From Wikipedia I read that non-ionizing radiation "does not carry enough energy per quantum (photon energy) to ionize atoms or molecules". https://en.wikipedia.org/wiki/Non-ionizing_radiation#Health_risks

From this I understand that a single photon from said non-ionizing radiation does not have enough energy to cause ionization (an electron to be removed from an atom). Isn't it possible for multiple photons (originating from the radiation source which does not have enough energy per single photon to ionize an atom with the energy of a single photon) to hit an atom (to be more specific to hit a single electron) in such short succession that the combined energy of all those photons causes ionization (when 'combined energy of photons' is greater than the ionization energy of said atom)?

According to this forum post, I quote:

Photons don't have amplitude. All photons of equal frequency have the same energy. Electromagnetic waves with different amplitudes basically consist of different numbers of photons, which is why their energies can depend on amplitude.

So if I understand correctly a wave with the same wavelength and different amplitude has more photons per second being emitted? If so, if we take for example a microwave and increase the amplitude infinitely the probability of enough photons origination from the microwave's radiation source hitting an electron enough times per second in order to ionize it would increase with the amplitude. Would it still (when the number of photons emitted per second increased toward infinity) not be possible for this radiation to cause ionization?

For a real world comparison: let's say the atom being hit with the radiation is a pinata and the photons of the radiation are children hitting the pinata. If a single child doesn't have enough force/energy to destroy the pinata in 1 hit, then the pinata could still be destroyed by a few of the children each hitting the pinata once. If increasing the amplitude translates to more children hitting the pinata in total, wouldn't increasing the amplitude of the microwave radiation result in a higher probability of ionization and eventually result in ionization from this microwave radiation?

I'm clearly misunderstanding some of the core concepts. What am I missing / what about my comparison is wrong?

  • $\begingroup$ @PM2Ring thanks for pointing it out. Should be fixed now $\endgroup$ Commented Jan 20, 2020 at 6:55
  • $\begingroup$ That's a very interesting question! A very similar question appears in the discussion of the photoelectric effect: a beam of low frequency light doesn't have enough energy to create free electrons, regardless of intensity. This question illuminates one reason why quantum theory was necessary. $\endgroup$
    – DK2AX
    Commented Jan 20, 2020 at 15:09

2 Answers 2


Isn't it possible for multiple photons ... to hit an atom

This is possible but has negligible probability. Furthermore, it would have to happen within an extremely brief time window, since after the first hit, the atom would deexcite.

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    $\begingroup$ If I uderstand correctly: So it is theoretically possible but very very unlikely for non-ionizing radiation to cause ionization even when the amplitude is very very high. Could you perhaps give an indication (or a way to estimate such a number) of how high the amplitude of such non-ionizing radiation would have to be in order to have any probability of causing ionization? Thank you! $\endgroup$ Commented Jan 19, 2020 at 21:58
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    $\begingroup$ @Maarten-Monicaforpresident you should consider two numbers: the energy needed for ionization of an atom (typically ~6 volts), and the size of the atom (~2 Angstrom). If the electric field of the non-ionizing radiation reaches levels on the order of 6Volts/2Angstrom = 300 MegaVolts/Centimeter, then you have a decent probability of ionization. $\endgroup$
    – KF Gauss
    Commented Jan 20, 2020 at 16:50

In the general environment of the Earth / where people live generally the density of photons is not high enough to have multiple photons interact with an atom or molecule near-simultaneously and cause an effect (i.e. ionization). However, there are some well known scientific techniques that do rely on multiple photon interactions for example: https://en.m.wikipedia.org/wiki/Two-photon_excitation_microscopy

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    $\begingroup$ I wonder how much power would be required? i.e. all those photons streaming onto the material presumably will have a thermal heating effect? If the material gets hot enough, it's going to vaporize and form a plasma, just from the heat. I wonder whether there is some situation in which we can fire enough non-ionizing photons onto something to cause ionization, without the shear power of the beam vaporizing/ionizing the material first? $\endgroup$ Commented Jun 9 at 18:09
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    $\begingroup$ That's a good point but yes it is possible to do this without overheating / vaporizing. One thing to note - it's usually not a continuous beam but a pulse, where you might have 1000 pulses per second, but each pulse is ~200 femtoseconds (200 x10^-15 s) in width. That gives plenty of time to dissipate the heat. $\endgroup$
    – dllahr
    Commented Jun 10 at 19:49
  • $\begingroup$ Sure. But just because the pulses are short does not mean that the local heating will be small. If you dump a lot of power in a short space of time, that could still give massive localized heating I feel? $\endgroup$ Commented Jun 11 at 7:51
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    $\begingroup$ I guess however that you're right that the shorter the pulse, the more likelihood that two photons will both strike the same atom, before the excitation from the first strike has decayed? $\endgroup$ Commented Jun 11 at 7:53
  • $\begingroup$ It's good to be careful with the language here, because it is two possibilities: (1) two photons interact with an atom simultaneously (2) one photon interacts with the atom and then before the atom decays from its excited state a second photon interacts. Also to clarify, this isn't a theoretical exercise - multiphoton spectroscopy has been around for more than 30 years $\endgroup$
    – dllahr
    Commented Jun 12 at 11:10

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