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Was the eye to be hit by a very intense source of pure UV radiation (in the hypothesis of absolute lack of any other stimuli), which color would the brain elaborate from said stimulation?

Basic black?

Blinding white?

Some kind of colourless shine?

What if the same stimulation was received in an already colourful setting? Would its intensity hijack the optic nervous system and thus remain the only stimulation to be computed by the brain?

A second layer to the same question, apparently completely different yet instead stemming from the same roots: can a light be so near the UV spectrum that (if devoid of lower frequencies) it could appear as an over-writing dark/black light which cancels out weaker radiations, thus appearing to the naked eye as a visible darkness?

Qmechanic
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FringesExplorer
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3 Answers3

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The human eyes cannot see UV light at all. UV light is outside of the visible spectrum. Directing a UV beam into the eye (of sufficient intensity) will probably cause pain and damage your eyes since lower wavelength light has a higher frequency and therefore energy.

A second layer to the same question, apparently completely different yet instead stemming from the same roots: can a light be so near the UV spectrum that (if devoid of lower frequencies) it could appear as an over-writing dark/black light which cancels out weaker radiations, thus appearing to the naked eye as a visible darkness?

No since as stated above UV light lies outside the visible spectrum. Our eyes can see light with wavelengths between 380 and 700 nanometers. But ultraviolet light has wavelengths shorter than 380 nm. That means they go completely undetected.

joseph h
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  • I would think that if theintensity of the UV was strong enough to start destroying the retina , signals would go to the optical nerve that could simulate light? – anna v Nov 27 '20 at 07:12
  • Hi Anna. I’m not sure. I imagine that specific point could be explained by people in medical sciences. Good point. – joseph h Nov 27 '20 at 07:46
  • Thank you both. Now my question evolves: the eye is not much sensitive of near-UV frequencies, with purple being a made-up color, while "true" violet (that of eggplants for example) is often almost-black to the eye. Could a radiation with only violet frequencies surpass other optical perceptions if shone with great intensity among other light sources, appearing as a radiant dark? Or would it be anyway openly light-violet-coloured, impairing the "appearing as black" perception? Which colour is, scientifically speaking, an exclusively near-UV violet light? How "dark" is it? Lilac? Or "eggplant"? – FringesExplorer Nov 27 '20 at 08:44
  • The human retina can see down to around 310 or 300 nm, but the lens of the eye blocks wavelengths under 380 nm or so. – PM 2Ring Nov 27 '20 at 11:56
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My simple question to you: Have you ever seen some "black light" coming out of your TV remote, or the microwave oven?

What you need to understand is : "Black" is not a color. In optics, black is the absence of a colour...and these are two different things which is exactly why you can't feel or see vacuum simply because vacuum is the "absence" of matter. Also, there is a difference between seeing and feeling. The two things must not be confused. UV light (as Dr Jh pointed out) is not in the visible spectrum for the human eye. This means that the electrons in the sensory pigments at the back of the retina cannot be excited by UV light in a way that can make our brain register the signal as a colour.

Yet, this doesn't mean that the light is getting reflected completely simply because it cannot excite the electrons in specific chemical pigments properly. A part of the light will get absorbed and will make the molecules in the eye vibrate and will hence heat up the portions of the eye and cause burn damage much the same as an intense beam of laser does in laser surgery. You'll only know it when your sensory pigments get totally damaged and you simply cannot see any more!

.Simply increasing the intensity will not change the properties of the light, it will only increase the number of photons hitting the eye per unit time. Light is seen by the eye because it can cause chemical changes in pigments. And whether or not a photon can cause a chemical change depends upon the electronic energy levels in the molecule being exposed, and the frequency of the photon, not the number of photons. So your question can be re-framed as : Is there a possibility that if I get an extremely powerful red laser and point it at a peice of white paper, the color of light reflected is blue rather than red?...or anything else other than red!?

