This installment of “It’s Obvious. Not!” looks at: Periodical: “Physical Review” Volume: 21 Page: 483, 486 Title: "A Quantum Theory of the Scattering of X-Rays by Light Elements" Author: Arthur H. Compton Excerpt from page 486: In the above excerpt, Compton discusses how to calculate the momentum of an electron that caused x-ray or gamma scattering. The momentum added to the electron is the momentum of the incident photon minus the momentum of the scattered photon. Problem: The angle of scattering, (theta), appears to be mislabeled in the above figure vs. the usage of the angle in formula 1. In formula 1, Compton calculates the magnitude of the electron momentum as the vector difference of the momentum of the incident and scattered photons. To subtract two vectors, you place their tails together, the resulting vector that points from the head of the second to the head of the first is the difference vector as shown below and described in this Wikipedia article . To get t...