PHYSICS

Superconductivity: Definition: The phenomenon of obtaining the property of zero resistance and infinity of conductivity is defined as  superconductivity. The substances which exhibit the property of superconductivity are called  Superconductors. Explanation: superconductivity was discovered by Kamerlingh in 1911. He observed that when a substance  is cooled to very low temperature, the resistance of the substance vanishes and the current flows through it without any abstraction and variation. The experiment was done with Helium and found that Helium will be a super conductor at the  temperature 4.2 K. Critical temperature ‘Tc’ (or transition temperature): The temperature where an ordinary substance becomes a superconductor is defined as critical  temperature.  It is denoted by Tc.  examples : For Mercury Tc=4.15 K and for Cd, Tc =0.56 K Tungsten     0.01K Aluminum  1.19 K NiBi        ...

BCS theory on superconductors. This theory was proposed by three scientists Bardeen, Cooper and Schrieffer in 1957.  This theory  explains about zero resistance of a superconductor. In normal substances the flow of free electrons is opposed by the vibration of ions or atoms in the  lattice due to the collision between them. Hence the normal conductors possess resistance. In the case of superconductors below the critical temperature (Tc), the atom or the ion is distorted  by the free electron during the collision.  The result produces a mechanical wave called phonon. During this collision the free electron exchanges an amount of its momentum with the lattice ion. Hence the momentum of free electron in reduced. It moves with less momentum. If another free electron collides with the distorted lattice ion (phonon), the second electron gains  an amount of momentum from the phonon. Hence the second free electron moves with greater momentum. But the ch...

Helium and Neon laser Construction :  Helium-Neon laser is a gas laser. A mixture  of helium and neon gas  in 10 : 1  ratio  at the  pressure of 1 mm Hg is filled in a discharge tube.  The diameter of tube is 1.5cm and the length 80 cm . The two end of the discharge tube are provided by Brewster  glass windows  .  Two mirrors are arranged on either side of the discharge tube. One f the mirrors is perfect reflector where as the second mirror is a partial reflector.  The two electrodes of the discharge tube are connected to a high tension voltage , frequency generator.  Working :  When high tension voltage is applied  the Helium atoms are excited to higher energy level.  These higher energy levels are meta stable states at 20.06 ev  The excited Helium atoms collide with neon atoms which are in the ground state.  The excited neon atoms  will reach to three meta stable excited state...

  Ruby laser   Ruby laser is a solid state laser. Ruby is a transparent crystal of aluminium oxide and its chemical composition is  Al 2 O 3 . An amount of aluminium atoms are replaced by chromium atoms with 0.5%. Then the crystal (Al 2 O 3 , Cr2O3) will attain a light pink colour.  Construction :   A ruby rod of length 4 cm and diameter 0.5 cm to 1 cm is used . The end faces of the rod are grounded  and polished to be parallel to each other. One face of the rod is silvered and it acts s perfect reflector. The other face is semi silvered. The ruby rod is surrounded by an optical flash helical pipe filled with Xenon gas. The two ends of Xenon pipe are connected to a high tension voltage for pumping the chromium ions to excited state. The whole arrangement is enclosed in an evacuated chamber.  Working  :   Ruby laser works  on 4 level pumping principle.  when the high tension is applied the xenon pipe relea...

 population inversion It is defined as the condition which is obtained in stimulated emission of radiation , the number of atoms In the metastable state is greater than the number of atoms in the ground state. This condition is necessary for producing a powerful laser , because maximum number of atoms in the metastable state must be stimulated to jump to the ground level. Lasing action: It is a chain process during stimulated emission in which every atom emits two identical photons and finally a powerful narrow beam of laser is released. To start the lasing process the condition of population inversion must be obtained. That is the number of atoms in meta stable state must be greater than that of ground level. In this process an external photon of selected wavelength incidents on an atom in meta stable state. Then the atom is stimulated to jump down to lower energy level by emitting two identical photons. These two photons are exactly equal to the incident photon...

  Spontaneous emission of radiation:   This is the phenomenon of emission of energy in the form of radiation when an atom jumps from excited energy state to the lower energy state after completing its life time in the excited state.   Explanation:  The atoms of a substance are excited to higher energy state from ground state by absorbing an amount of energy indenting on it.  But the atom in the excited level is not stable, because the life time of the atom in the excited state is about 10^-9 sec ( nano second).   After 10^-9 seconds the atoms jump down to the lower energy level releasing an amount of radiation.  The amount of energy released is equal to the difference in the two energy states.  Let the atom is excited to a higher energy state E2 and jumps to lower energy state E1 after its life time.  Then the amount of energy release E = hc/λ= E2-E1  Where λ= wavelength of the radiation and c= velocity ...

  LASER “Light Amplification by Stimulated Emission of Radiation”  Characteristics of Laser:  1. High ly coh erent:   All the electrons which are excited to the higher energy level jump to same lower energy level at the same time during lasing process(lasing actio n). 2. Laser is monochromatic:   The light consists single wavelength 3.Laser consists high intensity. They can travel through any longer distance with out dispersion.  4. It propagates in the form a narrow beam.  laser can penetrate through water. 5. Laser consists directionality :   ordinary light propagates in all directions but laser propagates along one direction only. 6. Laser is not absorbed by water.    ordinary light propagates in all directions but laser propagates along one direction only. 7. Laser produces heat and increase the temperature of the substance on which it incidents. A very high temperature about 100K can be prod...