The Electromagnetic Spectrum V - Visible Light

Crossposted at Dailykos.com

Now that we are past infrared, we can talk about something that we actually can see. Visible light occupies the region from about 400 nm (nanometers) to 800 nm, with violet on the short side and red on the long. Thus, red blends into infrared at around 800 nm.

Like most EMR, visible light may be generated by a hot body (the so called “blackbody” radiation) wherein the frequency distribution maximum is determined by the temperature. This is why we use terms in photography having to do with temperature, because the distribution is a function of temperature. As the temperature of the emissive body increases, the wavelength decreases. That is why stars with relatively low surface temperatures appear red, while hotter ones, like our sun, are yellow, and very hot ones appear blue.

Visible wavelengths also come from other processes, including low energy electronic transitions. More right away.

The spectrum of hydrogen in the visible is composed of four lines at 410 nm, 434 nm, 486 nm, and 656 nm. The first two are very in the blue, the third blue green, and the last one red. The implications of this series are of central significance to our understanding of the way that atoms are put together.

Most folks know that a hydrogen atom has a single proton (for the most part) and a single electron. At temperatures with which we are accustomed, that electron is the lowest energy level possible, the 1s orbital. It can not get any closer to the proton because of quantum mechanical restraints, but can get further away. Each step further away requires energy input, either as heat, electrical input, or other energy input. When energy is input, if it is sufficiently great, the electron may occupy a higher energy level, but those are limited in number, once again due to quantum mechanical considerations.

Let us look at the red (656 nm) line in the hydrogen visible spectrum. That emission occurs when an electron in the second excited state, with a quantum number of 3, drops to the next lower one, with a quantum number of 2. Both of those levels are not populated at “normal” temperatures, but can be populated easily enough in a “neon light” tube of by high temperatures. I know that this sounds a bit esoteric, but the ramifications are revolutionary.

The folks who figured out why this pattern happened were the giants just before quantum theory and the giants who developed it. Names like Balmer, Rydberg, and especially Bohr are legend. When Bohr dealt with it, he had a bit of difficulty with the discrete energy differences, but the evidence was just too much and he developed a cogent mathematical model that explains the spectra. By the way, there are other series wherein electrons fall back to the 1 level, or to the 3, or even 4 level, or that fall from other higher levels to ones other than 2. Actually, there is hardly a limit on how many transitions can occur.

I will get a bit more technical just for a second. The transition upwards with increase in energy is also bound by the Heisenberg uncertainty principle, and the Frank-Condon rules of vertical transitions. If there are questions I will address them, but that is out of the scope of this diary. The point is, even though these phenomena are quantized, there are more than only one quantum number, and each has to be considered. Most downward transitions are from the vibrational ground state of the excited electronic state, but there are exceptions.

What does all of this mean? Light is the closest thing that we have to magic. It allows us to see, and also allows us to interpret the physical universe. It is pushing the limit for posting for the daily cycle now, so I will do a follow up on other ramifications of visible light, such as the mechanism of vision, next time.

I will hang around a bit for questions and comments. Warmest regards, Doc.

Tags: EMR, Teaching, Visible