Osm Physics by Ashish

    When a photon of the proper frequency interacts with this system, three types of optical processes occur (i) Absorption (ii) Spontaneous Emission and (iii) Stimulated Emission (i) Absorption Let the atom initially be in the lower energy state Em . A photon of proper energy h ν = (En − Em) interacts with the atom. This photon is absorbed, and the atom makes a transition from the lower state m to the higher excited state n. This process is called absorption. Because it is caused by an external photon, it is also called stimulated absorption. The probability of such a transition depends on the energy density of radiation. P mn   ∝   u(ν) P mn   =  B mn   u(ν)                                    (1) where B mn is a proportionality constant called the Einstein absorption coefficie...

  Laser : L ight A mplification by S timulated E missions of R adiation. The Story of the Laser  In 1917 , a famous scientist named Einstein had a big idea. He explained the science behind how lasers would one day work — but back then, nobody actually built one yet. In 1954 , another scientist named Townes was the first person to actually use Einstein's idea. He made a machine that could make microwaves stronger. He called this machine a MASER (which stands for Microwave Amplification by Stimulated Emission of Radiation). In 1958 , Townes and Schawlow (and also two other scientists, Basov and Prokhorov) figured out how to use the same idea — but this time for light instead of microwaves. This was the first big step toward the laser. Finally, in 1960 , a scientist named Maiman built the very first working laser . He made it using a special ruby crystal . This was a huge moment in science hi...

Consider two macroscopic systems A and A′ that can interact and exchange energy. Together they form a joint system A* . The joint system A* is isolated. Therefore, its total energy remains conserved . Interaction between A and A′ can occur mainly in two ways: Thermal interaction Mechanical interaction A general interaction may involve both heat and work .   1. Thermal Interaction     If the external parameters remain constant , the energy levels of the system remain unchanged. Any energy exchange is then called thermal interaction .   Energy is exchanged in the form of heat . If the system absorbs energy: If the system loses energy: Since the combined system is isolated: Therefore, Thus, heat absorbed by one system equals the heat released by the other system . Key idea In thermal interaction, the energy levels remain fixed, but the distribution/probability of energy among the fixed levels changes ,...