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By increasing the frequency of electromagnetic wave in visible range, human eye sees it as a color change.

But how human eye will see transition after violet wavelength to ultraviolet? As far as I understand ultraviolet (at least middle and far ranges) is invisible for eye.

So if imagine some source that can smoothly change frequency of the radiation from violet to (far) ultraviolet, how human will see it?

Will it just smoothly disappear?

Stdugnd4ikbd
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2 Answers2

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The sensitivity of your cone cells (the ones that see color) varies with frequency, as shown in this nice graph from Wikipedia.

Cone sensitivity

If you had a monochromatic light source of constant intensity, but could smoothly vary the frequency, it would appear to your eyes as becoming brighter and dimmer across different frequencies.

On the far left of the graph you can see that the sensitivity of the S cones smoothly (but quite steeply) goes to zero. This is exactly the brightness you would see if you were to tune your light source from violet to ultraviolet, i.e. you would observe smooth dimming of the light until it is gone.

noah
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  • I am wondering is there such sources that can smoothly transit radiation frequency from visible to invisible – Stdugnd4ikbd Jan 18 '22 at 10:35
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    Some lasers can do that, yes. – noah Jan 18 '22 at 10:49
  • Lasers? Lasers, on the contrary, radiate almost monochromatic light. Maybe You meant due to temperature changes? What are that lasers? – Stdugnd4ikbd Jan 18 '22 at 11:24
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    Yes, when you asked about "varying the frequency" I assumed (and noted in my answer) we are talking about monochromatic light. Some laser setups emit (almost) a single wavelength, but you get to choose which one it is smoothly over some range. They are typically called tunable lasers. – noah Jan 18 '22 at 11:41
  • First time see it, very interesting – Stdugnd4ikbd Jan 18 '22 at 11:49
  • You should also point out that the rods have a spectral response curve. You may not sense the violet color, but until the rods cut out, you'll sense the existence of energy in the V -- NUV range. Rods may extend further than cones, see e.g. the graph at https://www.yorku.ca/eye/lambdas.htm – Carl Witthoft Jan 18 '22 at 15:19
  • I believe that "cone response" plot is somewhat simplified. ISTR that the "L" receptors actually have some response to the shortest visible wavelengths, which is why those wavelengths look more purple* than blue. http://www.color-theory-phenomena.nl/05.00.html – Solomon Slow Jan 18 '22 at 15:34
  • * Some call it "violet." – Solomon Slow Jan 18 '22 at 15:37
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Broad spectrum
It very much depends on the spectrum of the light source. If the spectrum is rather broad - e.g., a black body radiation that has a maximum that shifts towards the ultraviolet or even an ultraviolet lamp, then the source will remain violet but become pale and possibly invisible after some time (the latter scenario is less likely for a black body spectrum).

Narrow spectrum
On the other hand, if we talk about a monichromatic light source, then the transition will be more abrupt, as it will be determined not by the spectrum of the light, but by that of the photoreceptors in our eye - see How does light combine to make new colours?. Note also the related question: Is it possible that there is a color our human eye can't see?

Roger V.
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  • I am wondering is there such sources that can smoothly transit radiation frequency from visible to invisible – Stdugnd4ikbd Jan 18 '22 at 10:33
  • @ЙцуoееЦукен An incandescent bulb will do, especially if it has a fused silica envelope. – garyp Jan 18 '22 at 10:51
  • Wikipedia lists a variety of sources of ultraviolet. The sources specifically designed for ultraviolet have narrower spectrum, but more general sources are more likely to be tunable. In this sense I agree with @garyp - one can tune the temperature of an incadescent lamp by simply increasing the current. – Roger V. Jan 18 '22 at 10:54
  • Thanks for answer – Stdugnd4ikbd Jan 18 '22 at 11:52
  • No, it really won't be abrupt. Check the responsivity curves of both rods and cones – Carl Witthoft Jan 18 '22 at 15:16
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    @CarlWitthoft what I say is rather different from what your comment suggests. – Roger V. Jan 18 '22 at 15:26
  • @RogerVadim I was objecting to "...transition will be more abrupt..." Guess that's not unambiguous – Carl Witthoft Jan 18 '22 at 17:33
  • @CarlWitthoft if we talk about a monichromatic light source, then the transition will be more abrupt, as it will be determined not by the spectrum of the light, but by that of the photoreceptors in our eye. So your claim is that photoreceptors in an eye have an absorption spectrum wider than the spectrum of any typical ight source. Is that right? – Roger V. Jan 18 '22 at 18:54