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Many light sources like LEDs and lasers only emit a single wavelength of light.

Is there a light source that emits all wavelengths of visible light at the same time?

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After some discussion with Robert Cartaino, I'm reopening the question, since the community vote takes precedence. The discussion about whether it should end up closed or not can continue here:… – David Z Jan 13 '11 at 1:21
I also removed the excessive comments which are not relevant to this particular question. – Robert Cartaino Jan 13 '11 at 1:32

3 Answers 3

Blackbody radiation for a white hot object emits the spectrum from infrared to ultraviolet.


Graphite is a decent approximation of a blackbody radiator. So graphite heated to white hot will emit the full spectrum of visible light.

Note however, that the spectrum will not be flat. There will be more energy on the ultraviolet end of the scale. See the curves in the graph on the upper right of the Wiki page for more information.

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This is not coherent though! – Noldorin Jan 6 '11 at 18:37
Yes, blackbody radiation is not coherent. Most light sources are not coherent light. But I don't think the questioner's "at the same time" meant coherent. If they had meant coherent they would probably have used that specific word, as coherent is very unambiguous and precise and is not usually replaced with "at the same time" as a synonym. But perhaps s/he will add a comment to make it unambiguous. – inflector Jan 6 '11 at 19:27
@Noldorin: I don't think the asker meant to ask about coherent light. However, such a question would be a good one, and there are a number of broadband coherent sources. Starlight has a blackbody spectrum and is spatially coherent. Short laser pulses can have many tens of nanometers of bandwidth, and light produced by supercontinuum generation can span a wavelength range broader than the visible (!!) while still having a fixed phase relationship between its spectral components. – Colin K Jan 11 '11 at 21:57
Before people pounce on me, yes, I know that stellar spectra are not strictly blackbody. – Colin K Jan 11 '11 at 21:58

Sounds like you want a supercontinuum source:

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Dear Gway, LED lights are a nice discovery but if you want a superior light that really emits at all frequencies, light bulbs with wolfram are a choice. They're cheaper, emit at all frequencies, don't contain Hg or other serious poisons, and another advantage is that they can heat up your home. Unfortunately, light bulbs are getting banned at many "progressive" places such as the European Union.

Black body radiation is nonzero at each frequency. It is an idealized radiation - which may be approximated by black objects (in the conventional sense). The most accurate natural source of black-body-like radiation is the cosmic microwave background but its temperature is 2.7 K. Black holes emit thermal Hawking radiation, too: but it hasn't been observed yet. However, the high frequencies carry a higher percentage of energy from a black body than the low frequencies. This can be circumvented by having a black body radiation from a whole array of sources that have different i.e. variable temperature. By taking a right mixture of the low-frequency ones, you may obtain a pretty much flat (or another sufficiently convex) spectrum over the visible (or another) interval as well.

If some light contains all frequencies, it is not coherent. Coherence requires the frequency - and usually the direction - to be universal for all the light. So the light containing all frequencies is pretty much by definition maximally incoherent. Coherence means that the information about the relative phases of the electromagnetic waves at two different points in spacetime is predictable and periodic. For light that contains all frequencies, it is unpredictable and aperiodic.

Cheers LM

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For the Americans: Wolfram – Andrew Mar 18 '11 at 14:45
In your last paragraph regarding coherence, you say 'if some light contains all frequencies, it is not coherent'. Maybe we're using different definitions of coherence? To me, coherent means "can produce high-contrast interference". I use laser pulses with >1 octave of bandwidth to produce spatial and temporal interference all the time. In fact, I once used a ~13 meter delay line to interfere two consecutive pulses from a femtosecond oscillator. – Andrew Mar 18 '11 at 14:52
@Andrew: THe point Lubos is making is that different wavelength phases change at different rates, so that you need careful control on the optical path for different wavelengths to make coherence in non-monochromatic light. It is very difficult, and unless you are specifically arranging things to do it, it can be considered practically unrealizable. – Ron Maimon Apr 20 '12 at 8:35
@RonMaimon, we seem to be discussing a definitional issue. To do so, we need to agree on definitions. What definition of coherence are you using? – Andrew Apr 22 '12 at 16:25

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