Fri 27 Sep, 2013 09:32 pm
One problem in physics that has always intrigued me is color. What is color? Why is something one color, and another a different color even though the same light is hitting both objects? In grammar school and high school, I remember my teachers telling me that a leaf is green because it absorbs all other colors of light, and reflects only green light. This explanation never made sense to me and I never believed it. I have always wondered what happens to light on a microscopic level that causes only certain colors to bounce back into our eyes.
As you know, light from the sun is made up of all colors: red, orange, yellow, green, blue, indigo, and violet. Well, there are other “colors” as well, but the human eye cannot see them; these include ultraviolet, infrared, microwave, gamma rays, etc. In physics, we do not refer to these as colors but rather electromagnetic waves. The portion of the electromagnetic spectrum that humans can see is referred to as visible light, or color. Each color has a certain energy value and a certain frequency of oscillation (vibration) associated with it. The frequencies of visible light range from 7.5 x 1014 (violet) to 4.3 x 1014 (red).
When we begin to look at things microscopically, we begin to see that everything oscillates, or vibrates. Atoms, when bonded together to form a molecule, act as objects connected together by springs, free to bounce back and forth, or oscillate. What exactly are bonds between atoms?: Nothing more than electrons being shared between the atoms, in ionic cases. Each type of molecule has a unique pattern of atoms and bonds, making it oscillate in its own unique way. Furthermore, physics has discovered a property called the photoelectric effect. When light shines on an object, it excites (moves more rapidly than normal) the electrons on its surface. Why does this make sense? Well, light is made up of electricity and magnetism. Basically, throwing electrical forces at electrons makes them move. Physicists find this fascinating; I’m not surprised, are you? So we know two things: all materials vibrate microscopically and all materials are affected electrically by light. If we combine the two ideas, we can say that bonds are affected by light, and therefore light has an effect on the way molecules vibrate.
Still not seeing the big picture? Well, let’s pretend we are a ray of yellow light vibrating along through the air toward a banana (a ripe one). Fact: yellow light happens to vibrate 5.2 x 1014 times per second. The molecules in the banana are joined in a way that vibrating at 5.2 x 1014 times per second really tickles their fancy. In physics, this is called resonance. The banana molecules vibrate more easily at 5.2 x 1014 times per second than at any other frequency, so when yellow light hits them, they really start moving! This allows the yellow light to bounce off the molecules more efficiently than all the other colors of light. This is much like double-jumping someone on a trampoline; when you jump at the same time, you end up bouncing higher! So now, let’s imagine that we are yellow light vibrating along towards an orange (or tangerine, or cutie). Since the orange molecules don’t like to shake it like we like to shake it, our frequencies clash and the light just doesn’t bounce the same. This is much like dancing with someone who is off beat.
Here is the coolest part. Since color depends directly on chemical bonds and atomic structure, it can really tell us a lot about the molecular makeup of just about anything! You may have heard about this in your chemistry class. In chemistry (and physics), we use spectrometers to capture light that is bouncing off a substance to generate spectral lines. Each element in the periodic table has a unique set of spectral lines. These lines are nothing more than an indication of which colors resonate with a certain substance. In other words, the lines indicate which vibration frequency tickles the atoms or molecules fancy. To further investigate color, we can think about materials that are clear or translucent: like a diamond. Diamonds have a very rigid structure, making it difficult for the molecules to vibrate. If the molecules don’t vibrate, then we do not see any color. All objects resonate at more than one frequency, including frequencies that are not part of the visible spectrum. However, by using infrared or thermal imaging cameras, we can “see” those vibrations and translate them into a temperature reading.
How many other properties can be determined by color? To my knowledge, we have measured chemical makeup of stars, the speed of galaxies in the universe, temperature, electrical properties of materials, photosynthesis, and I am sure many more. Knowing the relationship between light and physical objects opens up many possibilities in science. Understanding it is the first step.
You may or may not be aware that the issue of color perception has been central to some modern themes in philosophy and cognitive science. Physics is only part of the story.
1. Consideration of color was a key issue that caused Ludwig Wittgenstein to reject his own celebrated Tractatus Logico Philosophicus
for a change in thinking found in his Philosophical Investigations
2. Empirical studies of color perception and physiology by Franscisco Varela (et al) have been the sub-area which has driven "embodiment theory" in cognitive science as an alternative to (failed) informational models which start from physical signals.
Not sure I understand what is so 'alternative' about all this, which is standard physics material. Yes, atoms and molecules resonate with certain specific wave lengths of light and the types of spectral rays a given object absorbs and emits tell us something about its chemical composition.
I find the biological, mental and artistic aspects of color more intriguing, personally. Like the fact our retina have three different color receptors (cone cells) activated with a broad enough spectrum that they overlap and we can see all wave lengths in the visual range. This is why those so-called 3 primary colors are enough to code for any color. TV screens can trick us to see violet or brown or black or white simply by mixing three colors because
our eyes work with approximately the same three colors. If our eyes had receptors for 4 colors, TV screens would probably need 4 colors to work for us. Primary colors are literally
in the eye of the beholder. And we can see millions of different colors just with these 3 types of cone cells.
Another fascinating and quite mysterious aspect is color as a mental object, a representation of wave lengths that looks attractive or not, joyful or sad, etc. We don't 'see' colors, we create those mental representations based on a combination of neuronal signals. Which is why we cannot be certain that everybody 'sees' (represents) colors the same way.
Finally, society and language play an important role. The division of the visual spectrum into distinct groups such ss red, orange, yellow, etc. is purely conventional. Some languages such as Persian do not distinguish between blue and green for instance.
Great explanation, I'm going to copy and save it (put it in my art files).
Speaking as a physicist, it is important to keep what we perceive as color (which is psychology rather than physics) from the frequencies of light. Sometimes the same words are used for both, but they aren't the same thing.
If you don't understand the difference, then tell me what frequency is the color brown.
There are things that are orange that don't reflect any light with the "orange" frequency. If something reflects light of the "red" frequency and the "green" frequency (at the same time) your brain will see that it is orange.
Thank you for embellishing my point 2 above.
Thank you for embellishing my point 2 above.
You misunderstood me. My take is based on the idea that senses and brain collect, display, classify and analyse information that exists objectively in this world.
And for someone who doesn't believe in selves, you come across as rather self-centred...
I haven't the slightest idea of what you're talking about.
I'm talking about the demise of informationalist models in AI and cognitive science. I thought perhaps because you cited linguistic factors in color perception you might understand some of its philosophical implications (expounded by Varela and Rosch. My apologies if you don't.
AI proponents in the 60s and 70s were just utterly naive, like today's neuronal scientists are, and like you are. Humbly recognizing how much mystery remains out there (and in our own bodies and minds) should be part of any scientist's training. Would save them and you some embarrassment.
This does not mean that the world is not full of information, or that my piece of Camembert magically disappears from the universe when I turn my attention elsewhere...
What are sympathies from someone who does not exist worth?