What is the “photoelectric effect,” the phenomenon for which Einstein won the Nobel Prize in Physics in 1921?

The emission of electrons from a material when it is exposed to light

The photoelectric effect, explained by Albert Einstein in the early 20th century, revolutionized the understanding of light and matter. Einstein demonstrated that light, whether perceived as a wave or a stream of particles called photons, can release electrons from a metal surface if the energy of the photons is sufficiently high. This discovery was fundamental to the development of quantum theory and showed that light possesses both particle and wave properties. The photoelectric effect has practical applications in many fields, including solar cells, which convert sunlight into electricity, playing a crucial role in advancing renewable energy technologies.