And Why Should We Care? Of Atoms, Mousetraps, and the Power of the Stars
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a heavier thing because there is no energy being added, whereas there is always the chance it can shed some energy and become lighter. Of course, the third option is that it does nothing and stays the same, and sometimes that is the case. For the hydrogen atom this means that the heavier version will eventually shed some of its mass. It does so by emitting a single particle of light, the photon we met earlier. For example, a next-to-lightest hydrogen atom will at some point spontaneously convert into a lightest hydrogen atom as a consequence of a change in the orbit of the electron. The excess energy is carried away by a photon.* The reverse process can occur too. A photon, if one just happens to be around, can be absorbed by the atom, which then jumps to a higher mass because the energy absorbed promotes the electron to a higher orbit. Perhaps the most everyday way of getting energy into atoms is to heat them up. This causes the electrons to jump up into the higher orbits and subsequently drop back down again, emitting photons as they go (this is the physics behind a sodium vapor street lamp). These photons carry an energy that is exactly equal to the energy dierence between the orbits, and if we could detect them, we would have a direct window into the structure of matter. Fortunately, we are detecting them all the time because our eyes are nothing more (or less) than photon detectors, and the energy of the photons is registered directly as color. The azure blue of an island-pitted tropical ocean, the jagged diamond yellow of Van Gogh’s stars, and the iron-red of your blood
* The energy taken away by the photon is equal to 13.6 eV minus 10.2 eV, which is 3.4 eV.