# Photon Energy

1. Sep 20, 2015

### 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?

2. Sep 20, 2015

### Staff: Mentor

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

3. Sep 20, 2015

### 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?

4. Sep 20, 2015

### e.bar.goum

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

5. Sep 20, 2015

### Staff: Mentor

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.

6. Sep 20, 2015

### e.bar.goum

7. Sep 20, 2015

### Joe_Limon

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?

8. Sep 20, 2015

### Staff: Mentor

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

9. Sep 20, 2015

### Joe_Limon

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

10. Sep 20, 2015

### Staff: Mentor

A battery in a magnetic field?

11. Sep 20, 2015

### Joe_Limon

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

12. Sep 20, 2015

### rootone

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

13. Sep 21, 2015

### Staff: Mentor

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

14. Sep 21, 2015

### nikkkom

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.

15. Sep 21, 2015

### 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?

16. Sep 21, 2015

### nikkkom

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").

17. Sep 21, 2015

### Joe_Limon

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

18. Sep 21, 2015

### Staff: Mentor

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.

19. Sep 21, 2015

### Joe_Limon

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

20. Sep 21, 2015

### Staff: Mentor

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

21. Sep 21, 2015

### Staff: Mentor

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