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(light tapping) - We've just seen the beautiful rainbows you get when you refract light with a prism. Now let's think about how we actually perceive the colors of the rainbow in the first place. What makes yellow look like yellow? Inside our eyes, we have special color receptors called cones, that are sensitive to specific wavelengths of light. For example, one kind of cone is most sensitive to red light, so when long wavelength light hits them, that is from the red end of the spectrum, they respond by sending a corresponding electrical signal to the brain, and the magnitude of this signal will depend on how much red light is present in the light. Our eyes have three main kinds of color receptors. One is sensitive to the longer wavelengths, redder light, one for medium wavelengths, or greener light. And one for shorter wavelengths, or the bluer light. The cool trick is our brain blends signals from these three receptors to represent any color. Colored television, and computer monitors, borrowed this same idea of color receptors from our eyes. If you zoom into a computer screen, you'll see it's made up of tiny rectangles, or pixels. Each of which contain a red, green, and blue region. To display colors, the monitor illuminates these three colors accordingly. To make the screen appear yellow, the monitor turns on only the red and green regions in each pixel. From far away, these blend into yellow. And that's how we can digitally create any color using just different amounts of red, green, and blue. In our next exercise, you'll try to create a color by mixing together different amounts of red, green, and blue on your monitor. For example, to get this shade of green. It's interesting to note that other animals have different cones in their eyes. For example, dogs only have cones that are sensitive to yellow and blue, so red light doesn't send signals to their brain at all. The mantis shrimp, on the other hand, holds the current record for the most number of color receptors in its eyes, 12 different kinds. And scientists still don't know how they're all used to perceive color. - Voila, he is clean. (exciting fun music) - Beyond that though, there's also a certain class of people, they're all females, it turns out, who we call tetrachromats. They have four different color receptors in the eye, whereas the vast majority of the population has three. And so we think, and expect, that their ability to discriminate colors, to basically tell the difference between two very, very similar shades, is probably much more precise for the tetrachromats. I wish I was one.