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Photoelectric Effect And Dual Behaviour Of Electromagnetic Radiation

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Photoelectric Effect And Dual Behaviour Of Electromagnetic Radiation - Lesson Summary

In 1887, Hertz performed an experiment in which he observed that when the surface of certain metals such as cesium, potassium, sodium, and rubidium exposed to a beam of light, then electrons were ejected. This is called as photoelectric effect. The electrons thus emitted are called photoelectrons.


Observations:

0 to 1 electrons were ejected from a metal, if the incident light has frequency of less than a certain minimum frequency value called the threshold frequency.

It is observed that the kinetic energy of the ejected electron depends upon the frequency of the incident light but not on the intensity of the light radiation.

Ex: when the metal potassium is irradiated with high, intense red light having frequency of 4.3×1014 to 4.6×1014 Hertz for hours together, no electron is ejected. But as soon as it is irradiated with even less intense yellow light having frequency of 5.1×1014 to 5.2×1014 Hertz, photoelectrons are ejected. As the threshold frequency for potassium metal is 5.0×1014.

Einstein explained photoelectric effect using Planck's quantum theory of electromagnetic radiation. To overcome attraction forces on electrons, the photons of incident light must possess a certain minimum energy so as to release an electron from the metal surface.

If the energy possessed by the photon is high then the energy transferred to the electron is also high and that makes high kinetic energy of the ejected electron.

Greater is the intensity of incident light or the number of photons that strike a metal, greater is the number of electrons ejected.

Ex: When more and less intense yellow light is made to shine on potassium metal separately, then relatively more number of electrons will be ejected from the potassium metal irradiated with high intense yellow light.

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