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Question about the Electromagnetic Spectrum

  1. May 28, 2012 #1
    As I understand it, there are these categories:

    infrared light
    visible light
    ultraviolet light
    gamma rays

    It seems that microwaves are really just radio waves that happen to have a very short wavelength, with the distinction that radio & microwaves are waves that can be directly produced by an electrical circuit. Is there any reason that physicists & engineers just decided that radio radiation any higher frequency than some arbitrary level would be known as microwaves?

    Then there is the light radiation (infrared, visible, ultraviolet). It would seem that the frequency is too high to be directly produced by an electrical circuit, so it is not considered to be a radio wave - an just happens to correspond to the radiation that stars and planets produce. Is this why it is called "light"? Or is there some other reason?

    Then there are the X-rays and gamma rays. I understand that gamma rays are generated by atomic nuclei (i.e., in a spontaneous release of energy, or from a fission or fusion), but what about X-rays? It seems that X-rays are generated by ionized electrons, although maybe this is simply due to the fact that ionized electrons have a super high temperature, and therefore X-rays are just the regular thermal radiation for that temperature.

    I guess my question is why is there a categorical limit point between microwaves and infrared light, and also between ultraviolet light and X-rays? I can understand why gamma radiation is in a class by itself, and other than the obvious categorization of light into visible, and less or greater than frequency than visible, I don't quite understand the categorizations.
  2. jcsd
  3. May 29, 2012 #2


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    It's ALL electromagnetic radiation. It's all the same phenomenon, the only property that is varying across the spectrum is the frequency (or, equivalently: the wavelength) of the radiation. ALL of these names/categories are somewhat arbitrary and historical. There is no "rule" as far as I am aware, that says that radio waves are specifically used to describe the portion of the spectrum that can be produced by electric circuits. In fact, electric circuits produce infrared radiation, and so do you and me, any anything else that is at room temperature. Some circuits even do produce visible light.

    Similarly, there is no rule that says that only visible, IR, and UV can be called "light." There is no particular reason why these are often called light and the others are not. Basically there are two ways in which the word "light" is used. The first usage is to use the word light to refer to specifically the visible portion of the EM spectrum. The second usage of the word "light" is as a synonym for EM radiation of any type (not necessarily visible). In this second usage, anything that consists of photons is light. So you could hear people talking about X-ray light or UV light etc. In astronomy I hear people talking about microwave light and radio light. It takes less time to say than "microwave radiation."

    Saying that "microwaves are just high frequency radio waves" is like saying that "UV waves are just high frequency light waves." Uh, yeah, that's true by definition. What's your point? Again, the divisions in the spectrum are all arbitrary/historical.

    That doesn't mean that the divisions are not *useful*. Because of the wide range in frequencies across the spectrum, the character of the EM radiation seems to change (especially in terms of the way the radiation interacts with matter). For instance, at the high end of the spectrum, (gamma ray, X-ray, and maybe far-UV), the radiation has the unique characteristic of being ionizing radiation, which makes it very dangerous. Furthermore, the discrete/quantized nature of the radiation is much more apparent at these wavelengths than it is at longer wavelengths. The reason is because a single photon "packs more punch" i.e. each photon on its own has enough energy to have some sort of measurable effect on matter. In contrast, at longer wavelengths, a single photon on its own isn't really that noticeable, and it's only an ensemble of a tremendous number of photons (which basically just manifests itself as a classical EM wave) that has a measurable effect. That's probably the reason why you rarely hear of someone talking about "radio photons," even though to do so would be perfectly legitimate and correct.

    It's obvious what makes visible light distinct (and why it is in a category of its own) from a human perspective. Visible and near IR are really not all that different. Infrared radiation is associated with heat, because everything that has a temperature will glow (i.e. will emit EM radiation), and the shape of the spectrum depends on the temperature. The higher the temperature, the shorter the wavelength at which the emission peaks. So, at room sort of temperature, these so called "thermal radiation" spectra tend to peak at infrared wavelengths. Infrared light is also efficiently absorbed by certain types of matter, because it can excite vibrational and rotational motions in the molecules of that matter. THat makes it effective at heating up the matter. (The same thing is true of microwaves and water).

    Radio waves and microwaves as you've pointed out are easily produced, sensed, and manipulated using technology, which makes them useful as a medium for the transmission of information. HOwever, this is increasingly true of visible and IR light as well.

    So in conclusion, the distinctions are historical, stemming from the fact that these different parts of the EM spectrum were discovered at different times by different people (and as you pointed out, were produced using different techniques). However, the distinctions are useful, because although these are all fundamentally the same type of radiation, the different types appear qualitatively different in character as a result of their different wavelengths. The categories are meant to reflect these differences.
    Last edited: May 29, 2012
  4. May 29, 2012 #3
    These are just loose, useful categories with no deep meaning. It's mostly a historical accident.
    If you want precision you can always use specific numbers.
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