Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The Colour of Light

  1. Aug 8, 2005 #1
    I have been pondering about the nature of colour and its philosophical status as either an irreducible, objective property of an object or rather as a derived quality, dependent on the perceiver. Basically, to put it in Lockeian terms, the question is whether colour is a 'primary' or a 'secondary' qualit. I know Locke and others considered it a secondary quality, but I have been thinking about a scenario where colour does indeed seem an intrinsic property.

    Normally people say colour is the result of different wavelengths of light being absorbed and reflected by the constituents of a body, giving it a particular colour. However, consider the following scenario. You are in a dark room, no light enters and therefore it is pitch black. Then, a light from a source at the back of the room is emitted. You perceive the light, and it has its usual colour (i.e. white or yellowish-white). The light reaches you straight from the source, so the colour you see is not the result of wavelengths of light being absorbed by a material and the rest being reflected back. It actually does seem the light is objectively coloured in this case.

    Does anybody seem any flaw or any important details I overlooked in this simple thought experiment which would invalidate the view that light is an intrinsic property of the world rather than secondary and derived?
  2. jcsd
  3. Aug 8, 2005 #2
    There's no such thing as 'light' as a distinct, separate entity. Light is just a specific range of frequencies of waves. Whichever range of waves the source of the light is emitting is the range that you will see.

    'White' light is the color of all colors together (black is the complete absence of light - and therefor color). When light bounces off of a surface the surface absorbs some colors, and reflects other ones, which is what you see. So if nothing is absorbed you will see the color of the original wavelength emitted... in the case of your example.. white/yellowish.

    I don't know if this belongs in the philosophy section.
    Last edited: Aug 8, 2005
  4. Aug 8, 2005 #3
    The question posed before is a philosophical question par excellence, and it is therefore appropriate under this section. Anyways, light is a stream of photons exhibiting a wave-particle duality, as noted by quantum physics. Your answer referred to seeing the colour of objects, but what I was referring to was the actual colour of light, as light itself does not have to be reflected off other bodies in order to be perceived. Is light (this stream of photons) already coloured in your view? Comments and other input by others regarding this question could be useful.
  5. Aug 8, 2005 #4


    User Avatar
    Gold Member

    The light travels as whatever wavelength it has, it does not need a medium for it to exist. If a red light wave is coming towards you, it does not need to reflect off anything to be perceived as red light in your eyes. Think of the photoelectric effect!
  6. Aug 8, 2005 #5
    Yes, as Pengwuino said, Light is colored, and does not require a medium to be perceived as a color.
  7. Aug 8, 2005 #6
    The colors that we see depend on the wavelength of the light that enters our eyes. The color of an object depends on its reflectance properties. I think it is not right to say that light is colored. You should say light has a certain wavelength (a certain band of wavelengths), and the colors that we perceive depend on those wavelengths.

    Consider an animal other than a human with differently built eyes. It will see different colors when you shine the lamp in its eyes.
  8. Aug 9, 2005 #7
    Whatever... the point is they're already wavelengths and don't need to bounce off of something.

    And I don't really see the point in your post, why can't we say it is colored. Color is the name we give to the reflectable attributes. And they're universal, someone/thing with different built eyes might see the light differently but they still recognize the different wavelengths the same way we do, so it doens't matter.
  9. Aug 9, 2005 #8
    Every time an electron jump to a lower orbit a photon is emited, depending from what orbit did the electron jumped, diferent the wavelenght of the photon and so it's color. You can check on neon lights, depending on the gas the electron are flowing to, diferent the color of the light, becouse of the electron distribution of it's atoms......
  10. Aug 9, 2005 #9
    The color of light is created in our brains. There is nothing inherent in the different frequencies that should make blue so different than red, and red than yellow. The differences in frequencies are simply faster or slower: all we can account for by the difference in frequency is shades of grey.

    The eyes and brain select different ranges of wavelengths and turn them into the spectacular, and pretty much fictional, qualitative experience we know as color.

