According to classical theory, as a wave such as an x ray penetrates matter. it affects every electron in the region through which scat- tering of the x ray is due to the combined effect of all these electrons. On the other hand. in the photoelectric effect a single quantum of light a photon causes the ejection of a single electron. Thus. Compton reasoned that from the quantum point of view. a single ray photon" should interact with a single electron.