How did Einstein's explanation of the photoelectric effect challenge classical physics and contribute to modern technological advancements?

Einstein explained that light is made of tiny packets called photons, each carrying a fixed amount of energy. When a photon hits a metal surface, it can give enough energy to an electron to leave the metal; if the photon energy is too low, no electron leaves, even if the light is bright, contradicting the idea that light’s intensity should matter. This showed that classical waves couldn’t describe the photoelectric effect, forcing physicists to accept that light behaves in both particle and wave ways. The new quantum picture later led to inventions like photodiodes and solar cells, where electrons are moved by light to produce electricity. Thus Einstein’s work not only overturned old physics but also paved the way for devices that turn light into useful power.