Lifting a rock upwards takes energy. Dropping that rock back down releases energy – energy we could use to, say, smash a walnut. Lift the rock again, exactly as high as before.
It takes exactly as much energy as the previous lift, and, when the rock drops, it releases exactly the same amount – the same quantity – of energy again. The energy is quantized. You and I, and everything around us, are made of atoms.
Every atom has a nucleus, surrounded by a cloud of electrons. Each electron tends to hang out at a certain distance from the nucleus. Among atoms of the same type (atoms of carbon, atoms of oxygen) these distances are always the same.
Much as it takes energy to lift a rock, it takes energy to move an electron farther from the nucleus – to “excite” an electron. Much as a falling rock repays the energy it took to lift it, an electron falling back to its original level in the atom repays the energy it took to excite it. The electron emits that energy as light, the specific hue depending on the amount of energy in each jump.
Red is relatively low-energy light; orange is higher, yellow higher yet, through the rainbow: Red-orange-yellow-green-blue-indigo-violet (the initials spelling out ROY G. BIV). Juggling several balls, the juggler plays catch with herself: She tosses the first ball just so high, giving herself time to toss a second ball upward, and to also catch a third ball, previously tossed.
Key to this game of catch is timing: The balls must be tossed to t.
