No, you can generate "light" with any frequency. The distinction between photons and waves is a more technical nature. Below about 10 GHz, EM waves are usually produced with feedback circuits and detected with rectifiers. In this area, you can measure amplitude and frequency of "wave trains". Until now, nobody could detect photons in this area.

Above 1000 GHz, you can *not* measure frequency or amplitude, but only the existance of energy packets, called "photons". Both regions do not overlap (Terahertz gap)

Your question is not clear. Need more information. However, let me give it a try. There is a process called pair annihilation which is the inverse of pair production and occurs when a positron is near an electron and the two come together under the influence of their opposite electric charges. Both particles vanish simultaneously, with the lost mass becoming energy in the form of two gamma-ray photons. The total mass of the positron and electron is equivalent to 1.02 MeV, and each photon has an energy of 0.51 MeV plus half the kinetic energy of the particles relative to their center of mass. The corresponding maximum photon wavelength is 1.2 pm. Electromagnetic waves with such wavelengths are called gamma rays.

So, the appearance of photons in some sense can be termed magical!

Dear Suhas.

As I understand this is debatable. It is evident that a wave cannot be larger than the universe itself; also it cannot be smaller than the planck scale (10^-35 m). So, this could be seen as the theoretical and observational bounds of a wavelength. But if you think space-time is a continuum, then there isn't any lower bound on the wavelength, and thus higher bound on the frequency of an EMW. However, there are measurement issues and uncertainty when you are dealing with extremely small length scales and very high frequencies. Recently, LIGO measured a distance of 10^-3 times of a proton's size, so one can assume that EMWs of that wavelength size should be measurable.

Dear Vikash Pandey,

Since below 10 GHz only EM waves exist (EM waves exist at f>0) and no photons exist as said by Sir Herbert Weidner, there must be a certain frequency at which photons magically appear, right?!

Let's avoid the term "Photon" and to talk of EM waves. These can be produced with any frequency: From ELF 80 Hz to far beyond the X-ray range. The minimum energy hf always corresponds to a wave packet of finite length (infinitely long wave trains possess infinite power). Nothing is limited to a single frequency. If lambda

Sir Herbert Weidner,

As you have mentioned above, at > 30 THz scientists only consider the particle nature of light and forgets its wave nature. While at < 10GHz, wave nature of light is only considered.  

My question: At what frequency does this particle nature of light begin?

In nature, most individual electromagnetic photons are emitted when electrons are captured by ionized atoms. For example, when an electron is captured by a proton to form a hydrogen atom, an  photon of energy 13.6 eV is emitted.

So that you can relate to all measurements units that this involves, here they are:

In joules, 13.6 eV is 2.178959988E-18 j

wavelength 9.11648E-08 m  (in nanometer, in the near uv range)

wavelength 3.288E15 Hz

This electron is captured on the rest state orbital about the proton, which is a locally fluctuating electromagnetic equilibrium least action state where it will stabilize permanently unless energy is provided to it to escape from this state.

If an amount of 13.6 eV or more of energy is communicated to it, it will permanently escape this proton, to eventually be captured by some other atom.

If less than 13.6 eV is provided, it will jump away from the proton to some authorized metastable orbital about the proton, from which it will almost instantly jump back to the rest orbital, emitting in the process an electromagnetic photon of the exact amount that was required to make it jump to the metastable orbital. All visible light and near lower and higher frequencies are generated in this manner.

This illustrates how photons are emitted whenever an electron is stopped in its motion . Look for the term bremmsstrahlung, you will find complete explanation of the process.

Electromagnetic photons can also be produced by forcing electrons to oscillate locally at any frequency. This is how radio waves and microwaves are generated in practical applications. Each time that the electrons that are made to oscillate come to the end of their run at each oscillation, an electromagnetic photon is emitted in synk with the oscillating frequency. Each electron generates one electromagnetic photon at each cycle. That's how coherent emission of electromagnetic photons is controled from the 10 GHz downwards that Herbert mentioned and even in the THz gap.

I specify "electromagnetic photon", because "virtual photons" are also often mentioned in physics and both must not be confused. Virtual photons have nothing to do with the electromagnetic spectrum.

Gamma photons can also be emitted by forcing electrons to oscillate like in FEL lasers or natural processes such as positronium decay that Vikash mentioned, or natural or controled high energy collisions between particles or natural activity at the atomic nuclei level.

@ Suhas

Your question: "At what frequency does this particle nature of light begin?"

Only if I have understood your question correctly, it is not the frequency which decides the "particle" nature of light, but rather your experimental set up! 

As there is none precise size scale to distinguish between the classical physics and quantum physics, same applies to wave-particle duality. Better see it as a painting which has black and white colors at either end and then there is a gradient of color scheme across the picture. At one end, black (classical, particle nature) dominates; at the other end it is white (quantum, wave nature). And in between the ends..hard to say..which is white and which is black..  just one color dominates over the other by varying amount!

Every physicist is convinced of the unity of nature. May be, at each frequency of the EM region, there are EM waves and photons. But how to be sure, since the detection devices are frequency dependent? When lambda 0.01 m, nobody could ever measure photons. It seems pointless, yet to speak of photons.

Problems arise, however, when some say that photons are always punctual. Even at wavelengths of about 100 m? In my opinion, this is pure fantasy and without any experimental evidence, because never photons were detected. Not even at wavelengths of light.

@Suhash: At what frequency does this particle nature of light begin?

The particle image begins exactly where you can *not* measure frequencies. Strange!?

Hi Herbert

Photons have been detected. I refer you to Einstein's photoelectric proof, which confirmed the particle nature of even visible light photons besides confirming that they have longitudinal inertia.

I understand your perception that radio wave and microwave behave like "waves" at our macroscopic level, because they do, and that all calculations made with wave behavior as an assumption are totally precise. This does not invalidate that when countless electrons in an emitter are made to oscillate synchroneously, that they send "waves" of identical photons moving at the speed of light and having the same frequency, waves that can then be mesured with our macroscopic instruments.

Note that I am not here trying to convince you here. I simply invite you to maybe dig more into the pool of experimental evidence that has been gathered over time about elementary particles if this issue intrigues you.

Suhas is totally free to make his own investigations and come to his own conclusions.

@André: Einstein calculated with energy units, not with particles named "Photon". That name was created 20 years later later by Lewis. And the longitudinal inertia (Radiation pressure) of light was calculated by Maxwell, 40 years before Einstein. The crucial proof of the particle-like nature of light is the compton effect at very high energies. What speaks against photons? It's the size. If they are really point-like, why can some pass through a waveguide with an inner diameter of 20 cm or more, others not? What prevents uncharged elementary particles to fly through a large pipe?