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I am an A-level student.

We have traditionally been taught that different types of EM waves exist only between certain ranges of wavelengths and frequencies.

However, I learned that electromagnetic waves of all kinds can exist in different wavelengths, not just the ones within the range they are typically associated with. For example, X-rays do not just exist within the range of $10^{-8}$ m to $10^{-11}$ m. They can exist at any given wavelength.

What, then, differentiates one type of EM wave from another? What are the criteria for the classification of EM waves? Where does this classification prove to be useful?

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    $\begingroup$ Where did you learn that? $\endgroup$
    – Wolphram jonny
    Commented Mar 23 at 19:30
  • $\begingroup$ I was learning about the Cosmic Microwave Background (CMB) radiation and how after its accidental discovery by Penzias and Wilson when they detected strong microwave signals at 7.35 cm, it became important to measure the signals at various wavelengths to recreate its Plank profile and see if it matched. $\endgroup$
    – Haram Tanveer
    Commented Mar 23 at 19:36
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    $\begingroup$ No one calls visible light x-rays. No one calls radio waves x-rays. No one calls visible light radio waves. Etc. $\endgroup$
    – Ghoster
    Commented Mar 23 at 20:25

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X-rays...can exist at any given wavelength.

That's not what "X-ray" means. In most cases,* when people say "X-ray," they are talking about a specific range of wavelengths/photon energies: Longer wavelength/less energy than "gamma rays," but shorter wavelength/greater energy than "ultraviolet rays."

But note! There are no internationally agreed-upon numbers that define the boundaries of the X-ray spectrum.

https://en.wikipedia.org/wiki/X-ray

* I spent about ten years of my career working alongside medical physicists who had a different opinion about what "X-ray" and "gamma ray" mean. Their idea was that "X-ray" means Bremsstrahlung radiation produced by bombarding a metal target with a high energy electron beam, and "gamma ray" refers to photon radiation that is produced by natural radioactive decay.

The spectrum of the medical physicists' "X-ray" completely contains the spectrum of their "gamma rays." It includes energies as low as 10 keV (e.g., as used in dental X-ray equipment,) and energies in the low tens of MeV (e.g., as used in radiation oncology treatments.)

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As was already pointed out, there is essentially no difference between different types of EM waves except their wavelengths (or, equivalently, the energy of the photons). However, there are certain properties that photons belonging to specific wavelength ranges exhibit, such as visible light (with wavelengths between 400 to 700 nanometers) being visible, or X-rays being able to penetrate matter which is opaque to visible light. Note that these names are rather arbitrary, and as was also pointed out, sometimes different fields assign the same name to different wavelength ranges. Which brings us to microwaves: The wavelength of what are called microwaves ranges from 1mm to 1m, which goes over 3 orders of magnitude. To properly measure the Planck spectrum of the CMB, you would have to take lots of measurements over many wavelengths in this range, but all of them are still microwaves.

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