How do we see colors?

This is related to the previous post about the speed of light. As I’ve said before, white light consists of a color spectrum from blue through green and yellow to red. These colors have different wavelengths, and these colors are reflected from objects that are bathed in white light because the objects absorb some colors and reflect what is not absorbed away. The reflected colored light is what we see.

But, how do we see separate colors? The answer is because of the marvelous design of the retina in our eyes. The retina is a section at the back of the eyeball that contains the sensor cells of the optic nerve, a nerve that extends back to he occipital region of the brain, which is at the back of one’s head. These sensor cells are separated into rods and cones. The rods and cones are interior to the surface of the retina. There is enough transparent nature to the nerve layers that cover the rods and cones to allow light to get through to them.

The major difference between rods and cones is that rods are more sensitive to intense light and the cones are more sensitive to low light. Cones have pigments that absorb particular colors. Rods have only one kind of pigment and play little role in color vision.

Cones give us trichromatic vision, or vision of three colors. The three kinds of cones are S-cones, M-cones and L-cones, corresponding roughly to blue, green and red colored light. The spectral sensitivity of these three cones overlap, allowing for a complete color spectrum vision, but it’s not exactly uniform. The sensitivities of the three cone types are different. For example, the S-cones peak in sensitivity in the 420-440 nm (nano-meters) wavelength and are more sensitive than the other two, but the corneal and lens absorbs some blue light, compensating for this.

The cones also give us more acute vision because each cone attaches to one optic nerve ending. This is where the resolution of vision happens because there are 4.5 million cone cells in a human retina.

There are 90-128 million rod cells and they are sensitive to light that peaks at 498 nm, which is half way between the peak sensitivity of the S and M cones. Rods provide us with vision in bright light situations and the nerve process is different, involving our peripheral vision. Since there are only a million optic nerve fibers, there has to be a certain amount of overlap of the connections. That’s where the occipital region of the brain comes in. This part of the brain processes vision in the form of nerve impulses from the optic nerve, and as you might have suspected there are two occipital lobes.

The occipital lobes do an amazing thing. They take the raw vision signals from the optic nerve and turn them into something that resembles a picture and it’s right side up and from left to right. This is amazing because the eye lenses focus on the retina upside down and reversed. This part of the brain also differentiates colors and makes us see in color. Some people suffer from color blindness, but it involves not differentiating greens and blues for example. This is a disease and has several causes and affects a significant percentage of the population. In most cases, this is a genetic disease. It points out how amazing our vision is and how we can see wonderful colors. What a dull and boring world it would be if we couldn’t.

Thanks for reading.