Towards the end of the 19th century, physicists were greatly satisfied that they were able to explain most of the phenomena on the basis of Newton’s laws of motion and Maxwell’s electromagnetic wave theory (according to which energy is emitted or absorbed continuously) . The former were used to explain the phenomena related to motion e.g. trajectories of planets, falling stones etc. and the latter to explain the phenomena related to energy transfer.

The branch of science based on Newton’s laws of motion and Maxwell’s electromagnetic wave theory to explain phenomena related to motion and energy is known as classical mechanics or Newtonian mechanics.

Classical mechanics was able to explain phenomena related to large size objects (macroscopic particles). How-ever, it fails when applied to small particles such as electrons, atoms, molecules etc. (microscopic parities) . For example, according to classical mechanics, it should be possible to determine simultaneously the position and velocity (or momentum) of a moving particle but this is contradicted by the Heisenberg’s uncertainty principle. Similarly, classical mechanics as-sumes that the energy is emitted or observed continuously whereas Planck’s quantum theory postulates that energy is emitted or absorbed not continuously but discontinuously in the form of packets of energy, called quanta. Further, the concept of quantum number was introduced arbitrarily to explain the atomic spectra.

A few phenomena for which the classical mechanics fails to give a satisfactory explanation are:
(1) Black body radiation
(2) Photoelectric effect
(3) Heat capacities of solids
(4) Atomic and molecular spectra

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