Presentation: quantum leaps

Let's quiz you on your knowledge of local landmarks. Where is this sign located, and what business is it for?

Where is this sign located, and what business is it for?

Where is this sign located, and what kind of business is it for?

Where is this sign located, and what business is it for?

Where is this sign located, and what kind of business is it for?

Where is this sign located, and what kind of business is it for?

Now turn to your neighbor--what are some things that you notice about all these signs?

All these signs have tubes with different types of gas atoms inside. Exciting the electrons of these atoms with electricity produces photons of different colors. If this is starting to sound (or smell) like chemistry, well, it is--and we'll be covering nearly two weeks of introductory chemistry in this presentation, and apply these concepts to the subsequent in-class activity.

But don't worry, as we can distill the chemistry we need to know for astronomy into two simple, fundamental rules.

First, the electron rule.

In this simple model of the simplest atom--hydrogen--an electron orbits the central proton in one of these circular orbits. But only in one of these orbits, and not "in-between." (The innermost allowed orbit near the proton is the "ground state.") The diagram is scaled in terms of energies rather than true distances, so the innermost allowed orbit has the lowest energy, and each subsequent outer orbit has a higher energy. (To keep things simple, we'll ignore the multitude of orbits above level 5.)

Second, the photon rule.

Typically the cost in energy to move an electron from a lower energy, inner orbit to a higher energy, outer orbit is paid for by absorbing a photon that has exactly that specific amount of energy--no more, no less, as "exact change is required" when comes to electrons in atoms.

For an electron to move from a higher energy, outer orbit to a lower energy, inner orbit, it must emit a photon that has exactly that specific amount of energy.

For the purposes of this course, we are merely taking these two rules as granted, and emphasize the consistent application of these "quantum leap" rules. If you are wondering about other types of quantum leaps not illustrated above in the subsequent in-class activity, there is an unspoken rule of quantum mechanics: "Anything not forbidden is allowed."

(As an aside, there is a train of thought that one should not question the fundamental rules of quantum mechanics and instead "shut up and calculate," for the ends justify the means.)

Consider hydrogen gas in a "discharge tube." Hydrogen is normally colorless. Let's turn it on.

When we zap the hydrogen in this tube with just enough electricity (or with enough heat) energy we can pay for the electrons to move up to outer orbits. When the electrons subsequently drop down to inner orbits, then they will give off certain photons corresponding to certain colors. In this case, the colors of these photons (violet, blue, and red) are perceived by our eyes as lavender. So these electrons are bombarded with electrical energy just to jump up, so they can jump down to give off photons. This is essentially a torture chamber for the hydrogen electrons, zapping them to get them to give off this color--if you listen closely to the discharge tube when it is on, you can actually hear the electrons screaming.

This actually occurs in a very thin outer part of the sun, which is normally not visible without special filters, or during a total solar eclipse, to block out everything but this thin layer of hydrogen that is at the right temperature for electrons to jump up, and give off that characteristic lavender combination of photons as they jump down. Outside of this zone it would either be too hot or cool, and electrons would not have the correct energies to be able to jump up to orbits in order jump down and give off photons. So keep in mind that the chromosphere is a special layer where the sun is torturing hydrogen electrons to make them emit this lavender color. In the next two presentations we will discuss other zones of the sun, and also other ways light can be produced.

If you have different atoms, their orbits will have different energies, and will absorb and emit photons of different colors. Going back to our landmark signs, colors other than lavender are produced by tubes with containing gases other than hydrogen. (Still more colors can be produced by other gases and mixtures of different gases, and by also coating the glass tubes as well.) Next time you see a large "neon sign," (note only orange-red is produced by neon) count how many unique colors there are, and think about all the different types of gases used in that sign.

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