Is false yellow true yellow when reflected?

  1. It is not uncommon knowledge that when we see the colour yellow on a display such as a cathode ray tube, LCD or LED screen it is not really yellow as V Sauce will testify. It is the pixel colours red and green on a black display that at the right intensity our brains perceive as yellow light.

    What if I shine that "yellow" light from a display on to say a sheet of white paper would the light projecting off the white paper sheet appear to my eyes as true yellow light of the light spectrum rather than the combined light of red and green?
  2. jcsd
  3. There is no such thing as a "true yellow". Whatever our brains perceive as yellow is yellow.

    You may be thinking of monochromatic yellow as "true", but even in this case there is no single electromagnetic frequency that is "the" yellow color; there is an interval of them.

    Now, if your question is that if given some non-monochromatic yellow, would a reflection off a white paper make it monochromatic? I think in this formulation the answer is obviously negative, because the white paper appears white precisely because it reflects all the optical frequencies (almost) equally.
  4. Okay you blew me away with science and I got no benefit, lol.
  5. ZapperZ

    ZapperZ 29,631
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    You did come to a science forum, didn't you?

    Try bringing a few friends with you to a paint store. Now, have each one of you pick what each thinks is the color "yellow" without the others seeing what each other picked.

    I will bet you that each one of you will pick a different color stick or sample. This is because there is a wide range of color that many of us perceived as "yellow". Same thing with red, blue, green, etc...etc. Each of these so-called colors have different shades, and different varieties. The name that's given gives no EXACT color.

    What is well defined is the "wavelength" or frequency of electromagnetic spectrum. Each color, even different shades of "yellow", have different wavelengths that can be accurately measured. So now, it is no longer just a matter of "quality" (i.e. giving it a name "yellow"), but now it is a matter of "quantity", i.e. now we have something that we can MEASURE that gives a number!

  6. BruceW

    BruceW 3,529
    Homework Helper

    So, essentially your question is "If I shine a red light and a green light on a white piece of paper, then will the paper appear yellow?" Yes, this is true for the same reason that when you shine red and green light on your eyes, you see yellow. If you think about it another way, what else would you expect to see on the paper? You have red and green light shining on it, so it doesn't make sense that you would perceive just one or the other. On the other hand, it depends on the paper. i.e. It might reflect one colour more strongly than the other. But, if you have a 'fair' piece of paper, then you should see yellow.

    Also interesting is the colour purple, which is made by using red and blue light. Red and blue are at opposite ends of the visible spectrum, so purple has no place on the visible spectrum, yet it is a colour that we can perceive. The point is that we have cells in our eyes that pick up the 'blue' and cells that pick up the 'red' and our brain interprets this as colour 'purple'. It does not matter that purple does not correspond to a given wavelength. Conversely, colours like yellow and cyan do correspond to a given wavelength (or at least a range of wavelengths), because red and green (giving yellow), are next to each other in terms of wavelength. And blue and green (giving cyan) are also next to each other on the wavelength scale.

    edit: I saw this interesting purple facts on some youtube video... It might have been vsauce as well.
  7. Pure ~400nm light looks fairly purple to me...

    (Granted, some shades of purple can only be made with red and blue combined, but at least to my perception, there's definitely a true, monochromatic light source that appears purple)
  8. sophiecentaur

    sophiecentaur 13,259
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    If all you leave with is an appreciation that Wavelength is not Colour, then you have benefitted from the experience.
    You will realise the difference between a single audio tone and a chord or actual Music. Colour and Wavelength have a similar relationship. One's in your head and 'experienced' and the other can be measured by an inanimate piece of equipment.
    Wavelengths can't change on reflection.
  9. The example of yellow is unfortunate because it may confound this discussion a bit... yellow is different from the other colors we see. At first I thought that was what the OP was asking about...

    What will be the other colors we see all originate in the sensory processing levels within the ten functional layers of the retina where most fundamental visual feature extraction occurs; extraction of the color yellow does not happen there, it is "synthesized" further up the processing stages, not even in the eye.

    As a result, we see the color yellow in a subtly different way than the other colors which is why it is used so effectively for signs and signals of danger, lane lines on roadways, advertising, etc.
  10. sophiecentaur

    sophiecentaur 13,259
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    I wouldn't say it was unfortunate. It's pretty much always the case that we percieve a colour that is not from a monochromatic source. There are an infinite number of 'colours' that can be produced in alternative ways. 'Yellow' is just the most obvious and it's a pity it is looked upon as an exception. The actual details of the processing are not really relevant to the underlying principle that the eye can't do more than three band analysis of what it receives. It's a popular notion that all colours are spectral when we virtually never see spectral sources. (At least, before they invented gas discharge tubes and lasers.)
  11. BruceW

    BruceW 3,529
    Homework Helper

    mm. you might be right about that. I've been in windows paint, using just blue only, and if you turn down the luminosity, I guess it looks a little bit purple-y. But still, I would definitely say it was blue if someone asked me. (or dark blue).
  12. sophiecentaur

    sophiecentaur 13,259
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    Gold Member

    It's very hard to be sure about the linearity of the phosphors / LCD filters at low levels. You could well find that the spectrum of the light from the 'primary' blue is wandering at low values of output.
    I've made this point before but, if you look at the 'named' colours on the CIE chromaticity diagram, they put the colour, known as Violet in the position of a desaturated purple, rather than at the position of spectral Violet. There must be a good evolutionary reason why we are so 'approximate' in our discrimination of colours in that (blue / purple) region, compared with other places on the chart.

    All the non-spectral colours are odd. Brown is an example. It's basically just a low luminance yellow but our brain assesses it in the context of the other, higher luminance areas of a scene and it 'looks' brown, whereas, exactly the same patch of colour would look yellow if its luminance is cranked up. The same thing happens with skin tones, aamof. Almost all skin, for all races, has the same narrow range of Chrominance values (hues), it's just the amount of neutral, dark pigment that differs from the very darkest to the very lightest skins. (Suck on that, Mr Griffin and friends).
  13. mfb

    Staff: Mentor

    They can, but it is certainly not something relevant for white paper or other everyday objects.
  14. sophiecentaur

    sophiecentaur 13,259
    Science Advisor
    Gold Member

    Very special circs, I think. What did you have in mind? Relativistic or non-linear effects?
  15. mfb

    Staff: Mentor

    Interactions with phonons/molecular vibrations (-> Raman spectroscopy or acousto-optic modulators), wavelength shifters, and various nonlinear effects.
    The Doppler effect is another option, but that gives really small effects in typical lab setups.
  16. The spectral response of the cones in the human eye overlaps completely between 450 nm and 630 nm, which happens to be the bigger and the brighter part of the visible spectrum. Any "color" in between stimulates at least two different types of cells, so it is as synthetic as yellow.
  17. Thanks for the replies guys. I have been reading through them all with interest and I guess what I was orginally trying to get to the bottom of in my question was as yellow say on a LED screen is points of red and points of blue light that seem to converge because they are such small points of light that Vsauce claimed that we were not seeing actual yellow. So I was asking that if this "false yellow" was emitted to and reflected off a sheet of white paper would it be "true yellow"? Instead of points of seperate red and blue light but real yellow light (Yellow spectrum).

    This is the Vsauce link on this topic
  18. mfb

    Staff: Mentor

    On a sheet of paper, there are no separated points, but the light can still be split into something appearing as green and red, if you send the light through a prism (for example).
  19. Yes, but be gentle.
  20. D H

    Staff: Mentor

    No. The spectrum of light reflected off the sheet of white paper is more or less the same as what goes into it.
  21. I'd be pretty surprised if your monitor was capable of reproducing pure 400nm light. You'd probably need a different light source to actually see what it looks like.
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