Light and Color

[Ensign Steve]

I've been stumped. What property of light causes different colors to have different wavelengths? The best that I could come up with (wild guess!) is that the wavelength is determined by the frequency at which the photons are ejected from the source of the color. Dorian thinks that can't be the case because even a single photon of light would have the property of color. But since any single photon would travel at the speed of light, I can't figure out what property of the photon would determine the wavelength or color.

[Hugo Holbling]

Originally Posted by Ensign Steve

The best that I could come up with (wild guess!) is that the wavelength is determined by the frequency at which the photons are ejected from the source of the color.

The colour is determined by the wavelength of the emitted photon. Since wavelength and frequency are related, you are quite correct.

Dorian thinks that can't be the case because even a single photon of light would have the property of color.

That's incorrect, alas, and all part of the wave/particle duality that seems such a mystery. Waves have colour, but not particles. Light is both, somehow.

But since any single photon would travel at the speed of light, I can't figure out what property of the photon would determine the wavelength or color.

You're asking the wrong question, i think. The colour of an object is determined by which wavelengths of light it absorbs, not any property of photons as particles. Light seems to have both a wave-nature and particle-nature, with wavelength being a characteristic of the former but meaningless when applied to the latter.

[Ensign Steve]

Originally Posted by Hugo Holbling

The colour is determined by the wavelength of the emitted photon. Since wavelength and frequency are related, you are quite correct.

Is that "emitted photon" singular? Can a single photon have its own wavelength?

That's incorrect, alas, and all part of the wave/particle duality that seems such a mystery. Waves have colour, but not particles. Light is both, somehow.

You're asking the wrong question, i think. The colour of an object is determined by which wavelengths of light it absorbs, not any property of photons as particles. Light seems to have both a wave-nature and particle-nature, with wavelength being a characteristic of the former but meaningless when applied to the latter.

Suppose, for example, that a short burst of light struck a colored object so quickly that only one photon of light had time to escape. Would the reflected light have the property of color? My friend thinks it would, but according to my theory (wild guess), it couldn't because it would take at least two photons to determine the wavelength. What do you think?

[Hugo Holbling]

A single photon does have a wavelength and the light would have colour. Not only that, but interference patterns still occur when only one particle is reaching a slit in the famous experiments that led to the wave-particle understanding of light.

Why do you think it would need two particles?

[The Lone Ranger]

How is it that light has wavelength, frequency, and color?


Wavelength:

Keep in mind that the photon in question is vibrating in addition to its forward motion. (Actually all particles vibrate and thus have wave properties. For macroscopic "particles" -- like baseballs, say -- the magnitude of the vibration is so small compared to the object itself, that their wave-like behavior is completely overwhelmed by their "particle-like" behavior.)

So, imagine that you're tiny -- roughly the size of an atom -- and that you can watch a single photon zip by, and that it leaves a visible trail as it goes. As the photon moves forward, it is also moving in the (let's call it) "up and down" plane. So, the trail made by its passage will look exactly like a sine wave.



The wavelength, then, is simply the distance the photon moves forward in the time it takes to complete one vibration -- that is, to go from "up" to "down" and then back to "up."


Frequency:

The more energy the photon has, the faster it vibrates. Therefore, the shorter will be its wavelength. That's why high-energy "light" like x-rays and gamma rays has very short wavelengths, while low-energy "light" like radio waves has very long wavelengths.

The frequency, of course, is a function of the wavelength, since it's the number of cycles per second. In the case of a photon, for all practical purposes, the frequency is the same thing as the vibration rate.


Color:

"Color" is simply how our brains interpret the wavelength(s) of light that are falling on the retinas of the eyes. For example, if a photon with a wavelength of about 700 nanometers hits an appropriate receptor cell ("cone") in the retina, the brain interprets this as "red." A photon of about 600 nm wavelength is interpreted as "orange." A photon of about 500 nm wavelength is interpreted as "bluish-green," and a photon of about 400 nm wavelength is interpreted as "violet."

"White light" consists of light of all (visible) wavelengths. (That is, that's how the brain interprets light of all wavelengths.) Objects that are emitting light may not emit light of all wavelengths, though, of course. So, for instance, if the burner on your stove is emitting mostly red light, you'll see it as glowing red.

For objects that are reflecting light, their "color" is determined by which wavelengths they happen to reflect. An object that absorbs all wavelegths of light but those that fall in the "green" portion of the visible spectrum will appear green, for instance, since it's returning green light to the viewer's eye, but not other wavelengths.


The Electromagnetic Spectrum




Hope this helps . . .

