The photoelectric effect was first documented in 1887 by the German physicist Heinrich Hertz and is therefore sometimes referred to as the Hertz effect. While working with a spark-gap transmitter (a primitive radio-broadcasting device), Hertz discovered that upon absorption of certain frequencies of light, substances would give off a visible spark. In 1899, this spark was identified as light-excited electrons (also called photoelectrons) leaving the metal's surface by J.J. Thomson. One of Hertz’' former assistants named Philipp Lenard went on to study this effect and was awarded the Nobel Prize in physics for his efforts. In 1905, Albert Einstein explained the photoelectric effect mathematically by proposing the concept of light quanta, or photons. This conclusion runs counter to the classic understanding of physics and is better understood in the context of wave-particle duality.