Classical and quantum interpretations of electromagnetic radiation

[Glenn G]

Hi,
So I can get the idea that the ac current in a radio transmitter produces radio waves of the same frequency of the ac supply, just like shaking a slinky sprung up and down but how does this translate into the radio waves as actually coming out as photons and for that matter other than knowing E = hf (so frequency is linked to photon energy is there any way of thinking about what frequency is in this context? Is it oscillations of the electric field within the photon itself. I've heard that thinking of the photon as a little wavelet i.e. Portion of a longer classical wave isn't useful)

Thanks for any guidance,
G.

 

[Dale]

You don't need quantum mechanics to understand radio waves. Classical EM is perfectly valid for that.

 

[Glenn G]

Hi Dale,
Thanks, I just wondered how to resolve what happens though through a quantum model, What does it mean to define a frequency for a photon other than just E/h does it represent anything?

 

[anorlunda]

What does it mean to define a frequency for a photon other than just E/h does it represent anything?

It does. We use the word radio for low frequencies, then microwave, infrared, visible light, and x-rays for photons of increasing frequency.

Edit: Also within the radio band, the frequency of the photons corresponds to the channel on the radio. For example, dial your radio to 90.7 Mhz or 107.9 Mhz.

 

[Glenn G]

Thanks anorlunda,
What is your understanding of the concept of frequency with photons, is it just an expression that equals E/h or do you think it actually represents the number of oscillations of an electric field within the photon per second?

 

[Dale]

Photons are eigenstates of a Fock space. As such they are not energy eigenstates. So a definite number of photons does not have a definite amount of energy nor a definite frequency. The states produced by a RF antenna are probably better approximated by a coherent state than either a Fock or an energy eigenstate.

It is all a useless exercise in unnecessarily complicating an already complicated phenomenon. Just stick with Maxwell's equations for radio waves.

 

[anorlunda]

Take @Dale 's advice. It is not helpful to think of photons with regard to radios.

Think of EM waves instead. The frequency of the wave determines the radio station. Modulation of the wave (AM or FM) determines the audio signals sent. The power of the radio station determines the strength of those waves. Trying to think of those things one photon at a time just leads you down a big hole of confusion.