The Experiment and Its Surprising Laws

Shine light on a metal plate and, under certain conditions, it ejects electrons. This is the photoelectric effect, discovered by Hertz in 1887. The setup is simple: a light beam strikes a metal cathode in a vacuum, and the ejected electrons produce a measurable current.

   light (frequency ν)
        \ \ \
         \ \ \        e-  ->  e-  ->  (current)
   ========[ METAL ]========

The observations were baffling for the physics of the time, which saw light as a continuous wave:

   What the classical wave predicts    What is actually observed
   --------------------------------    ---------------------------
   more light -> more energy           energy does NOT depend on intensity
   any light eventually ejects         below a threshold: NOTHING, ever
   it takes time to build up           emission is INSTANTANEOUS

Three facts resist any wave explanation. First, there is a threshold frequency ν₀: below it, no electron is ejected, even with very intense light. Above it, emission occurs even for faint light. Next, increasing the intensity increases the number of electrons, but not their energy. Finally, the energy of the electrons depends only on the frequency of the light.

A classical wave, whose energy depends on intensity, can explain neither the threshold nor the instantaneity. A new idea is missing — the one that will bring light into the world of quanta.