In the same way as the origin of X rays is the excitation of electrons, what is the origin of radio and infrared radiations in this respect?

• Infrared : natural thermal radiation of 'black bodies' , astronomical redshifted light, stimulated emission in lasers ... Surf on wiki else the answer is very long – user46925 Dec 18 '15 at 3:44
• You are thinking right. Even radio waves are made by electrons. Their wavelengths are between IR and X-rays. So radio waves are modulated radiations of zillion of photons.To get a specific frequency one need a generator and an antenna rid of specific length for effectivenest radiation with minimum energy loses. physics.stackexchange.com/q/135089 – HolgerFiedler Dec 18 '15 at 6:32
• – Urgje Dec 18 '15 at 10:25

They can be caused by the same principles as the X-rays, since the differences between them are just the frequency. When you have moving charges, they create a magnetic field that is propagating outwards at the speed of light. But if you accelerate them, the magnetic field changes. And changing magnetic fields create an electric field. Bassicaly you get light or electromagnetic waves.

The faster you are changing the speed of electrons, the bigger the frequency of that "light". Infrared frequency is light you would get if you had an AC current in a wire with the same frequency (roughly 10 to 14th). Obviously we can't reach those frequencies on a large scale, but we can do it on atomic levels where the speeds of electrons are much MUCH greater. If we would keep increasing the frequency we would eventualy reach the spectrum of visible light and you could see the wire emmiting light. Go even higher and you reach the X-rays. These are practicaly not achievable by these means, but if you heat up the matterial to let's say 5000 K, it Will glow red, because the speeds at which the atoms inside it are moving back and forth are on the order of that frequency of red light. Hope this explains sufficiently how light is created. Radiowaves are nothing different, they just have lower frequency.

Indeed, Xrays and infrared are just photons, of different energy. However, since they correspond to different frequencies and thus to different energies, they can be linked to other characterictic phenomena. For example while Xrays are commonly linked to their occurence in electronic spectra (the "Characteristic Xrays" mentioned in the question), IR radiation is commonly linked to lower energy phenomena like radiation of a black body at room temperature (cfr. Planck's law of radiation!). Even lower in energy is microwave radiation, which is famous for being produced 'by the universe itself' as cosmological background radiation. After microwaves, we have the radiowaves but these are so low in energy that I cannot come up with a 'natural' phenomenon in which they are produced! They are important in a lot of practical applications, like MRI. Extremely low energy phenomena, in any case.

• Yes, but what is the physical mechanism the produces radio waves? – garyp Dec 18 '15 at 3:28
• @garyp: The physical mechanism is electrons being accelerated, i.e. changing velocity. When charges move, there is a corresponding magnetic field. When their motion changes, the magnetic field changes. And the effect works in reverse. That starts a wave. It's called electro-magnetism. – Mike Dunlavey Oct 18 '16 at 17:31

There are a number of ways of generating radio waves.

The simple acceleration of electrons in a transmitting dipole will generate radio waves. I guess as you might have been using wi-fi to write your question, you knew this and are more concerned with "natural" sources of radio waves?

Basically, electromagnetic radiation of all wavelengths can be produced by bound-bound, free-bound or free-free transitions of electrons that may be bound in atoms or molecules.

Where bound electrons are involved, then it is typically the spacings between molecular rotational and vibrational levels that are responsible for infrared emission. Longer wavelength bound-bound transitions can occur between hyperfine transitions with small energy differences. e.g. The famous 21cm radiation comes from a hyperfine transition in hydrogen atoms.

The are also a number of mechanisms involving the acceleration of unbound electrons that produce continuum emission at long wavelengths. The most commonly occurring are bremsstrahlung -the acceleration of electrons in the electric fields of ions - which can produce all wavelengths down to a short wavelength roll-off that depends on the gas temperature; and synchrotron continua that are produced by electrons spiralling at relativistic velocities in magnetic fields.

Infrared and radio radiation has the same origin as X rays. When a photon 'crashes' with an electron, that electron gains the energy from the photon (if the photon has high frequency it gains a lot of energy, if it has low frequency it gains little energy). Then, the electron goes to a more energetic energy level, farther away from the nucleus. Then, the electron sends off a photon and goes back to the original energy level. That applies to all kinds of radiation. X rays, infrared and radio radiation are all photons, of different frequencies.

• Really? What molecule or atom has an electronic energy level 100 MHz from the ground state? – garyp Dec 18 '15 at 3:27
• Why this is not right teached today? Radio waves are modulated radiation. The wave generator switches on and off the radiation. The radiation by itself is from photons from accelerated electrons in the antenna rod – HolgerFiedler Dec 18 '15 at 6:55
• @HolgerFiedler So the question becomes how does quantum mechanics address the question. QM does it by defining a current operator, which in the case of electrons in a metal involves coupling between very closely spaced energy levels in the conduction band. Then adding coupling to the EM field modes. The state of the EM field that results from a sinusoidal drive is a quantum mechanical coherent state. So the quantum mechanical analysis can't really be covered in an elementary fashion (as far as I know). – garyp Dec 18 '15 at 16:48
• @garyp It's not acceptable how QM, which has successes (more or less) for inner atomic processes, was applied to macroscopic phenomena. It's obvious that electrons responsible for EM radiation and that the frequency of their photon emission in antenna rod has nothing to do with the wave length of the modulated radio radiation. BTW I could not stick your comment to my answer. Perhaps my English knowledge is not enough. – HolgerFiedler Dec 18 '15 at 17:49

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