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What part of the electromagnetic spectrum is not considered as light?

  1. Sep 25, 2011 #1
    I know that light is a form of electromagnetic radiation but I know that all electromagnetic radiation is not light. I know that light has four basic properties:

    1) Light travels in straight lines.
    2)Light can reflect.
    3)Light can bend.
    4)Light is a form of energy

    What I want to know is what part of the electromagnetic spectrum is not considered as light?
  2. jcsd
  3. Sep 25, 2011 #2


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    Technically, all of the spectrum is light. In terms of common usage, I would say that the ultraviolet, infrared and visible spectrums are usually referred to as light. But I only state that in terms of casual usage.
  4. Sep 25, 2011 #3
    But my optics teacher who is a Ph D in laser physics told me that all of the electromagnetic spectrum is not considered as light, are you sure about that all of the parts of electromagnetic spectrum can be refracted, reflected and travel in straight lines?
  5. Sep 25, 2011 #4


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    Sure. It's all a matter of scale. But that isn't the definition of light. Light is electromagnetic waves, which covers the entire spectrum by definition. But like I said above, especially being an optics major, in terms of casual reference people probably only call infrared to ultraviolet as being light. Outside of that they may call it radiation or waves. But the frequency doesn't change the basic physics of the wave for the most part (except for the divergence from classical electromagnetics that you are going to see as you go to higher frequencies).
  6. Sep 26, 2011 #5
    It has always been my understanding that "light" is that part of the electromagnetic spectrum that can be detected when it enters the eye. The relevant wavelenghts range from about 400 -750 nm. Electromagnetic radiation whose photons have wavelengths outside this range are therefore invisible to the eye and go by names other than "light" e.g. ultraviolet, x-rays, etc.

    The whole naming thing gets a bit confused when one talks about sunlight for instance. This term can apply to the unique mix of colors seen my the eye, or to the infrared and ultraviolet radiations that penetrate the atmosphere as well.

    One must take the use of the word light in the proper context.

    Hope this helps.
  7. Sep 26, 2011 #6


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    The term 'light' is used in practice to those e-m spectrum, which is ruled by the same optics (in practical sense) as 'visual light'. Telecom engineers using mostly 1550nm infrared, refer to it as to 'light' - and they use glass lens, silvered mirrors, Fabry-Perot filters, etc., virtually the same as in visual optics. Photolitography engineers use 200nm ultraviolet, and they still call it 'light'.
    But I can't see any more precise distinction than 'applicability of visual light optics' and 'not too-far extension of meaning of "visual light" '

    It would be rather unusual to call 1 nm X-rays or 1 mm-microwaves 'a light'.
  8. Sep 26, 2011 #7
  9. Sep 26, 2011 #8
    "Light" is any electromagnetic traveling wave, which includes the entire electromagnetic spectrum. "Visible light" is any em traveling wave of a wavelength that the human eye can detect, typically 390 to 750 nm. Aside from the wavelength, there is nothing physically different between visible light and other wavelengths. Radio waves can reflect, refract, travel, and carry energy just as well as a red laser beam if they encounter scale-equivalent objects. The confusion comes because historically, radio waves looked different at first than visible light, so the word "light" historically only meant visible light.
  10. Sep 26, 2011 #9
    Note, there are electromagnetic field configurations that are not traveling waves and therefore cannot be considered light, such as the electrostatic field created by static charges, the magnetostatic field created by static magnets or steady electric currents, or induction fields created by time-varying magnetic fields that vary slow enough to not induce a significant displacement current.
  11. Sep 26, 2011 #10
    The x-ray community does, at least in naming their light sources. "Linac Coherent Light Source", "Advanced Light Source".
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