Introduction: Quantum Physics | Modern Physics for IIT JAM PDF Download

Quantum mechanics, a science dealing with the behavior of matter and light on the atomic and subatomic scale. It attempts to describe and account for the properties of molecules and atoms and their constituents—electrons, protons, neutrons, and other more esoteric particles such as quarks and gluons. These properties include the interactions of the particles with one another and with electromagnetic radiation (i.e., light, X-rays, and gamma rays).

• The behavior of matter and radiation on the atomic scale often seems peculiar, and the consequences of quantum theory are difficult to understand and believe. Its concepts frequently conflict with common-sense notions derived from observations of the everyday world. 
• There is no reason, however, why the behavior of the atomic world should conform to that of the familiar, large-scale world. It is important to realize that quantum mechanics is a branch of physics and that the business of physics is to describe and account for how the world—on both the large and the small scale—is and not how one imagines it or would like it to be.
• The study of quantum mechanics is rewarding for several reasons. First, it illustrates the essential methodology of physics. Second, it has been enormously successful in giving correct results in practically every situation to which it has been applied. There is, however, an intriguing paradox.
• Despite the overwhelming practical success of quantum mechanics, the foundations of the subject contain unresolved problems—in particular, problems concerning the nature of measurement. An essential feature of quantum mechanics is that it is generally impossible, even in principle, to measure a system without disturbing it; the detailed nature of this disturbance and the exact point at which it occurs are obscure and controversial. 
• Thus, quantum mechanics attracted some of the ablest scientists of the 20th century, and they erected what is perhaps the finest intellectual edifice of the period.

History of developments in Quantum Mechanics

• The history of quantum mechanics began primarily with Michael Faraday's discovery of cathode rays in 1838 and Gustav Kirchhoff's formulation of the black body radiation problem in the winter of 1859–1860.
• Max Planck's 1900 quantum hypothesis proved a stepping stone in quantum mechanics. It states that any energy-radiating atomic system can theoretically be divided into several discrete ‘energy elements’ ε such that each of these energy elements is proportional to the frequency v with which they each individually radiate energy, as defined by the following formula: E = hν, where h is referred to as Planck's constant
  
  Question for Introduction: Quantum Physics

  Try yourself:Green light has a wavelength of 525 nm. Determine the energy for the green light in joules.

  • A.

    2.78 x 10^-19 Joules/ photon
  • B.

    3.78 x 10^-19 Joules/photon
  • C.

    3.38 x 10^-19 Joules/ photon
  • D.

    4.78 x 10^-19 Joules/ photon

  Explanation

  

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• Then, in 1905, Albert Einstein proposed, based on Planck's quantum hypothesis, that light itself is made up of individual quantum particles, which later came to be known as photons, to explain the photoelectric effect (1839), which states that shining light on certain materials can function to eject electrons from the material (1926). 
• Max Born introduced the term "quantum mechanics" in his 1924 paper "Zur Quantenmechanik."
• The further developments in quantum mechanics are represented in the timeline provided below.

Some Examples

Q.1. What is the difference between classical and quantum mechanics? 
Ans. Classical mechanics describes the behavior of macroscopic bodies, which have relatively small velocities compared to the speed of light. Quantum mechanics describes the behavior of microscopic bodies such as subatomic particles, atoms, and other small bodies. 

Q.2. The Quantum Mechanical Model of the atom was proposed by:
Ans. Erwin Schrödinger proposed the quantum mechanical model of the atom, which treats electrons as matter waves.

Q.3. The wavelength of the matter waves is independent of ______.
Ans. The wavelength of the matter wave is independent of charge.