Stern's experiment

Stern’s experiment involved measuring how the frequency of light emitted by electrons changes when they are accelerated. Using a setup similar to what is now called the photoelectric effect, he observed that electrons emitted from a metal surface under light exposure had energies that depended on the frequency of light, not its intensity. This demonstrated that light interacts with electrons in discrete packets, or quanta, leading to the idea that energy exchange occurs in fixed amounts. This experiment was foundational in establishing the concept of quantization in quantum mechanics, showing that energy levels are not continuous but discrete.