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Excitation and deexcitation: Day 1, P1
Let there be light!
Summary of what you've learned so far
1. Different atoms have different energy levels.
2. When an atom jumps from one energy level to a lower one, it emits a photon.
3. The energy of the photon is equal to the difference between the atom's energy levels. In other words, the photon carries the energy that was previously stored in the atom.
4. For visible light, the color of a photon is related to its energy. Blue photons have the most energy, green photons have less energy, red photons have the least energy of all. Infrared and ultraviolet photons are invisible. Infrared photons don't have enough energy to stimulate your eye, ultraviolet photons have too much.
For a real world example of all this think about heating an iron rod. As it gets hotter and hotter it starts to glow. At first it glows red because it only has enough energy to emit red photons. As it gets hotter it turns white because it is now emitting photons of all colors. And before it was hot enough to glow, you could feel the heat radiating from it in the form of invisible, infrared photons. (If you heated it enough the photons would become invisible again: they would be ultraviolet. But the rod would melt before it got that hot.)
Now here's a chance to play around and see if you can learn more. In the model below, try changing the energy levels of the atoms and see what happens. Can you make green photons? Can you make ultraviolet ones? Do you think energy is conserved in these interactions?
I'D LIKE TO BE ABLE TO SHOW THE OCCUPANCY NUMBERS OF THE ENERGY STATES IN THE ENERGY LEVEL DIAGRAM. IS THERE A WAY TO DO THAT? I'D ALSO LIKE TO GIVE THE KIDS A BUTTON WITH WHICH TO TURN ON A LIGHT SOURCE (AND WHEN IT'S ON THEY GET A SLIDER WITH WHICH TO CHANGE THE FREQUENCY OF THE LIGHT SOURCE)
What happens if you give an atom multiple energy levels? Try it and see!
In our model an atom can go from any energy level to any other. If it goes from a higher level to a lower one, it emits a photon, to go from a low energy level to a higher one it has to absorb a photon. In either case, the energy exactly balances -- the energy of the photon is always equal to the difference between the energies of the two atomic levels.
This means that every atom has a unique and characteristic set of photons that it can either emit or absorb, corresponding to all the possible differences between its energy levels. So just by looking at the photons that are emitted or absorbed by a bunch of atoms, we can tell which atoms they are! That's a very powerful thing to be able to do, and it will be the next thing we study in the unit on -- TA DAH!...
Spectroscopy.
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