Broadly, there are two types of mechanics called classical mechanics and quantum mechanics. Classical mechanics or physics explained successfully motion of the objects which can either be observed directly or can be made observable by instruments like microscope. But, the classical mechanics can not explain the mechanics of subatomic particles like electron.proton,neutron etc. Then there comes in picture the quantum mechanics, which explain the mechanics of these subatomic particles successfully.
Following examples will show that classical mechanics was inadequate to give explanation of observed facts:
(a) Photoelectric effect: when a photon of light falls on the metal surface and if the frequency of light is more than the certain minimum frequency (called threshold frequency), then electrons are ejected from metal surface. This effect is known as photoelectric effect.
According to classical theory of light, there should be no threshold frequency, the photoelectric current should increase with increase in frequency of light and kinetic energy of ejected electrons should increase with increase in intensity of light. There must also be a finite delay between the incidence of photons and ejection of electrons from metal surface.
But experimental results were totally opposite ,like, photoelectric effect is instantaneous ,and other observations were contradicted to aforementioned classical theory based observations.
Therefore classical mechanics failed to explain the photoelectric effect. These observations successfully later on by Einstein’s theory of photoelectric emission, which is based on Planck’s Quantum theory of Radiation.
(b) Stability of atom: Atomic model prepared by Rutherford was based on classical physics. This model assumes the atom to be consisting of a positively charged nucleus in the centre and negatively charged electrons revolved the nucleus in the circular orbit. Classical theory says that whenever a charged particle undergoes accelerated motion, it emits electromagnetic radiation. Therefore, an electron must emit energy continuously as its motion is accelerated. Thus, the orbital radius should decrease continuously and ultimately electron would fall in to nucleus. Therefore, an atom would collapse at the end.
But, this does not happen. As atom is stable and stability of atom could not be established by classical physics .This stability of atom was explained by Bohr’s quantum theory of atom.
Similarly, there were a lot of other observations like black body radiation, optical spectra, Compton Effect etc. which were not explained by classical mechanics. Therefore, it can be concluded that classical mechanics is inadequate to discuss the subatomic phenomenon. Hence, there is a need to introduce a new theory, which can deal with mechanics or motion of the subatomic or microscopic particles. Thus, the quantum hypothesis originated and the branch of physics dealing with motion of subatomic particles is called quantum mechanics.