Heinrich Rudolf Hertz was a German physicist who is best known for his work on the electromagnetic theory of light. He is particularly famous for his discovery of the photoelectric effect, which laid the foundation for the development of modern quantum theory. Hertz also demonstrated the existence of electromagnetic waves, confirming the theories of James Clerk Maxwell. His work has had a profound impact on the fields of physics and engineering, and the unit of frequency, hertz (Hz), is named in his honor.