By the end of the nineteenth century, scientists could describe most natural phenomena using Newton’s Laws of Classical Mechanics or Classical Theory. Matter and energy were thought to be separate and unconnected entities until then. Maxwell’s equations, developed by James Clerk Maxwell, a Scottish physicist, in 1873, enabled scientists to characterise the qualities of radiant energy. By the twentieth century, however, scientists had discovered that phenomena such as black body radiation and the photoelectric effect could not be described by classical theory or classical mechanics. During this period, the German physicist Max Planck proposed his idea of the quantised nature of electromagnetic wave energy.

Planck’s Quantum Theory

  • Until the late nineteenth century, Newtonian physics dominated the scientific paradigm. However, in the early twentieth century, scientists realised that the principles of classical mechanics did not apply at the atomic scale. 
  • Conventional light theories could not elaborate on the photoelectric effect because an elevation in light intensity did not end in a similar consequence as an increment in light energy.
  • Planck postulated that light energy is proportional to frequency, and Planck’s constant (h) is the constant that connects them. As an outcome of his studies, Albert Einstein discovered that light is composed of discrete quanta of energy known as photons.

Observations of Max Planck

  • When solids are warmed, they produce radiation of varying wavelengths. When an iron rod is fired in a furnace, it first gets dull red and gradually becomes much redder as the temperature rises.
  • As the temperature rises, the radiation released turns white, eventually blue as the temperature rises even higher. In terms of frequency, it indicates that the frequency of emitted radiation increases from a lower to a higher frequency when the temperature rises.
  • The red colour corresponds to the lower frequency area of the electromagnetic spectrum, whereas the blue colour relates to the higher frequency area.

Black Body and Planck’s Constant

  • A black body is an ideal body that produces and absorbs radiations of all frequencies, and the radiation released by such a body is known as black body radiation.
  • The precise frequency distribution of the emitted radiation (i.e., the intensity versus frequency curve of the radiation) from a black substance is determined only by its temperature.
  • At a given temperature, the intensity of radiation emitted increases as the wavelength decreases and achieves a maximum at a specific wavelength. It then begins to drop as the wavelength decreases further.
  • The wave theory of light cannot fully explain the aforementioned experimental results. Planck proposed that atoms and molecules could release (or absorb) energy only in discrete amounts and not in a continuous manner, contradicting a widespread assumption at the time.
  • The lowest amount of energy that may be released or absorbed in electromagnetic radiation was named a quantum by Planck.
  • This equation expresses the energy (E) of a quantum of radiation as a function of its frequency (v) as:

E= hv

The proportionality constant, ‘h,’ is also known as Planck’s constant, and it has a magnitude of 6.626 x 10–34 J s.

  • Planck successfully explained the distribution of intensity of black body radiation as a function of frequency or wavelength at various temperatures using this hypothesis.

Origin of Planck’s Quantum Theory

  • Electromagnetic Radiation (EMR) is a kind of energy that may go via a vacuum or a material medium and has both wave-like and particle-like qualities. Electromagnetic radiation includes radio waves, microwaves, infrared, visible light, UV-rays, X-rays, and gamma rays.
  • All things generate electromagnetic radiation based on their temperature. Low-temperature items emit radio waves or microwaves (low-frequency waves), whereas high-temperature objects release visible or ultraviolet light or even higher-frequency radiations.
  • A black body is a hypothetical item that can absorb all electromagnetic energy that comes into contact. It then begins producing thermal radiation in a continuous spectrum based on its temperature. Black body radiation is the radiation emitted by a black body.
  • The wavelength and temperature of the item influence the intensity of the radiation. The strength of the light at a given temperature changes with wavelength. Classical theory or Maxwell’s equation did not address this occurrence. As a result, Max Planck proposed his theory of quantisation of energy, often known as Planck’s quantum theory of radiation, to explain this occurrence.

Planck’s Quantum Theory Definition

  • Planck’s quantum theory discusses radiation emission and absorption. The following are the fundamental properties of Planck’s quantum theory:
  • Matter emits or absorbs energy in distinct amounts, unevenly in the manner of little packets or bundles.
  • Quantum is the tiniest bundle or packet of energy. A photon is a quantum of light in the context of light.
  • The energy of the absorbed or emitted quantum is exactly proportional to the frequency of the radiation. As a result, the radiation’s energy is represented in frequency.
  • A body or substance can emit or absorb energy in whole-number multiples of a quantum. In this case, n is a positive integer. So energy could be absorbed or radiated as hv, 2hv, 3hv, 4hv, etc., rather than 1.5hv, 2.5hv, etc.

E = hν 

  • Albert Einstein, a German scientist, examined Max Planck’s hypothesis and described the photoelectric effect.

Validation of Planck’s Quantum Theory

Many tests were carried out to investigate Planck’s quantum theory. The experimental findings confirmed and served as solid evidence for quantum theory. All of this demonstrates that the energy of electron motion in the matter is quantised. A prism may split light based on its wavelengths. If light behaves like a wave, the prism must generate a consistent rainbow. This also gave weight to Planck’s Quantum Theory. The emission spectrum of nitrogen gas also supports Planck’s quantum theory of radiation.

Conclusion

According to Planck’s theory, energy is not continuously released or absorbed but rather in packets known as quanta. It is referred to as a photon in the context of light.

Each photon contains energy directly proportional to the wavelength frequency, i.e. E is proportional to v. So, E = hv, where h is Planck’s constant.

Plank’s Quantum theory describes the quantum behaviour of energy through electromagnetic waves.