CoachThe retina (Chapter 10) carries two families of light-sensitive cells: the rods, which respond to faint light but ignore wavelength (night vision, in gray), and the cones, in three types most sensitive near (blue), (green) and (red). Human color vision is trichromatic: whatever the spectrum, the brain receives only three cone responses, and color is what it makes of that triple.
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Example 11.3 (Two yellows)
The line of a sodium lamp excites the red and green cones almost equally: the brain says yellow. A screen lighting its red () and green () subpixels gives the same yellow with no light near . A prism separates the two instantly; the eye never can.