Tuning the Laser When the photons are bouncing off the atom, the electron in the atom is absorbing the photon and jumping up to a higher level, and then it quickly jumps back down and spits the photon back out. Didn't we learn that you need to have exactly the right color of light to make an atom do that? Yes, each type of atom responds only to exactly the correct color. A big part of the difficulty in making laser cooling work was to get enough light of just the right color. For this you need lasers for which you can adjust the color very precisely to match the color that the atom wants. If the color is wrong the photons just go right through the atoms, and it is difficult to get the color just right! Yes, that is just what happens with real atoms and lasers, except that a real atom is even pickier than the ones in this demonstration. Just the vibrations in the floor of a normal building make the color of a laser change so much it doesn't match the atoms any more. Scientists have to work very hard to make the laser light stay the proper color.

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