In 1886, Heinrich Hertz, who was the first to demonstrate that electromagnetic waves can be artificially generated, noticed that a negatively charged electroscope could be discharged by shining ultraviolet light on it. Hertz’s observation caught the attention of J. J. Thomson, who inferred that the ultraviolet light was causing the electrode to emit electrons, thus restoring itself to electric neutrality. The emission of electrons from a substance due to light striking its surface came to be called the photoelectric effect. The emitted electrons are often called photoelectrons to indicate their origin, but they are identical in every respect to all other electrons.
This discovery might seem to be a minor footnote in the history of science, but it soon became a, or maybe the, pivotal event that opened the door to new ideas.