Photo Electrons

When light strikes a clean material’s surface, electrons are emitted from that surface under some specific conditions. This phenomenon is well known as the photoelectric effect. The electrons are called photo-electrons. When light strikes on the surface electrons of the material gain sufficient energy from light to overcome the work function of the material and hence emitted from the surface.

What is Work Function?

It is defined as the minimum energy required to remove an electron from its shell (outer most shell of the atom). It is measured in eV (Electron-volt). According to classical physics the number of emitted electrons and their momentum is independent of the frequency at which the light strikes the material, but it depends upon the intensity of the incident light energy.

But in actual experiment this statement of classical physics could not be observed. Instead, it is observed that for a particular incident light intensity, the kinetic energy of the emitted electrons varies with the variation of the frequency of the incident light. That means if the frequency of incident light is changed without changing its intensity, then the kinetic energy of emitted electrons or photo-electrons also changes.

What are Photo Electrons?

When light energy of sufficient intensity strikes on a surface of material, some electrons of the material very close to the surface, gain sufficient energy to overcome the work function of the material and are emitted from the surface with kinetic energy. These emitted electrons are called photo-electrons. It is also observed that, above a certain frequency of the incident light, the electrons better to say photo-electrons start emitting. This is the minimum frequency of incident light below which no photo-electrons is produced.

But above this minimum frequency, the kinetic energy of the electrons varies linearly with frequency. It is also observed that when intensity of incident light is changed by keeping its frequency constant, the rate of photo-electrons emission changes accordingly but the maximum kinetic energy of electrons remains unchanged. In 1900, German Physicists Max Karl Ernst Ludwig Planck stated that the heat radiation from a hot body surface is in the form of discrete packet of energy and these packets of energy are called quanta. This quantity of energy consisting in a single quanta or emitted packet of energy is hf where f is the frequency of radiation and h is the constant called Planck constant that is the quantity of energy,

donor impurity in semiconductor

Quantum energy is proportional to the frequency of radiation. The value of Planck constant is

In 1905 another famous German physicist Sir Albert Einstein stated that the light also consists of energy packet and this packets were referred as photons. The energy content in the photon is also given by same equation and that is
E = hf or E proportional to f.
Only difference is that here the frequency is the frequency of light.

As per this theory, when a light strikes on a material surface, photons with sufficient energy (when frequency is sufficient as E proportional to frequency) knock electrons on the surface of the material. The electrons gain sufficient energy to be removed from the surface, after overcoming the work function of the material. The rest of the energy in a photon is consumed to gain the kinetic energy of the electron. The maximum kinetic energy of an electron can be represented as

Where, hf0 is the work function or minimum energy required to remove the electron. h0 is the minimum frequency of incident light below which no electron is removed from the surface.
The photons energy or energy consisting in a photon of a particular light is proportional to the frequency of that light and hence that is inversely proportional to the wavelength of the light wave

Want To Learn Faster? 🎓
Get electrical articles delivered to your inbox every week.
No credit card required—it’s 100% free.

About Electrical4U

Electrical4U is dedicated to the teaching and sharing of all things related to electrical and electronics engineering.

Leave a Comment