J. J. Tomson and others had already taught that atoms have
parts. Planck and Einstein were at work to explain what those
parts were and how they worked.
Planck studied how hot objects emit light; he called the
packets of radiation emitted by black bodies “bundles”
or “energy elements.” Einstein studied how metal
surfaces that absorb light emit electrons; he calculated that
photons had to be quantized. All elecromagnetic radiation is
absorbed and emitted in discrete packets, that is, in quanta.
See also in The book of science:
Readings on wikipedia: