What distinguishes the different types of light from each other?


Are the different types of electromagnetic radiation just based on arbitrary divisions or is there specific properties each type has within their respective wavelength range?

In: Physics

Electromagnetic radiation vary in wavelenght/frequency. These waves have different properties depending on their wavelenght, but exactly how we divided them into category is mostly arbitrary. Technically speaking there is some objective aspect that shaped how we divided them, like visible light is what human can see, but even that have some variation in it depending on the person.

The biggest solid divisions of EM radiation are A) what the average human can and cannot see with the naked eye, and B) what can and cannot cause ionization of atoms. frequencies of EM radiation above a certain threshold are energetic enough to knock away electrons from molecules, and this can cause weird stuff to happen chemically. This is bad for things that, like us, are biology, because biology relies on *very specific* chemistry in order to live, and randomly ionizing a molecule can cause that chemistry to go awry in spectacular (read:awful) ways.

The opposite of this is also true to some extent though: sometimes the radiation can hit a certain molecule in *just* such a way that it causes a beneficial mutation in DNA that is inheritable and ultimately drives evolution, but 99% of the time you just get cancer and die.

If you look at just EM radiation from a physics point of view, it’s a spectrum – increasing frequencies / wavelengths of photons.

However, if you look at how EM radiation interacts with materials, you can start having categories, because interaction with materials is based on what’s absorbed by the atoms, what’s absorbed into molecules (and the intra-molecular bonds between the atoms in the molecule), and what’s absorbed or reflected or refracted by the spacing and the crystal (or pattern) structure of how the atoms are placed. Some effects are particle effects (absorption, including partial absorption like color effects, and transparency), others are wave effects (reflection, refraction, diffusion, etc.). Metals interact with radio and microwave frequencies, but not with visible light frequencies, etc. etc.

They do have some characteristic properties. The lines where one becomes other are
somewhat vague.

Gamma: Very high energy, very short wavelength. Ionizing radiation (=enough energy to strip electrons from atoms). Its creation usually involves nuclear reactions (radioactive decay, nuclear fusion, nuclear fission, annihilation). Goes through matter easily.

X-ray: High energy. Ionizing. Lower energy than gamma.
Gamma and x-ray are very similar. Under some definitions overlaps with gamma with the separation defining by the process though which they they are formed (x-rays coming from accelerated particles, for example in particle accelerators, while gamma comes from nuclear things). Goes through matter easily.

Ultraviolet: Quite high energy but low enough that something smaller than particle accelerator can make it (for example really hot things like welding arcs and our Sun or exited atoms). Ionizing. Not visible. Gets absorbed by matter so it doesn’t go through so easily.

Visible light: Human eyes can see it. Not ionizing. Does not penetrate matter easily.

Infrared: Not visible. warm and hot objects generate lots of this and it is absorbed by matter (causing it to heat). Does not penetrate matter easily.

Radio: Longer wavelength than infrared. Hot objects do not make much of radio waves. Goes through matter easily.

Microwaves: Radiowaves that have wavelength in the micrometer range.