Photoelectric Effect

If a metal surface is illuminated by visible or ultraviolet radiation, electrons are released - provided the frequency of the radiation exceeds a critical threshold. The number of electrons increases with the intensity of radiation, but the energy of the electrons does not. This effect was explained by Einstein assuming that the electromagnetic radiation is quantized.

In Einstein's model, it was assumed that a minimal energy, the work function $\phi$, must be given to an electron before it can escape from the surface of the metal. The work function is a property of a given metal and is in the range 1 to 10 eV. No emission occurs unless hn > f, but when the energy exceeds the work function, the electron can be emitted with kinetic energy (KE) as large as:

KEmax = h$\nu$ - $\phi$.

The value of KEmax can be measured using a simple circuit in which light of frequency $\nu$ is shone on a metal surface that has a retarding voltage Vr applied to oppose the emission of electrons.

We steadily increase Vr until the current vanishes. At that point the voltage will be the critical retarding voltage, Vro, where

eVro = h$\nu$ - $\phi$ = KEmax

where e is the charge of an electron, e = 1.6·10-19 C.

The current vanished when

Vr > Vro

 

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