done the experiment using a regular light bulb, I just looked at it throught a plate with two thin slits very close to each other (I couldn't do it for long, though, because the light was pretty intense).
I've also seen intereference patterns of sunlight going trought a curtain.