01 / The world before
Classical models did not explain all observations of light and electrons.
02 / The discovery
Einstein's explanation linked light energy to quanta.
03 / The people
Einstein and experimental physicists.
04 / The obstacles
Turning theory into devices took further engineering.
05 / The world after
The explanation strengthened quantum physics.
06 / Why it still matters
Photoelectric principles underpin some modern sensors.