Final answer:
Einstein's light quantum idea posits that light consists of particles called photons, whose energy is quantized and dependent on frequency, challenging the classical wave theory of light.
Step-by-step explanation:
Einstein's light quantum idea, which introduced the concept of photons, suggests that light behaves not just as a wave, but also as a particle, with energy that is quantized and proportional to its frequency. Recall that in interactions such as the photoelectric effect, light is seen as a stream of photons, each with energy E = hf, where h is the Planck constant and f is the frequency of the light. This has profound implications, as it showed that light intensity corresponds to the number of photons, and the energy of photons affects the kinetic energy of electrons ejected from metal surfaces when exposed to light above a certain frequency threshold.