Quantum Mechanics Notes: Introduction

4: Quantum Mechanics 1) Introduction The introduction of quantion mechanics was prompted (11) by the failure of classical physics in explaining (III) a number of microphysical phenomena that occur in nature. At the end of 19th century physics essentially consisted of classical mechanics thermo dynamics and electromagnetism, in which classical mechanics (iv) explained the dynamics of materials thermodynamics explained interaction between the matters and rediations electromagnetism provided framework to Study properties of radiation. (N) At twentieth century, classical physics had been quite unasiable was seriously challenged on hoo (VI) major fronts: (1) Einsten's theory of relativity which should Newson's (vii) law 's not valid at high speed (11) Classical physics ceases at microscopic level. Quantim mechanics formalism ig the orginal work of Louis de Broglie , Erwin Schrodinger, Werner Heisenberg Paul Dirac, Max Born, Albert Einstain and So on. Inadequacy of Classical mechanies__experimental - Evidence classical mechanics is based on Newton's law and the concept of mass, absolute time and absolute space PARE in

The inadequanics of classical mechanics includes : (1) Black body radiation : classical mechanics failed to explain the spectrum of black body radiation (ii) Atomic stability : Can't explain the stability of atom. (iii) photoelectonic effect : According to classical mechanics, the speed of ejected elections depend on the intensity But it actually depends on the frequency with which light is incident (IV) Atomic spectroscopy : oft could not explain the orgin of discrete spectral lines of atoms because in classical physics energy changes are always contracs (N) Specific heat of metals and gases It could not explain observed variation in specific heat of metals and gases. (VI) It does not hold in microscopic levels ie; in the region of atomic dimension (vii) Failed to explain compton effect, Raman effect etc. Black Body Radiation When a radiation falls on an object Some of it might be reflected and absorbed An idealized "black body " is a material object that absorbs all the radiation falling on it and hence appears black under reflection when illuminated from outside Also a black body is a perfect absorber as well as perfect emitter of radiations

One of the major unsolved problem during Second half of 19th century was to understand the conhnaas character of the radiation emitted by glowing object This problem consisted in essence of specifying the proper theory of thermodynamics that describes how energy get exchanged b/w matter and radiation. By mid- 1800$ a wealth of experimental data of black body radiation was obtained from all those result, the radiation has well defined conknous energy destribution A number of attemps aimed at explaining the orgin of the continous character of this radiation were carried out Black body : is a source of thermal radiation A body which absorbs radiations of all wavelengths falling upon it is called a black body. It neither reflects nor transmits any incident radiation. When the body is heated it emits radiation and is called black body radiation or full radiation. The radication inside a uniformly heated enclosure is people perfectly black'- Based on this principle Fery designed a black body. It consists of a hallow double walled metal sphere, blackened inside and nickel polished outside. If this sphere is heated then thermal sphere radiation comes out

of the opening The nature and intensity of radiation depends only on the temperature of the sphere. Variation of wavelength, a (x-anls) and E A Ts intensity of radiation E T4 (y-axis) emitted by the body 12 T2 T, 7 Explanation of the shape of the graph::- (1) W. Wien first studied the distribution of energy in the spectrum of black body using classical electromagnetic theory Acc to When's law, -5 -Ec,12T) Edd==?? = e da This is the spectral energy density E a dr of the radiation within the wavelength range a and a+d a. Here C1 and C2 are constants The law agreed well with law and also the displacement law Weens law was Found to agree with experimental data in the shorter waveleng th (ie; high Frequency) region but not get at longer wavength (low frequency) regions.