What is the longest and what is the shortest wavelength of light? How fast would a light-emitting object have to recede in order to shift the frequency of light to increase the wavelength from the shortest to the longest? (If there are limits)

  • 4
    $\begingroup$ There are no limits. $\endgroup$
    – ProfRob
    May 20 '15 at 16:59
  • 5
    $\begingroup$ Other than perhaps limiting to wavelengths between a Planck length and the size of the observable universe. $\endgroup$
    – ProfRob
    May 20 '15 at 20:13
  • $\begingroup$ Unless you mean visible light, electromagnetic waves of any frequency could be considered "light". And, if an object is traveling sufficiently close to the speed of light (compared to a given observer), "red shift" (or blue shift) could change pretty much any frequency to any other frequency, except possibly within the limits @RobJeffries notes. $\endgroup$
    – user21
    May 21 '15 at 14:16
  • $\begingroup$ The context of the question is astronomy. Stars don't emit EM waves in just any frequency and redshift is limited by the age of the universe. There must be a better answer than this. $\endgroup$
    – agtoever
    May 23 '15 at 7:24
  • $\begingroup$ @agtoever I agree $\endgroup$
    – Astrony
    May 23 '15 at 11:59

Regarding visible light, the nominal range is usually given as 400 nm (violet) to 700 nm (red). For light of the former wavelength to get Doppler-shifted to the latter means a z value of (700–400)/400 = 0.75, corresponding to a recession of 152,000 km/s, just over half the speed of light.

  • $\begingroup$ And ultra violet light get shifted into the visible light: 1225 nm -> 700 nm. $\endgroup$
    – LDC3
    May 24 '15 at 5:38
  • $\begingroup$ Thank you, but my question regarded the entire electro magnetic spectrum $\endgroup$
    – Astrony
    May 24 '15 at 8:21
  • $\begingroup$ Astrony, no, your question as it is currently written does not. You need to edit it if you want to ask this - currently you are asking about light, and Odysseus has answered exactly. $\endgroup$
    – Rory Alsop
    May 26 '15 at 9:12

Well, I don't think this question is entirely answerable. The true answer, I think, is that there really are no limits, as Rob Jeffries commented.

However, using Wikipedia as my only source, the crab pulsar holds the current record for most energetic gamma ray emissions at 80 TeV (wavelength of about $1.5\times10^{-11}$nm).

Whereas the longest detected wavelength would surely only be limited by our detector sensitivity. Any imaginable wavelength could be lengthened further by an arbitrary factor via, for example, scattering.

EDIT: Source: Gamma-ray Astronomy (Wikipedia)


Someone mentioned redshift. Note that the cosmic background radiation limits redshift we can detect now but there is also a much older neutrino background radiation even though we can't do science on it (at least not yet).

10^20 electron volts (approximately) is the most powerful cosmic ray ever detected. By an order of magnitude or two I think.

I once saw an electromagnetic spectrum poster that had DC electricity at 0 Hz.

  • $\begingroup$ I'm not well-versed in these things, so please correct me if I'm wrong, but I think the cosmic ray energy you quoted isn't from electromagnetic radiation? Rather it's the kinetic energy of the traveling particle (which isn't a photon)? $\endgroup$
    – user181339
    May 26 '15 at 19:44
  • $\begingroup$ Ah well, I misread cosmic sources for cosmic rays because I didn't read it thoroughly. Yes, that was probably a proton. I too do not know enough about modern physics to be sure if the passing electromagnetic field of the Oh-My-God particle* didn't discombobulate with it to produce a cromulent electromagnetic wave. *This is the correct term for it. See the title of the Wikipedia article. $\endgroup$
    – user6784
    May 26 '15 at 20:38

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