Friday, May 8, 2009

Color Vision

Color vision is one topic from biological physics that Russ Hobbie and I do not discuss in the 4th Edition of Intermediate Physics for Medicine and Biology. Why? Well, the book is already rather long, and it is printed in black & white. To do justice to this topic, one really needs color pictures.

The history of color vision is fascinating, in part because it illustrates the role that physics and physicists can play in the life sciences. The fundamental idea of trichromatic color vision was developed by two giants of 19th century physics, Thomas Young and Hermann von Helmholtz. Young was a fascinating intellectual, who Andrew Robinson describes in his book
The Last Man Who Knew Everything: Thomas Young, the Anonymous Genius Who Proved Newton Wrong and Deciphered the Rosetta Stone, Among Other Surprising Feats. Helmholtz was a leading figure in early German physics (see: Hermann von Helmholtz and the Foundations of Nineteenth-Century Science).

The Young-Helmholtz theory postulates three types of photoreceptors in the eye, corresponding to three different colors of light: red (long wavelength), green (intermediate wavelength), and blue (short wavelength). Other colors can be formed by a mixture of these three. For instance, yellow is a combination of red and green (which amazes me, because yellow does not look anything like what you might expect from a red-green mixture). You can make yellow light two ways: a single wavelength of light intermediate between red and green so it excites both the red and green receptors (their response curves overlap), or by two wavelengths--one pure red and one pure green--mixed together. Your eye cannot tell the difference: in each case the red and green receptors are both excited. However, you could easily tell which is which using a prism or diffraction grating. Cyan is a mixture of green and blue (and cyan does indeed look like what you might call blue-green). Magenta is a mixture of blue and red, and is a particularly interesting case because it is not one of the colors of the rainbow: you could not, for instance, have a magenta laser, because a laser outputs a single wavelength of light—one color—but in order to produce magenta you need to excite both the red and blue receptors without exciting the green receptor. There is no way to do this without using at least two wavelengths. Of course, if you mix all three colors (that is, excite all three receptors simultaneously) you get white light.

Color mixing is much easier to understand if you can visualize it. I suggest going to one of the excellent color mixing applets on the internet, such as this one, or this one. If none of this sounds much like what you learned when mixing paint in kindergarten, it is because there you were really doing color subtraction, rather than color addition.

Once you understand color mixing, you can understand color blindness. The most common type is red-green color blindness, where either the red or green receptor is absent. If the green receptor is not present, you cannot distinguish red from green or yellow (both excite only the red receptor), although you can still distinguish red from blue or magenta. Not sure if you are colorblind? There are many websites available that offer tests, including this one and this one. Not all animals have trichomatic vision. Your dog has only two receptors, making her a dichromat.

Another physicist that worked on color vision was James Clerk Maxwell, who is best remembered for his monumental work on electromagnetic theory (“Maxwell’s Equations”), as well as his work on the kinetic theory of gasses. But he also studied color vision by using wheels painted with more than one color, which when spun would produce a color mixture. Maxwell also produced the first color photograph.

We should keep in mind this admonition from Richard Feynman in Volume 1 of his famous
The Feynman Lectures on Physics: "Color is not a question of the physics of light itself. Color is a sensation, and the sensation for different colors is different in different circumstances." If you don't believe this, see this optical illusion. You can even play tricks on your eye by creating afterimages like this one. Color vision is a fascinating subject, and a great example of the interaction between physics and physiology.

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