Dual Nature of Radiation and Matter: Work Function of Metals, Photoelectric Effect (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The dual nature of matter and the dual nature of radiation were revolutionary concepts of physics.
  • At the beginning of the twentieth century, scientists unravelled one of the best-kept secrets of nature – the wave particle duality or the dual nature of matter and radiation.

Work Function of Metals

  • It is referred to minimum amount of energy required to take out one electron from the metal surface.
  • So, This energy can be supplied through various sources such as a strong electric field or irradiation of light with a suitable frequency.

Photoelectric Effect

  • It is the phenomenon in which the electrons are emitted from the metal surface when the light is illuminated in a suitable frequency.
  • The photoelectric effect is the conversion of light to electrical energy.
  • It follows the law of conservation of energy and is an instantaneous process.
  • Photoelectric current depends upon three factors:
  • The potential difference applied within the two electrodes
  • The incident light՚s intensity
  • The nature of the emitter material
  • Factors affecting stopping potential
  • The incident light՚s frequency
  • The emitter material՚s nature
  • The frequency of the incident light does not depend on the intensity.
  • Stopping potential is directly proportional to the kinetic energy of the emitted electrons.

Laws of Photoelectric Effect

The laws of the Photoelectric effect are as follows:

  • For a given metal and frequency of incident light, the photoelectric current is directly proportional to the intensity of incident light.
  • For a given metal, there is a certain minimum frequency, called Threshold frequency, below which there is no photo-electric emission.
  • The maximum kinetic energy of photoelectrons above a threshold frequency depends upon the frequency of incident light.
  • The photoelectric emission is an instantaneous process.

Heisenberg՚S Uncertainty Principle

  • Its Principle states that both the momentum and position of a particle cannot be determined simultaneously.
  • Mathematically, it can be expressed as ∆ x ∆ P ⩾ (h/4π) where ∆ x represents the Uncertainty in position, ∆ P represents the Uncertainty in Momentum.

Quanta or Photon

  • Light is composed of various discreet packs of energy which are called quanta.
  • Einstein explained the photoelectric effect on the basis of this theory. Each photon posses an energy E = (hv) and momentum and it depends on the frequency (v) of light incident and never on the intensity.
  • In metal surface, photoelectric emission occurs due to the photon absorption capability of an electron.

Electronic Emission

  • Minimum energy that is required to emit an electron from the surface of a metal can be supplied to the free electrons by either of the three methods given below:
  • Thermionic Emission: Required thermal energy is provided to the free electrons by suitably heating it so as to enable them to come out of the metal.
  • Field Emission: Electrons are kept under the strong influence of the electric field to emit the electron out of the metal.
  • Photo-electric Emission: When the light of appropriate frequency is made to illuminate a metal surface, electrons are emitted from it. These photo-generated electrons are called Photoelectrons.