longest, slowest wavelengths?

In regard to Why are red lights used in darkened control rooms and other low-light situations?, I have to nitpick because I’m not sure what Hawk meant:

“Red light, for those interested in the physics of these things, has the longest, slowest wavelength of any form of visible light and thus the least energy–as you shift toward the blue end of the spectrum, wavelengths shorten and energy levels rise.”

What does “longest, slowest” mean? Certainly, in a vacuum, all light travels at the same speed (approx. 3x10^8m/s), but in a medium such as air or glass or water, higher frequencies, shorter wavelengths (i.e., blues and violets) actually travel slower than their lower frequency, longer wavelength red siblings (and in air, it is virtually unnoticeable).

Maybe it should read, “Red light…has the longest wavelength and lowest frequency of any form of visible light…”


And I’m scratching my head over why he chose to say:

Infrared isn’t “heat” any more than visible light is “heat”.

Where do they get these SDSAB people anyway?

Infrared isn’t “heat” but people are used to seeing them associated - think “heat goggles”, for example. Every movie/TV show where they use IR pics you get this explanation that “hotter objects show in the infrared”. Which is true, just not a complete version of the truth.

That line about “the truth, the whole truth and nothing but the truth” cracks me up. Nobody knows the whole truth and it would take way too long to tell.

I was always taught that heat was transmitted in 3 ways - convection, conduction and radiation, i.e infrared radiation. If that is the case, I’d like to know whether other wavelengths of electromagnetic radiation also raise temperature (like, does red light just raise temperature to a lesser extent than infrared) or is it dependent on the material that the radiation hits, like microwaves heating water molecules, for instance.

I was always taught that heat was transmitted in 3 ways - convection, conduction and radiation, i.e infrared radiation. If that is the case, I’d like to know whether other wavelengths of electromagnetic radiation also raise temperature (like, does red light just raise temperature to a lesser extent than infrared) or is it dependent on the material that the radiation hits, like microwaves heating water molecules, for instance.

Spare me. Britannica says, “Invisible to the eye, [infrared] can be detected as a sensation of warmth on the skin.” Neither EB nor Hawk makes any claim that infrared radiation and heat are coterminous.

Later edit by Ed Zotti. Oops, pressed the wrong button and obliterated Brain Wreck’s comment to which the above was meant to be a reply. My apologies.

Every type of radiation carries energy, and therefore transmits heat. It just so happens that “hot” objects we are likely to encouter (fire, burning charcoal, light bulb, electric heater, etc) have temperatures between a few hundred and a few thousand degrees, and therefore they mostly emit infrared radiation.

Higher temperature objects emit at shorter wavelengths. The sun is about 6000 K, and therefore most of its radiation is in the visible light. So most of the heat from the sun is carried as visible light.

I assumed “slowest” was a reference to frequency, as you suggest. Not as precise as saying “lowest frequency”, but I think that’s somewhat understandable when writing for a general audience. (But on the other hand, is “frequency” really that technical a term?)

But I second Brain Wreck’s complaint about the whole heat = infrared thing; that seems like it’s perpetuating a common misconception.

scr4 and brain wreck are right, but I’d like to add something.

The wavelengths of absorption and emission are two different principles. When emitting light, most objects follow what is called a “blackbody spectrum,” and it doesn’t change too much among common substances. Ie, there is a sort of bell-curve of frequencies, and the peak moves toward higher frequencies (aside from also rising in amplitude) the hotter something gets. Things slightly above room temperature glow in the infrared, but heating things further will make them glow red, then white, then blue. People used to judge the temperature in a furnace based on the color of the thing glowing inside it.

When absorbing, the nature of the material matters a bit more. An electron in an atom will only absorb a photon of an energy such that it gets bumped from one orbital to another. Of course, it will also emit in the frequency also, so the nature of the material alters emission as well. However, because when absorbing you’re dealing with materials that are cool and have their electrons in mostly the same state, while when emitting you’re dealing with hot materials with electrons all over the place (and the phenomenon of emission deals with precisely those electrons which aren’t in the ground state), it just so happens that the nature of common materials affects the way they absorbs light more than it does how they emit it.

