Science Friday: Jumping Photons (2)

Last week I (foolishly) started blogging through George Gamow's Thirty Years That Shook Physics. (Foolish because I wonder whether my puny brain will even make it through the first chapter).

When last we met our hero (old quantum theory), Max Planck had just suggested that the energy of light radiation does not exist at every continuous quantity but that the higher the frequency of light radiation, the bigger the "packets" in which that energy existed. Planck didn't like it. The suggestion was an "act of desperation." He did it to make the theory match the experiment. But he hoped it wasn't part of the basic nature of energy but some quirk having to do with atoms.

But Einstein dashed his hopes to pieces, five years later in 1905. Einstein didn't win the Nobel Prize for relativity, special or general. Einstein didn't get the Nobel Prize for E = mc². Einstein won the Nobel Prize for demonstrating that Planck's "quantum" was in fact very much real. Even though light behaved like a wave on a large scale, on the smallest scale, light was little packets or "quanta" of energy, which Einstein called, "photons." The size of these quanta increased with frequency.

What Einstein did was show that Planck's quantum explained one of the puzzles of late 1800s physics: the photoelectric effect. The photoelectric effect is the fact that, below a certain threshold frequency, the electrons in the surface of a metal plate jump free with the same amount of energy no matter how intense of light radiation you shine on it. As you increase the intensity of the light on the surface at a lower frequency, the number of electrons jumping increases but the amount of energy each of them has does not.

However, once you reach the threshold frequency, which is different for different metals, the amount of energy the electrons have as they jump begins to increase. In fact, after not increasing at all, the amount of energy begins to increase evenly with the increase in frequency.

Planck's E_photon = hf explained this phenomenon. The energy of a photon increased evenly with the increase of frequency.

Next week: The Compton Effect