Energy of Photons: An Exploration

[Joe_Limon]

I was wondering about the amount of energy in Photons. Do they all have the same amount of energy? Or do they have a base amount which can increase depending on frequency or some other parameter? Also, I have read that photon photon collisions can yield positrons and electrons, both positrons and electrons have mass and thus energy via e=mc^2so does this interaction conserve energy?

 

[mfb]

Do they all have the same amount of energy?

Their energy is proportional to their frequency: E=hf. There is no "base amount".

Also, I have read that photon photon collisions can yield positrons and electrons, both positrons and electrons have mass and thus energy via e=mc^2so does this interaction conserve energy?

Every interaction conserves energy. The energy of the photons is converted to energy of electrons and positrons - the electron and positron masses are a part of this energy.

 

[Joe_Limon]

Ok, so does that mean there is a minimum frequency for the electron/positron conversion? Also, if you decrease the frequency of a photon to near zero can we still detect it? And do photons exert radiation pressure proportional to their frequency?

 

[e.bar.goum]

Ok, so does that mean there is a minimum frequency for the electron/positron conversion? Also, if you decrease the frequency of a photon to near zero can we still detect it? And do photons exert radiation pressure proportional to their frequency?

1. Yes. 1022 keV, which gives an electron/pair each with rest mass energy 511 keV. Further, to satisfy momentum conservation, this has to occur near a nucleus, so it takes some recoil.
2. Depends on how sensitive your detection system is.
3. Yes, photon momentum is given by p=hf/c

 

[mfb]

1. Yes. 1022 keV, which gives an electron/pair each with rest mass energy 511 keV. Further, to satisfy momentum conservation, this has to occur near a nucleus, so it takes some recoil.

Joe was asking about photon-photon collisions. Those are rare, but they don't need a nucleus.

Also, if you decrease the frequency of a photon to near zero can we still detect it?

At some point a description via fields becomes more useful. We can detect fields that are changing extremely slowly, and even static fields. Single-photon detection at very low energies is extremely problematic.

 

[e.bar.goum]

Joe was asking about photon-photon collisions. Those are rare, but they don't need a nucleus.
At some point a description via fields becomes more useful. We can detect fields that are changing extremely slowly, and even static fields. Single-photon detection at very low energies is extremely problematic.

That'll teach me to read the thread properly.

 

[Joe_Limon]

Joe was asking about photon-photon collisions. Those are rare, but they don't need a nucleus.
At some point a description via fields becomes more useful. We can detect fields that are changing extremely slowly, and even static fields. Single-photon detection at very low energies is extremely problematic.

I would imagine low energy detection would be problematic. Static fields is a concept I haven't heard of before, how do we sense them/create them?

 

[mfb]

With a compass, for example. Or with a voltmeter for electric fields.

 

[Joe_Limon]

With a compass, for example. Or with a voltmeter for electric fields.

Ok, separately that makes sense, but together... are there static electromagnetic fields?

 

[mfb]

A battery in a magnetic field?

 

[Joe_Limon]

A battery in a magnetic field?

Hmmm ok. Is it possible to induce photons into existence in a vacuum with stationary or oscillating electric and magnetic fields?

 

[rootone]

A flourescent light tube does that I think.
Well not a vacuum but a gas at very low density.

 

[Drakkith]

A flourescent light tube does that I think.
Well not a vacuum but a gas at very low density.

Indeed. The photons are created entirely by the gas particles, not the vacuum itself.

 

[nikkkom]

Hmmm ok. Is it possible to induce photons into existence in a vacuum with stationary or oscillating electric and magnetic fields?

Variable electromagnetic field *is* light (photons). If you wave a magnet, it emits photons. Very low frequency, low energy radio waves, but still.

Not very strong static electromagnetic field in a vacuum does not produce light.

Ultra-strong static electromagnetic field can produce electron-positron pairs, even in vacuum, and accelerate them, which will make their fields non-static (they are accelerating) and thus emit photons.

 

[Joe_Limon]

That is really cool. If you oscillated a single magnetically charged modlecule could you essentially create a low power laser which you could adjust the frequency?

 

[nikkkom]

That is really cool. If you oscillated a single magnetically charged modlecule could you essentially create a low power laser which you could adjust the frequency?

Ordinary radio transmitters do something similar - they emit their photons by moving charged particles (electrons) back and forth with the desired frequency.

Laser is a device which emits great numbers of IR, visible, or UV photons with the same phase and polarization. (Radio-wave "laser" is called "maser").

 

[Joe_Limon]

If vibrated in a fixed direction could you make a directional maser/laser without the use of optics?

 

[mfb]

That's exactly what antennas do.

Emission happens mainly orthogonal to the line of motion, however, to focus them you need a parabolic mirror or something similar.

 

[Joe_Limon]

That's exactly what antennas do.

Emission happens mainly orthogonal to the line of motion, however, to focus them you need a parabolic mirror or something similar.

Hmm ok, that changes my understanding substantially, I always assumed the photons released by radio transmission was due to dropping electron orbitals. Thanks!

 

[mfb]

I always assumed the photons released by radio transmission was due to dropping electron orbitals

That process is more typical for the emission of visible light (and some infrared and UV).

 

[Drakkith]

Hmm ok, that changes my understanding substantially, I always assumed the photons released by radio transmission was due to dropping electron orbitals. Thanks!

Nope. They are due to the back and forth acceleration of the electrons in the metal.