What really is Color (in Physics)?

I’ll begin this with a sticky note that I’m not at all good at explaining these biological “stuff”. But as I’ve read a lot of posts on vision, repeating the same bunch of words again & again, I’ve got some grasp on the topic. Well, it’s still worth a try. Moreover, this post would be a strong base for other future posts related to vision. Because, it’s often a misconception that color is an intrinsic property of light. This may be a reason why physicists are often tackled by color questions like, “Why doesn’t sky look violet?”, etc.

Okay, now define “color”…

First, let’s stick with the truth that light is made of the elementary particle, the quanta of electromagnetic radiation called photons. The theory of quantum electrodynamics (which I haven’t prepared for you, yet) best explains the behavior of these tiny objects that whizz around here and there, so fast that no known “thing” can overtake it. That’s enough for now.

What’s color? It can be defined in many ways. For now, (keeping aside, the technical parameters like hue, saturation, brightness, etc.) Color is the visual perception of things that emit, transmit or reflect light in the world. In other words, it’s simply the sensation produced in our eyes when electromagnetic radiation of certain frequency (i.e) specific photons impinge the retina of the eye. That’s why “color” means nothing to Physicists.

So, these  questions regarding perception, sensation, etc. should always target biologists (psychologists or physiologists, to be specific). I should remind you that there’s nothing so special in the different kinds of EM waves (as everyone think they’re). They’re not at all different. All are the same EM radiation. They’re classified as IR, UV, Gamma rays, etc. just because various range of frequencies have a wide variety of applications in different fields of study (stupid terminology). It should be noted that the “visible light” range is allotted in the spectrum only because that range (400-720 nm) supports our vision. If we were rattle snakes (IR sensors) or the bees (UV sensors) or intelligent creatures with antennae – ALIENS..!!! (at least our eyes are modified like that), then we’d have to redefine visible light. That’s why I suggest people not to believe such dodgy sayings that visible light is something very special, given a place in the EM spectrum.

Clockwork behind eyes (shallow’ish)

Objects in the outside world, say, green leaves aren’t really green. They absorb almost all wavelengths in the visible spectrum, while reflecting green. That’s why they appear green. If you shoot a natural scene, with an IR camera, you’ll arrive at the conclusion that these leaves reflect near-infrared too…

The job of our eyes is to detect the information that worldly objects reflect. These eyes don’t have much gears and wheels (at least in a physical point of view). There are two kinds of vision – photopic (normal daylight) and scotopic (diffusely illuminated). In order to work efficiently, our eyes have two kinds of photo-receptors. They are the cone cells (photopic) and the rod cells (scotopic). The iris does the job of directing light, whether to cones or to rods…


Rods are not so much important (despite their large number, around 120 million), as they don’t help us enjoy the taste of nature. They just show us black, white & gray (yuck..!!!). Because, rod cells (being tuned to scotopic vision) work only at dim light (dark room or say, at night), when the irises open up, letting more light into the retina. Hey, you should’ve experienced this as you shift from a well-lit room to a dark room, when you can easily notice the eyes adapting to the environment. As time passes by, the rods become very active (10,000 times over a period of 30 minutes) in capturing photons (unlike cones, which are active in triggering the optical nerve), but weak in detecting colors. Wonderful design…


We’re always used up with the well-lit view. Thanks to the 7 million cones for letting us enjoy the beauty of things around. Destroy those and you’ll go blind. Under photopic conditions, the irises are stopped from opening outward. Cone cells, being at the center of retina are mostly struck by light photons now. As an aside business, let’s see their classification. There are three types of cone cells (no special names) – Short, Medium and Long wavelengths (abbreviated S, M & L), each sensitive to a peak wavelength (i.e) “S” to 420 nm (bluish-violet), “M” to 534 nm (bluish green) and “L” to 564 nm (yellowish-green). Plotted below, is the response curves of the three cones (responsivity vs wavelength). For the sake of difficulty, understanding or whatever, let’s just say blue, green and red cones and go along with the mechanism behind perception…

So, what happens inside?

Different frequencies excite the cone cells (all the three) at different proportions. It’s our brain’s visual cortex that reconstructs these excitations into perceivable visualizations (the beauty). For example, if the light photons excite all the three cells equally, the color appears to be white. No excitation, it’s black. Or say, the photons smash (excite) the red and green cells equally, but unexpectedly, a lot of excitation happens at the blue cells (Duh, modulation is the fancy term for excitation). Now, there’s our blue. Note that there’s an overlapping in the response curves. What it means? No known light sources can generate something which excites only a single receptor. All the three are excited to some specific percentage. Just like you play around mixing different colors of light, the different proportions of excitations of cones correspond to different colors. Both are unambiguously analogous…

Still, there’s a necessity. While our rods (being so sensitive) can detect even a single photon, the cones are quite lazy (at least the brain and the optical nerves are), that they require a threshold of 50-100 photons (on average) arriving within a second to give a response.

Eyes of camera & humans

Most of the remotes use a diode for emitting near-infrared radiation to communicate with the television. The way the cones are excited modulated is very different than that of a digital camera. Don’t ever think that a camera reproduces the view of the environment exactly as seen by your eye. It just mimics the perception of eye. The colors you see in a camera and through the eye are similar, but not exact (You can notice the difference in a few photographs) Because, the natural cones in eyes and the artificial CCDs (charge-coupled device) in camera are very different. So, your eye sees apparently nothing, while the camera sees some color (purple, in this case) And, this depends on the camera you use, as different cameras make use of different filters. (Well, most of the cameras’ filters allow near-IR, which is enough for a remote) At least, this can be used to test a remote. So, are you still sticking to the definition of visible light?

Why isn’t the sky violet then?

If you still remember the XKCD comic, and you’re so much interested in the “Why?”, I’ll try to explain, having read the Physics FAQ article. I presume that you all know about Rayleigh scattering law (although Tyndall found it, “Lord” gets the credit), that the amount of scattering varies as inverse as the fourth power of the wavelength of light. According to this abstract statement, we should see violet sky, the violet in the rainbow, etc. There are possible reasons…

  • The Sun doesn’t emit radiation of constant frequency. The irradiance vs. wavelength plot shows that more photons are emitted in the 500 nm range.
  • As the incoming light passes through the upper O_2 and N_2 atmosphere, much of the violet light gets absorbed by the molecules, leaving only a few to penetrate the inner levels. Still, there should be a violet tinge on the observed blue sky. But, it’s not..!!!
  • So, all the remaining credits go to our eyes

Our blue and green cones are very sensitive to blue (Go, see the peak wavelength in the response curves again – bluish & bluish). Both are excited by the largely scattered blue light, in addition to the least scattered red and orange light (triggering the red cone), moderately scattered yellow & green light (green cone) and the highly scattered violet (blue cone). All these equalize each other out (I mean, the cells are almost equally excited), leaving blue cones to relatively much higher excitation.

There you go… Blue Sky…


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