Thursday, February 14, 2008

Everything is relative including the colors we see

We depend a lot on our perceptions, how we see things, how we hear sounds, how we feel things, how we smell things, how we taste things. More often than not people take these senses for granted and never for once think beyond the senses perceived to the act of perceiving. I recently happened to think a lot about one of these perceptions - sight. These were afterthoughts of a discussion with a person I met during a train journey recently.

When we see something we don't normally think beyond the thing that was seen. We don't think about the physical process that happens when we see the object we are looking at. We don't realize that light waves reflected (or originating) from the object travels through the air and passes through our eyes and falls on our retina. The light that falls on the retina triggers signals on the optic nerve which in turn causes the sensation of sight once these signal reach the brain.

Television engineering was one of my favorite subjects during my undergraduate engineering course. It was very interesting because I was always fascinated about how a television works - how the images are translated to pictures in the television camera and how these pictures are transmitted to the television set where these are converted back to images.

Once the course was over the operation of a television looked simple and plain to me. However I still cannot fathom how human eyes work. Simple questions like the number of pixel elements on the retina and the way the signals are transmitted to the brain. Research has been going on to decode the signals that get transmitted through the optic nerve. I am sure that the person who finally decodes the signals correctly is going to get a Nobel price as the discovery is going to eradicate blindness in humans.

Decoding the physical process of the generation and transmission of signals on the optic nerve is probably going to happen soon; but what about the decoding the process in the brain that gives the perception of sight? What is sight? It is so complex that I cannot even begin to imagine a physical explanation for the perception of sight. This is somewhat similar to the discussion I had on the form of human memory. What happens when the electric signals from the optic nerve reaches the brain?

Now let me come to the point I would like to highlight. How do you define a color? For example how do you define Red? How many primary colors are there? What do you mean by a primary color? What is infrared? What is ultraviolet?

Let me try to answer these questions one by one. A color, for example red, is defined as something we see when an electromagnetic wave of a particular range of wavelength(625–750 nm) falls on our retina. There are three primary colors - Red, Green and Blue. All other colors can be generated as a combination of these three different colors in different proportions and different intensities. Infrared is defined as the range above the wavelength of red. Ultraviolet is defined as the electromagnetic spectrum below the wavelength of violet.

These answers look simple and looks almost like taken out of a Physics text book. There is one thing that we are taking for granted here. An electromagnetic wave does not have a property called a color. There is no such thing as color. It is just a perception that we get when electromagnetic radiation of a given wavelength falls on our retina. If by some quirk of nature, my retina response curve gets reversed and starts generating the same signal that your retina generates corresponding to light of 650nm and 380nm respectively, then I would start seeing red when you see violet.

There is a natural explanation for why we see the colors we see for the range of frequencies as defined by the visible spectrum. These are the frequencies of light that get reflected by the elements and compounds and mixtures that we see in nature around us. In other words, the rest of the spectrum (mostly) gets absorbed by these physical objects.

If we were on a planet that had a different set of elements (that would require a different set of sub-atomic particles and different forces acting on these particles than those that we already know) with a different set of absorption spectra then we would probably have seen the same colors(there is no need to think otherwise) but for different frequencies of the electromagnetic spectrum.

Colors exist only in our brains. Also for a given wavelength of light the colors that each of us sees are unique to ourselves since the physical response of my retina is for almost for sure different from the response of your retina. You will never see the red color that I see and my red color is always going to be my own personal red color.


  1. Yea, it's interesting when you think about it. The perception evoked by the same wavelength could be slightly to totally different for me and you, and there is no way of knowing for sure until somebody cracks the brain's code:)