You make an excellent example of water and microwaves. However, the very phenomenon of color is that of absorption. If something is red, it means it’s not absorbing that frequency. I used to have a camcorder that could see in infrared, and some plastics (especially those used with devices that have remote controls), are clear in the infrared.

Another thing, food is heated using infrared/red heat lamps mostly because infrared will penetrate it better and won’t just crisp the surface.

So yeah, the connection between infrared light and heat is mostly circumstantial. Of course, heat is molecules moving, and the statement “infrared light is heat” is completely misleading when taken at face value.

Okay, if you want to really picky maybe a couple words could have been slightly improved upon.

But I liked it and look forward to seeing more Hawk.

I don’t think this is true. Most food items are opaque to infrared light, and there wouldn’t be any significant penetration. I believe heat lamps are used because it can heat uncovered food items from all directions, and is pretty safe, unobtrusive and efficient. (Microwave would be dangerous and expensive; visible light would have to be painfully bright; blowing hot air would make the room uncomfortably hot.)

Not at all. The spectrum of light which an object absorbs is identical to the spectrum it emits. Not even just really close, exactly identical. Any difference at all would lead to violations of the Second Law of Thermodynamics.

Most real objects will interact with some frequencies more strongly than with others, due to energy-level transitions, but even in the most idealized case (say, monatomic hydrogen), the spectral lines are not infinitely narrow. And most familiar substances are made up of molecules, which can have a very large number of very closely-spaced spectral lines. The net result of this is that any photon can be absorbed; some are just easier to absorb than others. But if an object is dense enough, and non-reflective enough, or has a sufficiently irregular surface, it’ll still end up absorbing most of the photons that hit it, regardless of frequency. Such an object appears (approximately) black, and when heated, it emits (approximately) a black-body spectrum.

Oh, and contrary to what Pink Floyd would have you believe, in most common media, redder light is, in fact, slower than bluer light. There may be some esoteric substance for which the reverse is true, but for your standard water, air, or glass, the longest wavelengths are, in fact, the slowest.

Kudos, Chronos. I’d never noticed that before. I’ll have to point it out to my (now-former) advisor, who uses the cover of “Ummagumma” to illustrate a certain concept to his set theory classes.

I’m sitting in a physics class right now, and my professor says you’re crazy. So does my textbook, and so does Wikipedia (at least the picture).

So, I’m not going to personally call the almighty Chronos crazy, but could you elaborate on this, and why every picture (and live demonstration) of a prism I’ve seen shows the light refracting like it does in the Pink Floyd logo?


The Floyd illustration and Wikipedia photo are correct. And Chronos is correct about red light being slower in most optical media.

The key is this: when going from a high speed of light medium to a low speed of light medium (e.g., air to glass), the faster wavelengths will be refracted less. In the other direction, the faster wavelengths will be refracted more. Sketch yourself a prism and do a little ray tracing, you’ll see that the color order is indeed correct.

[nitpick]Not a Pink Floyd logo as such, just part of the album cover art for Dark Side of the Moon.

Further to the Pink Floyd prism, it’s well known that it’s not strictly correct, ref:


And for the CD cover, the condemnation is stronger:

In all cases, the faster wavelengths will be refracted less. If, for a particular wavelength, the speed is the same in both media, that wavelength will not be refracted at all. If the speeds are very similar in both media, then that wavelength will be refracted by only a small amount. When one of the media is vacuum or something like it (as is the usual case with prisms), the fastest wavelengths will have the most similar speeds between the two media, so they’ll see the least refraction, going either way.

The Pink Floyd cover is, of course, just a drawing, so it’s not surprising that they got it wrong. I don’t know what’s going on with the Wikipedia image (originally from a NASA EPO page), but it looks like it might be at least partly “enhanced” as well.