I'm reading through George Gamow's Thirty Years That Shook Physics. This post finishes chapter 1, "M. Planck and Light Quanta." The previous posts were:
1a. Planck's Quantum
1b. Jumping Photons (Einstein and the Photoelectric Effect)
Today ends chapter 1 with "The Compton Effect."
The last part of chapter 1 is barely more than a page. It simply recounts the experiment of Arthur Compton in 1923. If on the smallest level light is made up of packets of energy, then these "photons" should bounce off of electrons.
But how to set up an experiment. Light at low frequencies in the best stream of electrons they could muster would be like dropping a baseball from a plane and trying to randomly hit a bird (sorry, my illustration). So Compton used X-rays, which because of their high frequency have very large quanta of energy. And he used carbon, which is relatively speaking a smaller element in which the electrons aren't as tightly bound.
The results of the experiment were consistent with Planck's hypothesis. Photons from the X-ray were absorbed by the electrons of the carbon and then expelled at a lower frequency in a deflected direction. Like it or not, Planck had been right.
Next Friday: Theories of the Atom