Mechanics deals with motion of bodies, which may be initially at rest or in motion. Newtonian mechanics could be able to explain the properties of observable bodies, hydrodynamics, transport phenomena, optical, electromagnetism, etc. but failed to explain micro system, that is sub atomic particles. The failure of Bohr theory to explain different intensities of spectral lines, splitting of spectral lines which required another theory.

Quantum Mechanics

Energy is transferred in quantum or small packets, example photons are Quantums of light energy. There is a concept of quantums of distance. In classical mechanics the future history of a particle is determined by its initial position and momentum together with force act upon it. In quantum mechanics the concept of probability and uncertainty principle which is significant at microscopic level.

Electromagnetic Radiation  

Matter and energy are inter-convertible according Einstein. E=mc2. The mechanical wave produced by vibrating particles needs a material medium for its propagation like sound. The electromagnetic waves are produced by accelerated charges and do not need material medium for propagation. Light is an electromagnetic radiation. It consists of oscillating electric and magnetic fields produced by accelerated charges. Electric and magnetic fields are mutually perpendicular.

Nature of Electromagnetic waves

Velocity of em waves is c=3*10*8 m/s. A moving charge has both electric and magnetic fields associated with it. If E is along y axis and B is along z axis then direction of propagation is along x axis. When the charge is accelerating there is change in electrical and magnetic energy which causes disturbance in space, and the disturbance is called electromagnetic wave.

Quantum Theory

According to Max Plank quantum theory oscillating particle radiate or absorb energy in small bundles called quanta. A radiation of frequency v is emitted a quanta of energy E=hv where h is plank constant. This quanta is given the name photon which can not be subdivided. En=nhv, n=1, 2, 3, ….it implies that the particle can emit or absorb em energy in multiples of hv. we know mc2=hv. = E

Momentum of one photon is hv/c from the above, or h/ wave length. Energy has dual nature wave nature and particle nature. With this theory many microscopic phenomena can be explained, including Einstein photo electric effect.