Answer to your second question:
Two waves of different frequencies (however small the frequency difference be) can never completely cancel each other, though two waves of the same frequency but different phases can. This is exactly the reason why physicists find it difficult to perform interference experiments, because it is very difficult to find two light sources with exactly the same frequencies. As a result, they have to divide the beam change the phase of one part and then make them interfere rather than using two different sources altogether.

As a bonus, watch this YouTube Video where this guy builds an intense IR source focused to a thin beam effectively so strong that even though invisible to the eye completely, it can simply put things on fire in a jiffy, and imagine someone pointed it to your eye!! Your retina will burn before it can see and if it doesn't you wont know even whether the light is on or off because retina doesn't detect IR!

So, yes playing with intense EM sources beyond the visible spectrum is exceptionally dangerous mainly because they are invisible, so the person involved will not know till the eye is damaged.

Martin Medro
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The lens in your eye absorbs ultraviolet radiation, so a beam of uv directed into your eye would not reach your retina and hence cannot be seen. However there are various medical conditions that require the lens to be removed, and if you have had this done then the uv is not absorbed by the lens and does reach the retina.

In fact someone who had this done commented to my reply to Is it possible that there is a color our human eye can't see? to say:

I had the lens of my left eye removed as a kid and I can confirm this. It's most apparent looking at black light you can find in clubs: On the right eye, it looks dark blue, on the left eye it looks light blue.

So we have a person with first hand experience telling us that uv light looks light blue, which is a bit disappointing really!

John Rennie
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  • If UV appeared light blue, then I wonder what light blue would have looked like! It may be that the two are perceived as same, but the Wikipedia article here makes things more clear I guess: https://en.wikipedia.org/wiki/Cyanopsia

    "The eye's lens is normally tinted yellow. This reduces the intensity of blue light reaching the retina. When the lens is removed because of cataract, it is usually replaced by an artificial intraocular lens; these artificial lenses are clear, allowing more intense blue light than usual to fall on the retina, leading to the phenomenon." Wikipedia

     – Martin Medro Nov 27 '20 at 11:06
  • It also appears that the quoted comment says: dark blue light (as it appeared to normal eye) appearing light blue (in the eye with lens removed) and not UV light appearing light blue. – Martin Medro Nov 27 '20 at 11:23
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    @CrazyGoblin the comment says black light appears as light blue. Black Light is near uv used in clubs to make people's clothes fluoresce. – John Rennie Nov 27 '20 at 11:42
  • Here's another personal anecdote from someone who can see near UV after having their lens removed. https://www.newscientist.com/lastword/mg24432591-000-super-seers-why-some-people-can-see-ultraviolet-light/ According to that article, most people can't see light with wavelength less than around 380 nm, but the retina can detect light down to 310 or 300 nm. I once heard of an elderly astronomer who had special quartz (which is transparent to near UV) spectacles made after he had cataract surgery so that he could view the stars & planets in UV. – PM 2Ring Nov 27 '20 at 11:54
  • Oh didn't realise that, never saw "Black Light"..sorry. Maybe I should go to clubs more! :) @JohnRennie – Martin Medro Nov 27 '20 at 12:57
  • Not disappointing to me... fascinating! Thank you all by now... But maybe said perception comes from a modification in the perceived stimulus because of the lens removal... I think I must reshape my question: can there be an "eggplant"-kind-of-violet light? Or violet light is inherently always lighter and more lilac-like than an eggplant? I am actually talking about how wavelenghts appear... can there be a very dark violet light? Often eggplants look almost black... which was the core of my question so it still holds some interest to me! – FringesExplorer Nov 27 '20 at 16:38
  • @FringesExplorer The violet of an eggplant may appear darker to you simply because it doesn't reflect as much...and not because there is a change in its frequency. As I said in my answer ...black is not a color in optics..it is the lack of it. So light of any color when dimmed infinitely will lead to a perception of blackness. A light itself cannot be black, thought it can be "transparent" to answer it the way you are asking... – Martin Medro Nov 27 '20 at 17:37