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# Photoelectric Effect and System of Particles The phenomenon of emission of electrons from the surface of the metal when the light of suitable frequency is incident on the surface of the metal is called the photoelectric effect. The electrons that get emitted in this process are called photoelectrons. The process of emission of the photoelectrons is called photoemission. The photoelectric effect helps in the clear understanding of the concept of the quantum nature of light.  The photoelectric effect is based on the principle of the law of conservation of energy.

### Photoelectric effect

During the photoelectric effect, the electrons on the surface of the metal absorb the energy from the incident light, and it is used to overcome the attractive force that binds the electrons to the nucleus. The phenomenon of the photoelectric effect is explained by considering light as a stream of particles called photons. The energy of the photons can be related to the frequency of the light by Planck’s equation.

Ephoton = hν = hc/λ

In the above equation,

• E is the energy of the photon
• h is Planck’s constant
• ν is the frequency of light
• c is the speed of light
• λ is the wavelength of the light

From Planck’s equation, it can be seen that different frequencies of light will have photons of different energy. The minimum energy of the photons required to remove the electrons from the metal is called the threshold energy. For the photons in the incident light to have an energy equal to the threshold energy, the incident light should have a frequency equal to the threshold frequency. The threshold frequency is the minimum frequency of the light that is needed for the phenomenon of the photoelectric effect. The relation between the threshold frequency and the threshold energy is given by the relation

Φ = hνth

Here, Φ is the threshold energy

νth is the threshold frequency

The kinetic energy of the electrons that get emitted from the surface of the metal due to the photoelectric effect can be related to the energy of the photons by the following equation.

Ephoton = Φ  + Eelectron

⇒ hν = hνth+ (½)mv2

If the frequency of the incident photons is lesser than the threshold frequency, there will not be any emission of photoelectrons. If the frequency of the photons is exactly the same as the threshold frequency, then the kinetic energy of the emitted electrons will be equal to zero.

The value of the threshold energy is different for every metal. This is because the attractive force between the electrons and their nucleus is different for different metals. The phenomenon of photoelectric effect can also be observed in non-metals, but the threshold frequency for non-metals is extremely high.

### Characteristics of Photoelectric Effect

1. The value of threshold frequency is different for different materials.
2. The intensity of the incident light will determine the value of the photoelectric current.
3. The stopping potential depends on the frequency and the process is instantaneous.

Do check BYJU’S for more physics topics such as System of Particles. The system of particles can be either discrete or continuous. The particles are separated from each other in a discrete system of particles, and in a continuous system, the separation between the particles is very small.