At the end of the 19th century, several experiments produced results that classical physics could not explain. Physicists were forced to rethink basic ideas about light, energy, and matter.

A blackbody is an ideal object that absorbs and emits radiation perfectly. Classical theory predicted that hot objects should emit infinite energy at high frequencies, which is impossible. Max Planck solved this by proposing that energy is emitted in discrete packets, $E = hf$, where $h$ is Planck’s constant and $f$ is frequency.

When light shines on a metal surface, electrons can be ejected. Classical wave theory said brighter light should always eject electrons, but experiments showed that only light above a threshold frequency works. Einstein explained this by treating light as particles called photons, each with energy $E = hf$.

Atoms emit light at specific colors rather than a continuous rainbow. This means atomic energy levels are quantized. Niels Bohr used this idea to explain the hydrogen spectrum, even though his model was not the final answer.