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Cool Human Physiology Fact of the Day

This is absolutely, totally, highly cool! How come I never knew this before?

Hoisted from comments:

Grasping Reality with Both Hands: Brad DeLong's Semi-Daily Journal: Applied Philosophy: Representation, Metarepresentations, and Qualia:

About purple...

I noticed an interesting thing while looking at the sensitivities of the receptors in the eye. The blue (beta) and green (gamma) receptors tail off to insignificant sensitivity much more sharply than the red (rho). The red sensors are actually more sensitive to very short wavelengths than the green, and begin to rival the blue ones.

As you approach the limits of human vision near the ultraviolet, the ratio of sensitivity of the red sensors to blue sensors is increasing, not decreasing.

So, photons of purple light (and yes, there is indeed a wavelength that is purple) stimulate the receptors in exactly the same way that combinations of red and blue light would. Had it not been for the quirk of the long tail in the red receptor sensitivity, we may never have had the concept of purple. The color wheel might have been a color bar. Blue would have just covered a larger wavelength range like red does. This might need explaining. While most texts say human sight goes to about 700 nm, it really goes much further (I have no problem seeing 790 nm). It's just that everything looks the same shade of red because the green and blue receptors are of negligible sensitivity. If there were not a long tail of sensitivity in the red receptors at the blue end of the spectrum, everything we see shorter than 420 nm would look exactly the same shade of blue.

Posted by: Njorl | December 28, 2006 at 11:35 AM