One Finite Planet

One Finite Planet

Photons: What is electromagnetic radiation?

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Radio waves, microwaves, millimetre waves, infra-red, visible light, ultraviolet, x-rays, gamma rays…
…… can seem so different, yet they are all electromagnetic radiation.

All are composed of the same fundamental particle: the ‘photon’.

All photons, travel at the ‘speed of light’ (or the ‘speed of photons?’), have the properties of a wave, and interact with electrons.

Photons can be absorbed by an electron, and when this happens, the electron takes on additional energy, and with sufficient additional energy electrons move to a higher energy state. Electrons can then release energy and drop back to a lower energy state, emitting a new photon when they do.

This cycle of being absorbed by electrons, and emitted by electrons, is the main interaction between photons and other things. It is the basis of how we see, how heat is radiated, how radio signals are transmitted and received, and how microwave ovens work. There are countless photons everywhere, being absorbed, and being re-emitted.

Every ‘photon’ has carries a specific amount of energy, which determines the ‘frequency’ of that photon. The total extra energy delivered to an object by a stream of photons is the energy per photon multiplied by the number of photons. Any object with energy will also be radiating some energy out by emitting photons. So an object which is otherwise stable, will either gaining energy, which will most often indicated by an increase in heat, or lose energy, typically be getting cooler, depending on whether there is more energy in the photons being absorbed, or more energy in the photons being emitted.

In considering energy arriving from absorbed photons, a useful analogy to consider bullets being fired at a wall. You can deliver the same total force to the wall by either doubling the number of identical bullets, or doubling the mass for the same number of bullets. If the bullets are so small they just bounce of the surface of the wall, then increasing the number of bullets will still result in all the bullets just bouncing off, even though the total push against the wall has doubled. Similarly, while doubling the number of identical photons will increase the energy or heat delivered, if those photons do not do enough to the electrons to change the structure of the molecules, receiving more photons still will not change the structure. This is why the frequency of the photons is so important. It is not just the total energy that determines the impact, it is also the power of the individual photons.

Note that 1Mhz is 106 Hz, 1Ghz = 109 Hz, 100Ghz – 1011 Hz etc.

Generally, individual photons with an energy much higher than visible light are considered ‘ionising photons‘. While photons below this energy level can make us feel warm, photons above this energy level can damage our cells. We definitely know that higher energy UV, or X-Rays or Gamma rays can cause cancer. We aware of no side effects visible light photons at the level we experience on a sunny day, and generally it seems that this same number of photons, or less, at even lower frequencies should pose even less risk. However, there is always the risk of something we do not know. In terms of risk, the higher the frequency the photons, the more care warranted in ensuring the quantity is not too high.


  • *2023-06-16 Fixed incomplete paragraph.
  • 2020-09-01 First “seed”

Table of Contents


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