What is Quantum Mechanics ?

Quantum Mechanics is the study of motion of microscopic particles like molecules, atoms , subatomic particles, elementary particles etc.  The motion of microscopic world is quite different from the motion of macroscopic world. Now, let us discuss the origin of quantum physics. The history of quantum physics started from the disagreement between observed and theoretical curves of the spectra of black body radiation. It was a big problem for physicists at that time. It was German physicist Max Planck who modified the Wien's law and proposed an idea of energy packets. This new idea was able to explain the spectrum of black body radiation. Using this new idea Einstein explained the photoelectric effect and Neil Bohr proposed modified version of Rutherford's Atomic model, which later known as Bohr's atomic model. But what was the new idea? According to Planck , the exchange of energy between radiation and matter takes place in the form of energy packets. The energy packet was called as Quantum. The energy of one quantum is given by E=hν  , where h is a constant known as Planck constant and ν is the frequency of radiation. This idea  of Planck was a revolutionary idea and gave birth to quantum physics. Later French physicist de Broglie combined two equations E=hν and E=mc^2 ..and concluded that light has dual nature wave as well as particle. Then he generalised this concept to every moving material particles. According to him every moving material particles posses wave nature. The wave associated with material particle was called matter wave. The wavelength of a matter wave is given by the formula 

λ=h/mv=h/p

where p=mv  is the momentum of the particle. This concept of matter wave was other pillar of quantum physics. Two American physicists Davisson and Germer demonstrated an experiment to find out experimental evidence for the existence of matter waves. They discovered the diffraction of electrons and since diffraction is the property of wave so it was concluded that electron has also wave-particle duality. This was the first experimental evidence of the existence of matter waves. Later German physicist Heisenberg proposed another fundamental principle of quantum physics, namely,  Heisenberg's Uncertainty Principle. According this principle , It is impossible to determine simultaneously both the position and momentum (velocity) of a particle with accuracy. In other generalized form this principle is stated as " The values of two canonically conjugate variables cannot be simultaneously measured with accuracy (zero error) for a microscopic particle. The product of uncertainties in the simultaneous measurement of conjugate variables always has a value above a certain minimum (h/2π)" . After that  Austrian theoretical physicist Erwin Schrödinger discovered the wave equation for matter waves and Max Born interpreted the de Broglie-Schrödinger waves as waves of probability.  Thus quantum physics was developed into another category of mechanics known Quantum Mechanics to study the motion of microscopic particles. To understand quantum mechanics, two things are very important for physicist first higher mental imagination and second is knowledge of higher mathematics.