Note 2 — Chapter IV · DISCOVERY AND NATURE OF X-RAYS
Reading edition
When ultra-violet light is allowed to fall upon a metal it causes the metal to emit electrons and thus to acquire a positive charge, the velocity of the emitted electrons being exactly proportional to the frequency of the incident light. Or when light of X-ray type falls upon the surface of almost any substance, it takes hold of an electron in the atoms of that surface and hurls it out into space with a speed exactly proportional to the wave length of the light. This phenomenon is known as the photoelectric effect.