Cheers,

Michael

[Shake]

Yes, frequency and wavelength are related by: c = <frequency in Hz> * <wavelength in m>, where c is the speed of light (in m/s, (3*10^8, roughly)).

Wow, this "quick edit" thing ROCKS!

[Ensign Steve]

Hugo, I thought it would need two particles if the frequency of the wave was determined by the frequency (in time) of the ejection of photons from the color source. It would take at least two to determine the length of the cycle, ya know? But like I said, that was out of left field, and Michael's explanation makes a lot more sense.

Thank you, Michael, that does help a lot. Believe it or not, I learned a whole bunch of that very same stuff today in school, but it was about radio waves, not visible light. Of course, good answers lead to even more questions.

1) I thought that a light wave (or any EM wave) was 90 degrees to itself, you know, in three dimensions, not 2 like a sine wave. How does the up-down movement of the photon correlate to the up-down-side-side motion of the EM wave?

2) Do non-visible forms of radiation also have that wave/particle duality, or just visible light? I think that photon means light-particle, do other EM waves also have particles that behave the same way? I think gamma rays have particles which is why they are so dangerous, but I never thought about any others.

[The Lone Ranger]

Hi JD,

Doubtless, a real-live physicist could explain things better than I, but I'll do my best.

Light indeed consists of an electromagnetic wave with two components -- an electrical field and a magnetic field at right angles to each other. The photon is said to be the "carrier" of the electromagnetic force, as its motion generates the wave.

At the level of individual photons, there's no clear distinction between the wave-like properties and the particle-like properties of the photon. Still, for practical purposes, the moving photon (but then, photons are always in motion, more or less by definition) generates the fields. Or maybe the moving fields generate the photon. After all, light is both an electromagnetic wave and a photon. Whether it behaves more like a wave or more like a particle depends upon what you're trying to measure. (Somebody once said that if you think you understand quantum physics, you're wrong.)


Here's a neat little java applet that shows how the electrical and magnetic fields oscillate at right angles to each other and to the direction of motion.



All electromagnetic waves are "light." It's just the case that we can see only a tiny portion of the electromagnetic spectrum. The less energy a photon has, the slower its "vibration," and so the longer is its wavelength (and, consequently, the lower its frequency).

Photons with the least energy have wavelengths measured in kilometers for the very lowest energy levels, up to wavelengths of about a meter for relatively high energy levels. This portion of the electromagnetic spectrum is what we call "radio" waves. It's simply low-energy light.

"Microwaves" are somewhat more energetic, and so have wavelengths in the centimeter to millimeter range.

"Infrared" radiation is more energetic still, but not as energetic as visible light.

"Visible light" is that very narrow band of the electromagnetic spectrum to which our eyes happen to be sensitive. It ranges from least energetic (red) to most energetic (violet).

More energetic than violet light is "ultraviolet."

Beyond ultraviolet light are "x-rays," and finally, the highest-energy (and thus shortest wavelength) light waves are "gamma rays."


High-energy light is dangerous precisely because it has so much energy. This makes it quite good at penetrating into matter. Ultraviolet radiation can penetrate into the skin, where it damages DNA molecules, sometimes causing skin cancer in the process. Higher-energy x-rays can go right through flesh, but are stopped by denser bone. Since they penetrate deeper into the body than does ultraviolet radiation, x-rays can cause damage to cells deeper in the body.

Gamma rays have such small wavelengths that to them, the human body is mostly empty space. That's fortunate in a way, because they can pass right through your body without hitting anything in the process. So, an individual gamma-ray photon is actually less likely to cause damage than is an x-ray photon. On the other hand, gamma rays are so energetic that they can damage individual atoms, should they impact them. This means that any gamma photon that happens to be absorbed by body tissues will do quite a bit of damage in the process.


Cheers,

Michael

[Clutch Munny]

Colour is only co-determined by wavelength. TLR is right to point out that colour is a matter of how our visual systems respond to light conditions. That response (ie, the perceived colour) is affected by a bunch of other things, including ambient light conditions, the prior state of the visual system, and dynamical properties of the input (as, for example, when the wavelength changes rapidly). Even under "normal" viewing conditions there are discontinuities between wavelength and perceived colour.

[Ensign Steve]

Thanks, guys, that helps a lot. That aplet is neat. I had seen something like it on video before, but now I have it to point people to.

[livius drusus]

Great posts, all. Not only do I know far more about light and color than I did a week ago, but Michael also cleared up a few things for me about how Bruce Banner became the Hulk. Thank you.