Chemical laser and tunable laser.

In a chemical laser instead of atoms the molecules are brought in the metastable state by a chemical reaction. These lasers are very strong and efficient. In a chemical laser, hydrogen and fluorine gases combine to form the  hydrogen fluoride gas. It is used to obtain an infrared laser beam of nearly 2 MW power.

Another chemical laser is the carbon dioxide laser. It is used to obtain a laser beam of nearly 100w power. Its main application is in sergery to close he small blood vessels while cutting the tissues by the evaporation of water the through the infrared beam. In the industrial area carbon lasers of more power are used for cutting the metal sheets.

The other chemical lasers me the dye lasers in which organic dyes are used. The energy levels of molecules of these dyes close due to which are very Laser beam can be obtained from them in a continuous range of wavelength. The Wavelength range of these lasers is from the visible region to the infrared region. By changing the separation between the mirrors at the two ends of the resonant cavity, a dye laser can be tuned for the desired of wavelength range. This is why the dye lasers are also called the tunable lasers.

What is Dielectric ?

Dielectric- Material such as glass ceramics polymers and paper are non-conducting materials. They prevent flow of current through them. Therefore they can be used for insulative purposes when the main function of non-conducting materials is to provide electrical insulation they are called insulators. When non-conducting materials are placed in an electric field. they modify the electric field and themselves undergo appreciable changes as a result of which they act as stores of electrical chages. When charge storage is the main function the materials are called dielectrics. For a material to be a good dielectric, it must bw an insulator. Hence any insulator is a dielectric.

Physical process of depletion layer.

Physical Process of Depletion Layer- We know that a p-type materials has holes as majority carriers negatively charged impurity atoms called negative ions (or acceptor ions). The n-type material has free electrons as  majority carriers and positively charged impurity atoms called positive ions (or donor ions). When we combine these two materials, following process takes place-

(i)The holes from p-region diffuse to the n-region where they combine with the free electrons.
(ii) The free electrons from the n-region diffuse to the p-region where they combine with holes.
(iii) The diffusion of holes (from p-region to n-region) and free electrons (from n-region to p-region) takes place due to the reason that there is a difference of concentrations in the two regions. (iv) The diffusion of holes and electrons across the junction takes place for a short time. After a few recombination of holes and free electrons in the vicinity of the junction, a restraining force is automatically Setup This force is produced due to depletion région. which exists on either side of the junction. As result of this further diffusion of holes and free electrons from a one region to the other is stopped by this depletion layer.

Motion of charged particles in crossed electric and magnetic fields.

When Uniform electric and magnetic fields are perpendicular to ed oderand act over the same region they are said to be in crossed configuration. When electrons pass through the region, they are defected simultaneously both the fields. Let two charged plane parallel plates set up a uniform ele field E in the y-direction and a uniform magnetic induction B is also set up the same region between the plates in z-direction. The magnetic feld is sud to act into the page. The fields are so oriented that they are perpendicular to the direction of motion of electrons which is say, x-direction. Let a Step lectrons enter the crossed field configuration with a velocity v.
The electric field deflects the electrons upward where as the m field deflects them downward.
The force due to the electric field is and the force due to due to the electric field is -
FE=eE.

Limitetions of Geiger Muller.

One main disadvantages of G.M. counter is its dead time. Dead time refers to the time taken by the tube to recover between counts. it require about 200 IS for the tube to recover Ifa lot of particles enter the G.M. tube of rapid rate, the tube will not have time to recover and some particles may nd be counted.

History of Superconductivity.

Superconductivity is a state exhibited by many conductors when cooled below their superconducting transition temperatures. Superconductivity is one ofthe most fascinating physical phenomena having a vast potentiality in practical applications. A superconducting material exhibits zero electrical resistivity and complete diamagnetism. The superconducting state is influenced by temperature, magnetic field, and current. There exist critical values for these three parameters above which values the material passes into normal State.

In 1908, the Dutch Physicist Kammerlingh Onnes succeeded in his efforts to liquefy helium, Gaseous helium turned into liquid helium at 4.2K at atmospheric pressure. Using liquid helium as the coolant the variation of  electrical resistance of metals at low temperatures was studied by Chines In one theory, the electrical resistance of pure metals should decrease continuously and vanish at absolute zero temperature. In another theory it was expected to increase exponentially as temperature approaches 0K. However, in the year 1911, onnes discovered that the electrical resistance of highly purified mercury dropped abruptly to zero at 4.15 K. Sudden drop in resistivity was not in accordance with the expectations and was recognized by Onnes to be an emirelynew phenomenon. He called it superconductivity Subsequently super conductivity was discovered in lead. tin, zinc, aluminium and other metals as well as in a number of alloys.

What is Soler cell?

solar cell is a device which is used for converting radiation energy into electrical energy. These cells generate a voltage proportional to electromagnetic radiation intensity and are called the photovoltaic cells because of their voltage generating capability.
selenium and silicon are the most widely used materials for solar cells though gallium, arsenide, indium, arsenide, and cadmium sulphide are also used. The construction and cross-section of a typical power solar cell for use as an energy converter. The surface layer of p-type material is extremely thin so that light can penetrate to the junction. The nickel-plated ring around the p-type material is the positive output terminal and the plating at the bottom of the n-type material is the negative output terminal, Power solar cells are also available in flat strip for efficient coverage of available surface area.