Photoelectric effect

Overview

Diagram showing the basics of photoelectric effect

A metal surface when exposed to radiation energy (like x rays, Gamma rays, UV rays, etc.) liberates electrons which can constitute electric current if a the is connected to a suitable conductor such as a wire. This phenomenon of emission of electrons due to exposure of a metal surface to radiation is called photoelectric effect.

History

German Physicist Heinrich Hetz

In 1887, German physicist Heinrich Hertz was experimenting by shining beams of ultraviolet lights on metals. He know this that the UV rays caused the metal surface to shoots sparks. This did not surprise him much though what did surprise him was that the frequency of light which wood cause the metals to shoot sparks was different for different metals.

He noticed carefully that increasing the brightness of the light caused increased production of electrons even though the energy of the electrons did not increase full stop on the other hand increasing the frequency of the light cause the emission of electrons of increased energy even though the number of electrons emitted did not increase. This phenomena could not be explained by henrich but later by a young scientist name Albert Einstein in 1905.

Albert Einstein was later awarded the Nobel prize for his achievement in explaining the photoelectric effect completely.

Explanation

Before you understand the photoelectric effect we shall understand a few important terms at first:

1. Planck's constant => a fundamental physical constant denoted by letter 'h'. The frequency of a photon when multiplied by planck's constant gives us the energy of that Photon. So,

Let, f

requency of photon = v

Plank's constant = h

Total energy of Photon (E) = vh

Where,

Plank's constant (h) = 6.62607015 × 10^-34J/s

2. Threshold frequency => The minimum frequency that a photon must have to cause emission of electrons from metal surface viz. well cause photoelectric effect. It is different for different metals because of their work functions.

Remember that photoelectric effect is also possible for non-metals to a limited extent due to their high threshold frequency and sometimes although rarely with fluids such as air and water.

3. Work function => The minimum amount of energy required to move an electron to a infinite seperation from the surface of a solid usually a metal is called the work function of that solid. 

More is the work function, more will be the threshold frequency, infact for photoelectric effect to occur the threshold frequency must always be equal to the work function of the solid of not more.

So, here's what the photoelectric effect is all about -

We all know that light consists of photons. Suppose a break off light falls on a metal surface such that,

Frequency of photon = v

Plank's constant = h

Work function of metal = W

Threshold frequency = f

For photoelectric effect to actually happen,

'v' must always be greater than 'f' i.e. v > f 

Then the prom will be able to knock off an electron from the metal surface given,

v > W

So, to take the electron or of the metal surface 'W' amount of energy is used up and the electron so emitted is now left with (vh - W) amount of energy. This electron is not free and can constitute current if allowed to flow by use of conductors such as wire. This current can be harvested as electricity.

Factors affecting photoelectric effect

The photocurrent i.e. the current produced from the emission of electrons due to photoelectric effect depends mainly on for factors :-

1) Frequency of Incident radiation 

Increase in the frequency of incident light causes emission of electrons of more energy although there is no noticeable increase in the number of electrons emitted.

2) Intensity of Incident radiation

Increase in the intensity of incoming radiation causes increase in the number of electrons emitted although there is rise in the energy levels of the electrons emitted.

3) Potential difference across electrodes

With an increase in the potential difference between the electrodes, the net flow of photocurrent increases.

4) The material used

Metals are preferable because they can loose electrons more easily, although glass is also sometimes used.

So, to increase the overall efficiency of a photoelectric system increasing in the frequency and intensity of the radiation as well as the potential difference across the electrodes and a good metal used like copper will yield the best result.