Anatomy and Health
Photoreceptors: How We See Roy.G.Biv
We learned about the 7 flavors of visible light in the electromagnetic spectrum since preschool days: red, orange, yellow, green, blue, indigo, and violet (They Might Be Giants wrote a song just for remembering the order). Visible light is actually a continuum of colors, though. So how do we see all of these colors? Is there a cell in our eye for every possible color? And what happens in our eye and brain that turns a certain wavelength of light hitting our eye, into a color?
We see light using photoreceptors, special cells in the retina of our eye. Light, as we learned in this chapter, is an electromagnetic wave. When light enters our eye, photoreceptors translate this electromagnetic wave into a signal that our brain can process and turn into the sensation of color.
There are two types of photoreceptors, cones and rods. Rods are very sensitive to light – as few as 6 photons can activate a rod cells to send a signal to our brain that says LIGHT. But rods don't see color, they just say, "Yes, light" or "No, light." Because they are sensitive to even just a tiny bit of light, rods are very important for seeing in the dark. This is why things seem to lose their color when it is dark. After dusk, we see mostly with our rod cells and since they don't see color, colors seem to fade.
The type of photoreceptors that allow us to see color are called cones. We don't have a cone for each of the hundreds of colors we see. Instead, they come in just three flavors that essentially correspond to the colors red, green, and blue. The rainbow of colors we see are generated by our brain mixing the colors red, green, and blue.
Each of these three cone receptors has a certain wavelength of light that activates it. We call the "red cone" red because the light that cone absorbs best is the wavelength of red light–564 nm. The "red cone" also gets activated by light with wavelength that are a little bit longer or a little bit shorter. The same for blue, except the blue cones absorb light with shorter wavelength, around 420 nm. The green cones are in between. Green cones are activated by light with a 534 nm wavelength.
How do we see turquoise which has a wavelength of about 500 nm?
What happens is that 500 nm is between 420 nm (blue) and 534 nm (green), so 500 nm light activates both the blue and the green receptor and the green receptor a little more than the blue because 500 nm is closer to green than to blue. The red cones don't get activated at all. Our brain reads this particular combination of green and blue as turquoise.
All of the colors we can see are just three types of cone photoreceptors signaling by different degrees. TVs and computer screens take advantage of this. The color pictures generated by televisions are made from just three colors, red, green, and blue. To make a rainbow of colors TV folk just mix different numbers of green, red, and blue pixels.
Check out a TED-ED video for a cartoon explanation of how we see color.
People who are colorblind have defects in their cone cells. People that can't see color at all have defects in one to all of the three cone types. The most common form of color blindness is caused by a defect in the "green" cone cells, which makes it difficult to distinguish green and red. A rarer form of color blindness makes people not be able to tell the difference between blue and yellow; these people have defective "blue" cone cells.