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QUANTUM THEORY

What is Quantum Theory?: An Overview About It..

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INTRODUCTION OF QUANTUM THEORY

The cornerstone of modern physics is quantum theory, also known as quantum physics or quantum mechanics. This theory is basically the physics of the microscopic world, i.e. the world inside atoms. This branch of physics started at the very beginning of the 20th century. In quantum theory we encounter phenomena or truths that are beyond our ordinary experience, that seem unreal at first glance. Even the scientists who laid the foundations of quantum theory were not the least surprised by several conclusions reached through theoretical analysis. All of the experiments that have been done to date to test this theory have strengthened its foundation.

QUANTIZED QUANTITY & QUANTUM

There are many figures in everyday life that can only be of a few selected values. These values are usually obtained by multiplying integers with some initial quantity. Such a quantity is called a quantized quantity and the primary quantity is called a quantum of that quantity.

In 1900, scientist Max Planck introduced his quantum theory. Despite the incredible success of the wave theory of light, it was not possible to explain some phenomena like blackbody radiation, photoelectric effect, atomic spectra, etc. with its help. Originally, Planck developed quantum theory while explaining the spectrum of blackbody radiation. Einstein’s later introduction of the photon particle concept further established this quantum theory.

Quantum Theory
The Structure of Atom

PROPERTIES OF ELECTRON

(i) CHARGE: Electron charge is negative. Its value is e = 1.6 X 10-19 C (SI) = 4.8 X 10-10 esu (CGS). [1 esu = static-electric unit of charge].

Various experiments show that the charge of any given particle is quantized and the quantum of charge is the electron charge e. That is, an impurity value can be 2e, -5e, 1000e, etc., but such values as 1.5e, -2.8e are unrealistic.

Quark: Quarks are elementary particles that have no independent existence in nature. According to the quark model, particles like protons, neutrons etc. are composed of quarks.

(ii) REST MASS: Constant mass of electron, m0 = 9.1 X 10-31 kg (SI) = 9.1 X 10-28 g (CGS). If the speed of the electron is sufficiently less than the speed of light, there is no difference between its constant mass (m0) and effective mass (m). Then it can be said, m = m0.

(iii) KINETIC ENERGY OF ELECTRON: ELECTRONVOLT: When attracted by a positive charge (+ve), the electron’s velocity increases, so its kinetic energy also increases; On the other hand, the electron’s kinetic energy decreases when it is repelled by a negative charge (-ve). The kinetic energy of an electron is usually expressed in units of electron-volts (eV).

Definition: The amount of increase in kinetic energy of an electron when it passes through a potential difference of 1 V is called 1 eV.

1 eV = Charge of an electron X 1V

        = 1.6 X 10-19 C X 1 V

        = 1.6 X 10-19 J

        = 1.6 X 10-19 X 107 erg

        = 1.6 X 10-12 erg

eV is a very small unit compared to erg or Joule; So, it is used only in atomic science, it is not used elsewhere. 1 KeV = 10³ eV; 1 MeV = 10⁶ eV.

OBSERVATION OF HERTZ

  • In 1887, the scientist Hertz observed that when ultraviolet rays (UV) were strikes to the electrode of an almost airless vacuum tube, a large number of sparks began to be generated in that region. He hypothesizes that the UV rays produce a large number of radioactive particles in the tube, which are identified as the cause the formation of spark.
  • Almost contemporaneous experiments by other scientists revealed that exposure to ultraviolet rays reduced the negative charge on the cathode plate. This can only happen when negatively charged particles are released from the cathode surface. In addition, it can be seen that, instead of ultraviolet rays, a similar phenomenon occurs when violet, blue, green light of a relatively short wavelength belonging to the visible spectrum is cast on the cathode made of alkali metals such as sodium, potassium, etc.

This phenomenon of emission of negatively charged particles from a metal surface under the influence of light is called photoelectric reaction.

Definition: When light of suitable wavelength is incident on a metal surface, electrons are ejected from it, a phenomenon called photoelectric reaction or photoelectric emission.

Electrons emitted in photoelectric reactions are called photo-electrons or photoelectrons. A unidirectional current of photoelectrons can be generated with suitable arrangements; The resulting electric current is called photoelectric current.

WORK FUNCTION

Definition: The minimum energy at which an electron can escape beyond the surface of a metal is called the work function of that metal.

This function depends only on the nature of the metal, not on how the electron gained energy. The work-function is usually measured in units of eV. Alkali metals such as sodium, potassium etc. have lower work function than other metals, but nowadays some alloys are used which are more convenient for emitting light.

FAILURE OF THE WAVE THEORY OF LIGHT

Light (or any electromagnetic radiation) travels in the form of waves—this theory cannot explain photoelectric action.

(i) Maximum Kinetic Energy of Photoelectrons: As the intensity of the light increases, so does the energy carried by the light waves.  Thus, if very intense light is shone on a metal surface, the kinetic energy of the emitted photoelectrons should be very high, i.e. there should be no upper limit to this kinetic energy.  However, the value of the extinction potential V0 implies that the kinetic energy of the photoelectron can never be greater than eV0.

(ii) Start Frequency: A light wave with a frequency lower than the initial frequency 𝛎0, if intense enough, also carries a large amount of energy. This energy should be sufficient to cause the emission of photoelectrons. But in this case no photoelectric emission occurs. On the other hand, weak light rays of higher frequency than 𝛎0 are able to cause that emission.

(iii) Instantaneous Behavior of Photoelectric Action: The energy that the incident light wave brings to the metal surface takes some time to concentrate into the tiny spheres of each electron. So according to wave theory there should be some time lag between the incident light beam and emission of photoelectron. Whereas photoelectric action is instantaneous, there is no time lag between incident and emission.

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