The Photoelectric Effect | Einstein's 'Nobel' Contribution To Quantum Physics

In 1900, German Physicist Max Planck established the relationship between the energy of a particle of light and its frequency using a fundamental constant known as the Planck's constant. Though he was able to explain the black body spectrum using his formula, he thought it is useless. It has no practical applications.

But in 1905, Albert Einstein used Planck's ideas to explain the photoelectric effect, which is the phenomenon of emission of electrons from a metal surface when light of some frequency shines on it. The photoelectric effect was discovered back in 1887 by Frank Hertz. There were three experimental facts about the photoelectric effect that we knew:

1. Photoelectric effect is an instantaneous process
2. There exists a cut off frequency (wavelength) above (below) which the photoelectric effect cannot take place.
3. The kinetic energy of the emitted electrons only depends on the wavelength of light used. It does not depend on the intensity of light.

Huygens wave model could not explain the photoelectric effect. Firstly, according to the wave model, the electron could take minutes to be knocked off the metal's surface if we are using a weak source of light. But, it takes place on the timescale of nanoseconds.

Secondly, if we are using a bright source, the kinetic energy of the emitted electron should be more. But it does not depend on the intensity of the source. It only depends on the wavelength of the light that is being used.

Einstein explained the photoelectric effect by considering light to be made up of particles. He described it as a collision between two particles. Einstein won the 1921 Nobel Prize in Physics for his explanation of the photoelectric effect.

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[physicsjourney]

If the frequency of the source and cut of frequency of metal are equal. Kinetic energy is equal to zero, then how the photo electric effect is possible ?

[PradeepKumar-ugut]

You said that if we continue to illuminate a metal surface with a source that has frequency less than cut off frequency, the photoelectric effect will take place sooner or later, how Einstein explained this?

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[debdip]

Hi! Explanations are excellent. Can you pls explain - what if light of frequency greater than the threshold is incident, and then the intensity of this source is incident?? Doesn't it mean that more no of photons are being incident on the surface per unit time? Does that increase the photocurrent?