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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?

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2 Answers 2

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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.

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  • $\begingroup$ I am wondering is there such sources that can smoothly transit radiation frequency from visible to invisible🤔 $\endgroup$
    – Stdugnd4ikbd
    Jan 18, 2022 at 10:35
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    $\begingroup$ Some lasers can do that, yes. $\endgroup$
    – noah
    Jan 18, 2022 at 10:49
  • $\begingroup$ Lasers? Lasers, on the contrary, radiate almost monochromatic light. Maybe You meant due to temperature changes? What are that lasers? $\endgroup$
    – Stdugnd4ikbd
    Jan 18, 2022 at 11:24
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    $\begingroup$ 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. $\endgroup$
    – noah
    Jan 18, 2022 at 11:41
  • $\begingroup$ First time see it, very interesting $\endgroup$
    – Stdugnd4ikbd
    Jan 18, 2022 at 11:49
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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?

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  • $\begingroup$ I am wondering is there such sources that can smoothly transit radiation frequency from visible to invisible🤔 $\endgroup$
    – Stdugnd4ikbd
    Jan 18, 2022 at 10:33
  • $\begingroup$ @ЙцуoееЦукен An incandescent bulb will do, especially if it has a fused silica envelope. $\endgroup$
    – garyp
    Jan 18, 2022 at 10:51
  • $\begingroup$ 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. $\endgroup$
    – Roger V.
    Jan 18, 2022 at 10:54
  • $\begingroup$ Thanks for answer $\endgroup$
    – Stdugnd4ikbd
    Jan 18, 2022 at 11:52
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    $\begingroup$ @CarlWitthoft what I say is rather different from what your comment suggests. $\endgroup$
    – Roger V.
    Jan 18, 2022 at 15:26

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