From billb@mail.eskimo.comMon Oct 16 05:59:04 1995 Date: Wed, 11 Oct 1995 22:25:56 -0700 (PDT) From: William Beaty To: William Beaty Subject: Re: misconceptions! On Sun, 24 Sep 1995, Alistair B. Fraser wrote: "Alistair B. Fraser" > You send mixed signals in your items, sometimes treating the word color as > a surrogate for frequency and sometimes (begrudgingly) acknowledging that > it is actually an issue of perception. Alas, the first is wrong; the second > is right. Most physicists have it wrong; probably the elementary textbooks > do a better job than Ph.D. physicists. > > Color is a perceptual response to a spectrum --- it is what you see, not > what you measure with a spectrometer: even Newton (Optic, 1730) recognized > this, "For the Rays [of light] to speak properly are not coloured. In them > there is nothing else than a certain Power and Disposition to stir up a > Sensation of this or that Colour." The phenomenon is even better understood > today, and can be summarized by noting that if you start with a spectrum > you can determine what color will be seen (with some provisos which need > not bother us for the moment). However, if you start with a color, you > cannot determine a spectrum. Many spectra can produce the same sensation of > color. This ambiguity even has a name: metamerism; different spectra which > produce the same color are said to be metameric. > > Thus, when you assert that "RED LIGHT PLUS GREEN LIGHT DOES NOT CREATE > YELLOW LIGHT", you are categorically wrong, indeed, the reverse it true. > While you are correct that there is no wavelength of, say, approximately > .55 micrometers created when monochromatic red and green are combined, that > is irrelevant: yellow has been created. It is not that it is an oddity of > human vision: human vision is what DEFINES color. It is yellow. (full > stop). > > The mistake was to believe the tables which identify a color and a > frequency (or wavelength) which are often published. They are correct only > in one sense: if there is a monochromatic frequency, the table will tell > you the color which will be seen. However, if there is a color, it will not > tell you the frequency. > > If you want to understand color, go to a psychologist not a physicist --- > the latter generally don't know squat about it. > > Now, about your statement that, "THERE ARE NOT SEVEN COLORS IN THE > RAINBOW," I have only one quibble, and that is with the word, infinite. > Conceptually, there may be an infinite number of frequency (intervals) > across the spectrum, but color is a perceptual phenomenon, and there is not > an infinite number of colors. You're right. I would imagine that our eyes' ability to resolve separate frequencies limits the number of colors to under a hundred. The sun, being an extended source, does blur the rainbow pattern, though I don't immediately see that this would reduce the number of perceived colors. Might you know of any articles on this subject? How many adjacent stripes of uniform color in that angular width are needed before individual stripes wouldn't be percieved? > Your statement that, "RED LIGHT IS NOT WARMER THAN BLUE LIGHT", has > problems. I know what you are getting at, but your categorical approach > causes you to make silly assertions. You're right, and I never caught myself doing this. I think I'm reacting to conversations with teachers, who counter my arguments about misleading explanations with: "but is it WRONG?" I've fallen for the temptation to say "yes," rather than to say "it's misleading, not actually wrong, but is the wrong way to explain it!" If you encountered the statement "Red light is warmer than blue light" in a textbook's physical science manuscript, how would you explain to the author why their statement is wrong? > > For example, warmer has a number of meanings and you don't acknowledge > that, nor do you tell your reader in what way you are using the term. If I > am in an art class, it IS correct to say that red is warmer than blue, so > how can you claim catagorical that it is not? > > Further, the claim that, "Red and blue light are equally warm," is nonsense > because it is categorical. I have no difficulty in creating a red light > which will be warmer (be more effective in heating a surface) than a blue > light --- I just turn up the brightness of the red. > > So the best you can say is if the red and blue light carry equal energy, > they have equal ability to warm (give or take the absorptivity of the > surface). But, then that is just a tautology, so it is kind of silly to > offer it. > > Your statement that, "We do perceive red light as looking warmer, but this > is psychology", is, I believe, also wrong. It is convention, but not > psychology. It is not a built in component of our perception system, but a > convention which has been adopted as a result of a series of accidents in > our surroundings and by the assertions of art teachers. Some of these > accidents are: > 1) We rarely ever see anything which is blue hot (much hotter than red hot) > and so as an object is heated it glows red to begin with (when it is hot to > the touch but not hotter than the sun), we associate red with warmth. > 2) When a person is cold, the body reacts by constricting the passage of > arterial blood to its surface. This cuts down on the energy loss, but also > leaves the person looking somewhat blue (venal blood). The reverse happens > when a person is too hot, energy is dumped by sending arterial blood to the > surface. > 3) Shadows outdoors are bluish (for the reason that they are illuminated by > blue light of the sky). > > Finally, I return to Newton: "the rays are not coloured." So, speaking of > red light is to speak of a perceptual rather than a radiometric phenomenon. > But you treated it as if it were just a radiometric description (but then, > I dare say, so did the textbooks you criticize). > > You are right to criticize any assertion that just because we speak of the > warm colors as being reds and yellows, we are making a statement about the > actual heating ability of various portions of the spectrum, but you went > wildly overboard. > > OK, there is my offering on color. Do you want more? Yes! And I greatly appreciate the time you took for such in-depth answers. .....................uuuu / oo \ uuuu........,............................. 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