What you are looking at
Light strikes a metal plate (the
cathode) sealed inside a vacuum tube. If electrons are
knocked loose, they fly across to the
collector on the right, completing a circuit that
registers as a
photocurrent on the meter. A battery lets you put a voltage on the
collector to help or hinder the electrons. This is the experiment that forced physics to accept that
light comes in particle-like packets.
Light arrives in photons
Einstein's 1905 explanation: light of frequency f delivers its energy in discrete
photons,
each carrying
E = h f = h c / λ
One photon gives all its energy to one electron. To escape the metal an electron must pay an energy cost
called the
work function φ. Whatever is left over becomes the electron's kinetic energy:
KE_max = h f − φ
So there is a
threshold frequency (and threshold wavelength λ₀ = hc/φ): below it, no single
photon has enough energy and
no electrons come out, no matter how bright the light. This is
the crucial clue that classical wave theory could not explain — a bright low-frequency beam should
eventually shake electrons loose, but it never does.
Frequency versus intensity
Turn up the
intensity and you send more photons per second, so more electrons are freed and
the
current rises — but each electron's energy is unchanged. Turn up the
frequency (shorter wavelength) and each photon is more energetic, so the electrons come out
faster (higher KE_max) — but the number is unchanged. Energy depends on colour; quantity
depends on brightness.
Stopping voltage
Make the collector negative and it repels the electrons. Increase this retarding voltage until even the
fastest electron is turned back and the current drops to zero. That
stopping voltage
directly measures the maximum kinetic energy:
e V_stop = KE_max
The graph below plots KE_max against frequency. It is a straight line whose
slope is Planck's
constant h (the same for every metal) and whose x-intercept is the threshold frequency. Measuring
that slope is exactly how Millikan pinned down h in 1916. Switch metals and watch the line shift sideways
while keeping the same slope.