what is the photoelectric effect?
Answers
The photoelectric effect refers to what happens when electrons are emitted from a material that has absorbed electromagnetic radiation.
Light with energy above a certain point can be used to knock electrons loose, freeing them from a solid metal surface, according to Scientific American. Each particle of light, called a photon, collides with an electron and uses some of its energy to dislodge the electron. The rest of the photon's energy transfers to the free negative charge, called a photoelectron.
Before Einstein, the effect had been observed by scientists, but they were confused by the behavior because they didn't fully understand the nature of light. In the late 1800s, physicists James Clerk Maxwell in Scotland and Hendrik Lorentz in the Netherlands determined that light appears to behave as a wave. This was proven by seeing how light waves demonstrate interference, diffraction and scattering, which are common to all sorts of waves (including waves in water.)
So Einstein's argument in 1905 that light can also behave as sets of particles was revolutionary because it did not fit with the classical theory of electromagnetic radiation. Other scientists had postulated the theory before him, but Einstein was the first to fully elaborate on why the phenomenon occurred – and the implications.
For example, Heinrich Hertz of Germany was the first person to see the photoelectric effect, in 1887. He discovered that if he shone ultraviolet light onto metal electrodes, he lowered the voltage needed to make a spark move behind the electrodes, according to English astronomer David Darling.
Then in 1899, in England, J.J. Thompson demonstrated that ultraviolet light hitting a metal surface caused the ejection of electrons. A quantitative measure of the photoelectric effect came in 1902, with work by Philipp Lenard (a former assistant to Hertz.) It was clear that light had electrical properties, but what was going on was unclear.
According to Einstein, light is made up of little packets, at first called quanta and later photons. How quanta behave under the photoelectric effect can be understood through a thought experiment. Imagine a marble circling in a well, which would be like a bound electron to an atom. When a photon comes in, it hits the marble (or electron), giving it enough energy to escape from the well. This explains the behavior of light striking metal surfaces.
While Einstein, then a young patent clerk in Switzerland, explained the phenomenon in 1905, it took 16 more years for the Nobel Prize to be awarded for his work. This came after American physicist Robert Millikan not only verified the work, but also found a relation between one of Einstein's constants and Planck's constant. The latter constant describes how particles and waves behave in the atomic world.
Further early theoretical studies on the photoelectric effect were performed by Arthur Compton in 1922 (who showed that X-rays also could be treated as photons and earned the Nobel Prize in 1927), as well as Ralph Howard Fowler in 1931 (who looked at the relationship between metal temperatures and photoelectric currents.)
When light of some suitable frequency falls on a metal surface, electrons are ejected out from the metallic surface. This process is called as photoelectric effect.
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