    "In 1957, ninety-odd years after Maxwell's famous demonstration, Edwin Land - not merely the inventor of the instant Land camera and Polaroid, but an experimenter and theorizer of genius - provided a photographic demonstration of color perception even more startling. Unlike Maxwell, he made only two black-and-white images (using a split-beam camera so they could be taken at the same time from the same viewpoint, through the same lens) and superimposed these on a screen with a double lens projector. He used two filters to make the images: one passing longer wavelengths (a red filter), the other passing shorter wavelengths (a green filter). The first image was then projected through a red filter, the second with ordinary white light, unfiltered. One might expect that this would produce just an over all pale-pink image, but something `impossible' happened instead. The photograph of a young woman appeared instantly in full color - `blonde hair, pale blue eyes, red coat, bluegreen collar, and strikingly natural flesh tones,' as Land later described it. Where did these colors come from, and how were they made? They did not seem to be `in' the photographs or in the iluminants themselves. These demonstrations, overwhelming in their simplicity and impact were color `illusions' in Goethe's sense, but illusions that demonstrated a neurological truth - that colors are not `out there' in the world, nor (as classical thery held) an automatic correlate of wavelength, but, rather, are constructed by the brain."

    Oliver Sacks

    An Anthropologist On Mars
    p.24 Vintage Books, 1995
  11. Aug 9, 2005 #10
    Ah Zooby, I knew there was a reason I liked you!
  12. Aug 9, 2005 #11
    The point is that only the observer makes them colors. The observer interprets the combination of wavelengths as colors. Light in itself is not colored. Color is an interpretation, based on the wavelengths in the light originating from the object you assigned the color to and also on the wavelengths originating from its surroundings.

    Land's experiments corroborate that.
  13. Aug 9, 2005 #12
    The color of a certain part of a scene is probably a function of the spectral content of the whole scene.

    Finding this function is the (currently not yet achieved) aim of color vision research.
  14. Aug 9, 2005 #13
    Just to throw my ha'penny in (stingy bast that I am), not only is 'colour' merely an interpretation of the frequency of the EM wave but also, to contradict Smurf's statement "[w]hichever range of waves the source of the light is emitting is the range that you will see", two different observers may disagree as to the colour of that light. In cosmological redshift, for instance, the frequency of light emitted (i.e. in a reference frame in which the source is at rest) is greater than that in which it is received by receding observers in distant galaxies (i.e. in a reference frame in which the observer is at rest). Therefore 'colour' cannot be said to be an intrinsic quality of light, since even frequency and wavelength are relative properties.
  15. Aug 9, 2005 #14
    Curious6, I have long thought that color is intrinsic. There are a number of reasons that I think this way.

    First, photons are messenger particles or wave particles if you prefer. They have no intrinsic wavelength or color of their own but carry the wave length as a function of the energy state of the electron that emitted them. Some are also absorbed by the medium through which they pass carrying that information with them also. If we measure that light we can learn a great deal about the emitter as well as the intervening gases or medium. I'm sure that you are aware of all of this, but the point is that photons carry information and do not have any specific wavelenght or color themselves.

    Second, we did not and do not invent color nor change the color that we see.
    Its true that our minds try to compensate for background color and contrasts so that our color perception changes from what is actually received, seen,but humans are not the only animals that see color and the color that we do see for the most part is perceived the same or nearly the same for everyone. If this were not the case paintings photographs and TV would not look the same for everyone and wouldn't make any sense. For us to see and perceive the same color it must be a property of that which we are seeing and not merely some mental perception of our minds.

    Third, we are evolved animals and as I said we did not invent color but evolved to detect and see that which is already there and a property of the world around us. We developed color perception because color is there and is useful information. It is an intrinsic part of our environment.

    Forth, take our favorite example of color, the rose; roses breed true to their color and by interbreeding or controlling pollination we can make hybrids of different colors but we cannot breed a blue rose because roses do not have a "blue" gene. This proves that there is some property of roses that is intrinsic and genetic and we call that property color. If color were just perceived then why or how could roses or any other flower, bird, fish etc develop a gene that determines their color and passes it on to their off spring. It must therefore be intrinsic.
  16. Aug 9, 2005 #15

    Yes, but how can you be sure that light itself is actually coloured, and not that the stream of photons composing it is actually colourless and that it just acquires its colour, as we observe it, through the interplay between the properties of the photons (i.e. wavelength and frequency) and the receptor (the eye/brain system)? Basically, what I am trying to say is that the properties of the photon can trigger a system when perceived by the eye that creates the colour 'yellow' or 'white', for instance, in our brain.
  17. Aug 9, 2005 #16
    Very interesting indeed. However, if that experiment was supposed to definitely prove colours are an illusion created by the brain and not an 'automatic correlate of wavelength', why is there still a controvery or debate among the philosophical community about the objective nature of colour?

    Also, could you please explain in slightly more detail what exactly this experiment proved. Is it saying that wavelengths trigger a mechanism in our brain which account for us seeing a particular colour for each particular wavelength?
  18. Aug 9, 2005 #17

    Could be the result of evolutionary factors. Sight and the ability to assign a particular colour to different wavelenghts undoubtedly provided a competitive edge, thereby allowing this trait to be passed on to subsequent generations, until it became widespread and common to all humans.

    But is it really an intrinsic property of the world? Again, as other posters in this thread have noted, is a colour not just an invention by the brain used to differentiate between varying wavelenghts?

    Again, not necessary. Roses could just have developed in a way that its petals are constituted of particles that would absorb and reflect the necessary wavelenghts to produce the impression of the colour rose in our minds. The reason then which would explain why roses would not have a distinct 'blue' gene is that the constituents of the petals would all have to change in order for the wavelengths of light to be absorbed and reflected back to create the colour blue.
  19. Aug 9, 2005 #18


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Perceived, qualitative color is a function of the activity of the visual processing stream in the brain. Any number of visual illusions will show this to be the case. Light waves themselves do not have some sort of qualitative color; they merely have various wavelengths. The brain constructs color perceptions from various combinations of light wavelengths that strike the retina, but it is not true that there is always a simple, straightforward mapping from light wavelength of the visual scene to perceived color.

    One can find a good demonstration of this here. This site features three different color illusions, wherein we can plainly see that color perception of objects is fluid and depends largely on the context of the given scene. Note that the objective properties of your computer monitor, including the wavelengths of light striking your retinas, are held constant in these demonstrations. What is changing is the context in which certain features of the scene are viewed, and this changes the manner in which we perceive the colors of these features. For each masked and unmasked scene, is one color perception 'right' and the other 'wrong'? If so, which is 'right,' and why? I think the answer is clearly that there is no 'right' way to perceive these colors. Both ways of perceiving the relevant colors are just interpretations of the visual scene created by the brain. If this were not the case, we should not expect our color perceptions to change so fluidly and radically by simply changing some of the surrounding visual context.
    Last edited: Aug 9, 2005
  20. Aug 9, 2005 #19
    Damn, i won't belive in my eyes anymore!!! or should i say my brain....
  21. Aug 9, 2005 #20
    You'll have to address that question to the philosophical community.
    All the experient proved was that color isn't an objective property of light, but an experience primarily created in the brain (via wavelength selection by the eyes). It didn't explain how.

    The next generation of experiments explored how the experience of color is dependent on context, as detailed by hypnagogue in his excellent post. Any accomplished artist can verify this: the effect of a color can be grossly modified by what colors you place next to it.

    The important thing to realize is that there is nothing about wavelengths of light between, say 400 and 430 nm that requires it to be percieved as blue rather than blue, or blue.

    The wavelengths reflected by a particular object are objective and measurable, but what our brains make of those frequencies, how it processes them into the experience of color, is splendid fiction.

    Compare to sound. Take the example of a note on a clarinet. A smooth increase in the frequency of a clarinet tone results in a smooth increase in pitch. If we we percieved sound the same way we percieve color we would encounter "thresholds" where an increase in frequency would result in the clarinet suddenly, inexplicably changing in total sound quality, so that it sounded, variously, like a violin, a trumpet, a bag pipe, a flute, and so on. There would be nothing measurable to account for these drastic shifts in qualitative experience beyond shift in frequency and wavelength. Therefore, the brain must be selecting these different ranges out and presenting them to consciousness as much more different from each other than they actually are.

    The story where I got the quote about Edwin Land is about an artist who became completely colorblind after a blow to the head in a car accident. Nothing whatever happened to his eyes, only a part of his brain was damaged. It's the first chapter of that book by Sacks, An Anthropologist on Mars and is entitled The Case of the Colorblind Painter. I think most libraries carry his books.

    If you're interested in color perception you might also read his book: The Island of the Colorblind, which is about a remote pacific island where a genetic predisposition to total colorblindness affects a large part of the population. The difficulties this condition causes the sufferers underscores why natural selection would favor any and all born with color vision.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook