WHAT IS ESCAPE VELOCITY

escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero. It is the speed needed to "break free" from the gravitational attraction of a massive body, without further propulsion, i.e., without spending more fuel.
For a spherically symmetric body, the escape velocity at a given distance is calculated by the formula

where G is the universal gravitational constant (G = 6.67×10⁻¹¹ m³ kg⁻¹ s⁻²), M the mass of the planet, star or other body, and r the distance from the center of gravity.
In this equation atmospheric friction (air drag) is not taken into account. A rocket moving out of a gravity well does not actually need to attain escape velocity to do so, but could achieve the same result at any speed with a suitable mode of propulsion and sufficient fuel. Escape velocity only applies to ballistic trajectories.

The term escape velocity is actually a misnomer, and it is often more accurately referred to as escape speed since the necessary speed is a scalar quantity which is independent of direction (assuming a non-rotating planet and ignoring atmospheric friction or relativistic effects).

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WHAT IS ENERGY

Energy is the capacity of a body to do work .

energy is a property of objects, transferable among them via fundamental interactions, which can be converted in form but not created or destroyed. The joule is the SI unit of energy, based on the amount transferred to an object by the mechanical work of moving it 1 metre against a force of 1 newton.

Work and heat are two categories of processes or mechanisms that can transfer a given amount of energy.

The second law of thermodynamics limits the amount of work that can be performed by energy that is obtained via a heating process—some energy is always lost as waste heat. The maximum amount that can go into work is called the available energy. Systems such as machines and living things often require available energy, not just any energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations.

There are many forms of energy, but all these types must meet certain conditions such as being convertible to other kinds of energy, obeying conservation of energy, and causing a proportional change in mass in objects that possess it. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, the potential energy stored by virtue of the position of an object in a force field such as a gravitational, electric or magnetic field, and the thermal energy comprising the microscopic kinetic and potential energies of the disordered motions of the particles making up matter. Some specific forms of potential energy include elastic energy due to the stretching or deformation of solid objects and chemical energy such as is released when a fuel burns. Any object that has mass when stationary, such as a piece of ordinary matter, is said to have rest mass, or an equivalent amount of energy whose form is called rest energy, though this isn't immediately apparent in everyday phenomena described by classical physics.

According to mass–energy equivalence, all forms of energy (not just rest energy) exhibit mass. For example, adding 25 kilowatt-hours (90 megajoules) of energy to an object in the form of heat (or any other form) increases its mass by 1 microgram; if you had a sensitive enough mass balance or scale, this mass increase could be measured. Our Sun transforms nuclear potential energy to other forms of energy; its total mass does not decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.
Although any energy in any single form can be transformed into another form, the law of conservation of energy states that the total energy of a system can only change if energy is transferred into or out of the system. This means that it is impossible to create or destroy energy. The total energy of a system can be calculated by adding up all forms of energy in the system. Examples of energy transfer and transformation include generating or making use of electric energy, performing chemical reactions, or lifting an object. Lifting against gravity performs work on the object and stores gravitational potential energy; if it falls, gravity does work on the object which transforms the potential energy to the kinetic energy associated with its speed.
More broadly, living organisms require available energy to stay alive; humans get such energy from food along with the oxygen needed to metabolize it. Civilisation requires a supply of energy to function; energy resources such as fossil fuels are a vital topic in economics and politics. Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun (as well as the geothermal energy contained within the earth), and are sensitive to changes in the amount received. The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency. The vernacular terminology is not consistent with technical terminology. For example, while energy is always conserved (in the sense that the total energy does not change despite energy transformations), energy can be converted into a form, e.g., thermal energy, that cannot be utilized to perform work. When one talks about "conserving energy by driving less", one talks about conserving fossil fuels and preventing useful energy from being lost as heat. This usage of "conserve" differs from that of the law of conservation of energy.

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WHAT IS VELOCITY

Velocity is the rate of change of the position of an object, equivalent to a specification of its speed and direction of motion, e.g. 60 km/h to the north. Velocity is an important concept in kinematics, the branch of classical mechanics which describes the motion of bodies.
Velocity is a vector physical quantity; both magnitude and direction are required to define it. The scalar absolute value (magnitude) of velocity is called "speed", a quantity that is measured in metres per second (m/s or m·s⁻¹) in the SI (metric) system. For example, "5 metres per second" is a scalar (not a vector), whereas "5 metres per second east" is a vector.
If there is a change in speed, direction, or both, then the object has a changing velocity and is said to be undergoing an acceleration .

Equation of motion

Velocity is defined as the rate of change of position with respect to time, i.e.

where v is velocity and x is the displacement vector. This will give the instantaneous velocity of a particle, or object, at any particular time t. Although the concept of an instantaneous velocity might at first seem counter-intuitive, it is best considered as the velocity that the object would continue to travel at if it stopped accelerating at that moment.
Although velocity is defined as the rate of change of position, it is more common to start with an expression for an object's acceleration and from there obtain an expression for velocity, which can be done by evaluating

which comes from the definition of acceleration,

Sometimes it is easier, or even necessary, to work with the average velocity of an object, that is to say the constant velocity, that would provide the same resultant displacement as a variable velocity, v(t), over some time period Δt. Average velocity can be calculated as

The average velocity is always less than or equal to the average speed of an object. This can be seen by realizing that while distance is always strictly increasing, displacement can increase or decrease in magnitude as well as change direction.
In terms of a displacement-time graph, the velocity can be thought of as the gradient of the tangent line to the curve at any point, and the average velocity as the gradient of the chord line between two points with t coordinates equal to the boundaries of the time period for the average velocity.

Constant acceleration

In the special case of constant acceleration, velocity can be studied using the suvat equations. By considering a as being equal to some arbitrary constant vector, it is trivial to show that

with v as the velocity at time t and u as the velocity at time t=0. By combining this equation with the suvat equation x=ut+at²/2, it is possible to relate the displacement and the average velocity by

.

It is also possible to derive an expression for the velocity independent of time, known as the Torricelli equation, as follows:

where v=|v| etc...
The above equations are valid for both Newtonian mechanics and special relativity. Where Newtonian mechanics and special relativity differ is in how different observers would describe the same situation. In particular, in Newtonian mechanics, all observers agree on the value of t and the transformation rules for position create a situation in which all non-accelerating observers would describe the acceleration of an object with the same values. Neither is true for special relativity. In other words only relative velocity can be calculated.

Quantities that are dependent on velocity

The kinetic energy of a moving object is dependent on its velocity by the equation

where E_k is the kinetic energy and m is the mass. Kinetic energy is a scalar quantity as it depends on the square of the velocity, however a related quantity, momentum, is a vector and defined by

In special relativity, the dimensionless Lorentz Factor appears frequently, and is given by

where γ is the Lorentz factor and c is the speed of light.

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WHAT IS DISTANCE

Distance, or farness, is a numerical description of how far apart objects are. In physics or everyday usage, distance may refer to a physical length, or an estimation based on other criteria (e.g. "two counties over"). In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, and is a concrete way of describing what it means for elements of some space to be "close to" or "far away from" each other. In most cases, "distance from A to B" is interchangeable with "distance between B and A".

Distance is a scalar quantity . It has only magnitude not direction . Distance is a very fundamental  unit .
Example of distance - distance between hodal and delhi is 90 km ,distance of faridabad from hodal is70 km , etc

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WHAT IS PHOTOELECTRIC EFFECT

The photoelectric effect is the observation that many metals emit electrons when light shines upon them. Electrons emitted in this manner may be called photoelectrons.
According to classical electromagnetic theory, this effect can be attributed to the transfer of energy from the light to an electron in the metal. From this perspective, an alteration in either the amplitude or wavelength of light would induce changes in the rate of emission of electrons from the metal. .

  The electrons are only dislodged by the photoelectric effect if light reaches or exceeds a threshold frequency, below which no electrons can be emitted from the metal regardless of the amplitude and temporal length of exposure of light. To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hf. This shed light on Max Planck's previous discovery of the Planck relation (E = hf) linking energy (E) and frequency (f) as arising from quantization of energy. The factor h is known as the Planck constant.

Light–matter interaction

Mathematical  derivation -

The maximum kinetic energy of an ejected electron is given by
where is the Planck constant and is the frequency of the incident photon. The term is the work function (sometimes denoted , or ), which gives the minimum energy required to remove a delocalised electron from the surface of the metal. The work function satisfies
where is the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is then
Kinetic energy is positive, so we must have for the photoelectric effect to occur.

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WHAT IS OHM'S LAW

This law relates the current and potential difference with resistance as the constant of proportionality.

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance,  the usual mathematical equation that describes this relationship:

where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

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GOD PARTICLES wrt MASS

One of the most active large research projects today is the search for an extremely small but energetic particle that is thought to be the key to understanding how mass appeared shortly after the Big Bang. The Higgs boson is a hypothetical elementary particle that has not been observed but, if found, would dramatically advance the 70-year development of a model of elementary particle interaction. Its existence was predicted along with other particles by the so-called Standard Model. The Standard Model describes how leptons, quarks, gauge bosons, and the Higgs particle fit together and explains how the Higgs mechanism takes place, which in turn explains why elementary particles exhibit mass. The discovery of the Higgs boson would finally validate the Standard Model, since it’s the only elementary particle predicted by it that hasn’t yet been observed.
Experiments to find the Higgs boson are currently being performed using the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Switzerland, . The LHC is expected to be able to answer the question of whether or not the Higgs boson actually exists. One possible signature from a simulated proton-proton collision that would demonstrate the Higgs boson’s existence .The Higgs boson is believed to decay almost immediately after such a collision into two jets of hadrons (composite particles made of subatomic elementary particles held together by strong nuclear forces) and two electrons, . In December 2011, two experiments at the LHC independently reported that their data hint that the Higgs particle probably exists with a mass of about 133 proton masses. The range of mass for the Higgs particle is now thought to have been narrowed considerably to between approximately 122 and 138 protons. It is expected that the LHC will have a definite answer by the end of 2012.

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WHAT ARE HIGGS BOSONS OR GOD PARTICLES

The Higgs boson or Higgs particle is an elementary particle initially theorised in 1964, whose discovery was announced at CERN on 4 July 2012.^[7] The discovery has been called "monumental"^[8][9] because it appears to confirm the existence of the Higgs field .:
  The "God particle" is the nickname of a subatomic particle called the Higgs boson. In layman’s terms, different subatomic particles are responsible for giving matter different properties. One of the most mysterious and important properties is mass. Some particles, like protons and neutrons, have mass. Others, like photons, do not. The Higgs boson, or “God particle,” is believed to be the particle which gives mass to matter. The “God particle” nickname grew out of the long, drawn-out struggles of physicists to find this elusive piece of the cosmic puzzle. What follows is a very brief, very simplified explanation of how the Higgs boson fits into modern physics, and how science is attempting to study it.
The Higgs boson (or Higgs particle) is a particle that gives mass to other particles. Peter Higgs was the first person to think of it, and the particle was found in March 2013. It is part of the Standard Model in physics, which means it is found everywhere. It is one of the 17 particles in the Standard Model. The Higgs particle is a boson. Bosons are particles responsible for all physical forces except gravity. Other bosons are the photon, the W and Z bosons, and the gluon. Scientists do not yet know how to combine gravity with the Standard Model.
It is very difficult to detect the Higgs boson with the equipment and technology we have now. These particles are believed to exist for less than a septillionth of a second. Because the Higgs boson has so much mass (compared to other particles), it takes a lot of energy to create one. The Large Hadron Collider at CERN is the equipment scientists used to find it. The collider has enough energy that it is able to make Higgs bosons. When you smash particles together, there is a small chance a Higgs Boson will appear, so the Large Hadron Collider smashed lots of particles together to find it.
Higgs bosons obey the conservation of energy law, which states that no energy is created or destroyed, but instead it is transferred. First, the energy starts out in the gauge boson that interacts with the Higgs field. This energy is in the form of kinetic energy as movement. After the gauge boson interacts with the Higgs field, it is slowed down. This slowing reduces the amount of kinetic energy in the gauge boson. However, this energy is not destroyed. Instead, the energy is converted into mass-energy, which is normal mass that comes from energy. The mass created is what we call a Higgs boson. The amount of mass created comes from Einstein's famous equation E=mc², which states that mass is equal to a large amount of energy (i.e. 1 kg of mass is equivalent to almost 90 quadrillion joules of energy—the same amount of energy used by the entire world in roughly an hour and a quarter in 2008). Since the amount of mass-energy created by the Higgs field is equal to the amount of kinetic-energy that the gauge boson lost by being slowed, energy is conserved.
Higgs bosons are used in a variety of science fiction stories. The physicist Leon Lederman called it the "God particle" in 1993. He used this name to get attention and support for experiments to detect the particle. However, most scientists do not like this name, because the particle has nothing to do with any kind of god and the nickname might confuse people.

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WHAT ARE THE PROPERTIES OF PHOTON

Einstein explain photoelectric effect by  assuming that electromagnetic radiation travels through space in discrete quanta called photons . Thus photon is a quantum of electromagnetic radiation which always travels along straight paths with velocity equal to that of light and zero rest mass .

Properties of photon  

                         The energy of a photon of frequency v is hv .

                        A photon behave like a particle of rest mass equal to zero .

                         It possess not only a definite energy hv but also a definite momentum equal to p=  mc =mcc/c =E /c .

                       Photons are electrically neutral and so are not affected by electric and magnetic field .

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WHAT ARE BLACK BODIES AND BLACK BODY RADIATIONS

Black body

A body which completely absorb radiations of all wavelengths incident on it .

A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the tempera
A black body in thermal equilibrium has two notable properties

1. It is an ideal emitter: at every frequency, it emits as much energy as or more energy than any other body at the same temperature.
2. It is a diffuse emitter: the energy is radiated isotropically, independent of direction

or
 An ideal body is now defined, called a black body. A black body allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the black body is a perfect absorber for all incident radiation .

Black body radiation

The radiations emitted by black bodies are called black body radiations . 

Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.
The thermal radiation spontaneously emitted by many ordinary objects can be approximated as blackbody radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium.
A black-body at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. At higher temperatures, black bodies glow with increasing intensity and colors that range from dull red to blindingly brilliant blue-white as the temperature increases.
Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, and it is believed that they emit black-body radiation  with a temperature that depends on the mass of the black hole.
The term black body was introduced by Gustav Kirchhoff in 1860.  Black-body radiation is also called complete radiation or temperature radiation or thermal radiation.

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WHAT IS QUANTUM MECHANICS

Quantum mechanics  deals the objects at microscopic level . means in QUANTUM MECHANICS  we study the particles which are of very small size i.e  which are of atomic size where lengths are of the orders of about one angstroms to fermis .Quantum mechanics was found successful in giving satisfactory explanation to many observed facts related  to micro objects .

The development of quantum mechanics took place  in two stages -

• First stage - This stage began with the Max Planck's hypothesis .

  According to this hypothesis the radiation is emitted or absorbed by matter in in discrete packets or quanta each of energy hv , where v is the frequency of radiation and h is Planck's constant .
  
  

  • Second stage - This stage began with Erwin Schrodinger .

    This mechanics combines the earlier ideas of classical wave theory with Louis de -Broglie's wave particle duality relationship .

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WHAT ARE THE APPLICATIONS OF LASERS

There are lot of applications of lasers in the every field of life .These can be discussed as -

• LASER WELDING 

This type of welding competes very well with the other standard techniques , like arc welding ,resistance welding  electron beam welding etc .
The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density for welding and cutting.
The automobile industry makes extensive use of carbon dioxide lasers with powers up to several kilowatts for computer controlled welding on auto assembly lines.
Garmire points out an interesting application of CO2 lasers to the welding of stainless steel handles on copper cooking pots. A nearly impossible task for conventional welding because of the great difference in thermal conductivities between stainless steel and copper, it is done so quickly by the laser that the thermal conductivities are irrelevant.

• LASER CUTTING 

Metals can be cut with output power of atleast 100W to 500W provided that material heated by laser beam is blown with a jet of oxygen so that cutting products are side by  side removed . It gives fine and precise cut . Chemical purity is maintained in laser cutting . And cutting speed is quite high.

•  MEDICAL USES OF LASERS

The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density. This makes it useful as a cutting and cauterizing instrument. Lasers are used for photo coagulation of the retina to halt retinal hemorrhaging and for the tacking of retinal tears. Higher power lasers are used after cataract surgery if the supportive membrane surrounding the implanted lens becomes milky. Photo disruption of the membrane often can cause it to draw back like a shade, almost instantly restoring vision. A focused laser can act as an extremely sharp scalpel for delicate surgery, cauterizing as it cuts. ("Cauterizing" refers to long-standing medical practices of using a hot instrument or a high frequency electrical probe to singe the tissue around an incision, sealing off tiny blood vessels to stop bleeding.) The cauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver.
Lasers have been used to make incisions half a micron wide, compared to about 80 microns for the diameter of a human hair.

• SURVEYING AND RANGING

Helium-neon and semiconductor lasers have become standard parts of the field surveyor's equipment. A fast laser pulse is sent to a corner reflector at the point to be measured and the time of reflection is measured to get the distance.
Some such surveying is long distance! The Apollo 11 and Apollo 14 astronauts put corner reflectors on the surface of the Moon for determination of the Earth-Moon distance. A powerful laser pulse from the MacDonald Observatory in Texas had spread to about a 3 km radius by the time it got to the Moon, but the reflection was strong enough to be detected. We now know the range from the Moon to Texas within about 15 cm, a nine significant digit measurement. A pulsed ruby laser was used for this measurement.

• LASERS IN COMMUNICATION

Fiber optic cables are a major mode of communication partly because multiple signals can be sent with high quality and low loss by light propagating along the fibers. The light signals can be modulated with the information to be sent by either light emitting diodes or lasers. The lasers have significant advantages because they are more nearly monochromatic and this allows the pulse shape to be maintained better over long distances. If a better pulse shape can be maintained, then the communication can be sent at higher rates without overlap of the pulses. Ohanian quotes a factor of 10 advantage for the laser modulators.
Telephone fiber drivers may be solid state lasers the size of a grain of sand and consume a power of only half a milli watt. Yet they can sent 50 million pulses per second into an attached telephone fiber and encode over 600 simultaneous telephone conversations (Ohanian).

•  LASERS IN HEAT TREATMENT

Heat treatments for hardening or annealing have been long practiced in metallurgy. But lasers offer some new possibilities for selective heat treatments of metal parts. For example, lasers can provide localized heat treatments such as the hardening of the surfaces of automobile camshafts. These shafts are manufactured to high precision, and if the entire camshaft is heat treated, some warping will inevitably occur. But the working surfaces of the cams can be heated quickly with a carbon dioxide laser and hardened without appreciably affecting the remainder of the shaft, preserving the precision of manufacture.

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BASIC TYPES OF LASER

The classification of laser is done on the basis of active material used in them .Accordingly various types of lasers are :
     

•     SOLID STATE LASER
•     GAS LASER
•    SEMICONDUCTOR

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There are many types of lasers available for research, medical, industrial, and commercial uses.  Lasers are often described by the kind of lasing medium they use - solid state, gas, excimer, dye, or semiconductor.
Solid state lasers have lasing material distributed in a solid matrix, e.g., the ruby or neodymium-YAG (yttrium aluminum garnet) lasers. The neodymium-YAG laser emits infrared light at 1.064 micrometers.
Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of a visible red light. CO₂ lasers emit energy in the far-infrared, 10.6 micrometers, and are used for cutting hard materials.
Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudomolecule or dimer is produced and when lased, produces light in the ultraviolet range.
Dye lasers use complex organic dyes like rhodamine 6G in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.
Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, e.g., the writing source in some laser printers or compact disk players.
 

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WHAT IS THE PRINCIPLE OF LASER ACTION

To make a laser we need three things :

1. Active material
2. Pump source
3. Resonator                 

This can be explained as -
the block diagram  can be drawn as

Fig. 2: Basic Laser system

A representative laser system is shown in Figure (2). It consists of three basic parts.

• An active medium with a suitable set of energy levels to support laser action.
• A source of pumping energy in order to establish a population inversion.
• An optical cavity or resonator to introduce optical feedback and so maintain the gain of the system overcoming all losses.

Brief description of each of the above components and their basic function are given below. or gain medium: Laser medium is the heart of the laser system and is responsible for producing gain and subsequent generation of laser. It can be a crystal, solid, liquid, semiconductor or gas medium and can be pumped to a higher energy state. The material should be of controlled purity, size and shape and should have the suitable energy levels to support population inversion. In other words, it must have a metastable state to support stimulated emission. Most lasers are based on 3 or 4 level energy level systems, which depends on the lasing medium. These terms can be explained as -

1. Excitation or pumping mechanism: Absorption of the energy by the atoms, electrons, ions or molecules as the case may be, of the active medium is a primary requisite in the generation of laser. In order to excite these elements to higher energy levels, an excitation or pumping mechanism is necessary. It is well known that under the equilibrium state, as per Boltzman?s conditions, higher energy levels are much less populated than the lower energy levels. One of the requirements of laser action is population inversion in the levels concerned. i.e. to have larger population in the upper levels than in the lower ones. Otherwise absorption will dominate at the cost of stimulated emission. There are various types of excitation or pumping mechanisms available, the most commonly used ones are optical, electrical, thermal or chemical techniques, which depends on the type of the laser gain medium employed. For example, Solid state lasers usually employ optical pumping from high energy xenon flash lamps (e.g., ruby, Nd:YAG) or from a second pump laser or laser diode array (e.g., DPSS frequency doubled green lasers). Gas lasers use an AC or DC electrical discharge through the gas medium, or external RF excitation, electron beam bombardment, or a chemical reaction. The DC electrical discharge is most common for 'small' gas lasers (e.g., helium-neon, argon ion, etc.). DC most often pumps semiconductor lasers current. Liquid (dye) lasers are usually pumped optically.
2. Optical resonator: Optical resonator plays a very important role in the generation of the laser output, in providing high directionality to the laser beam as well as producing gain in the active medium to overcome the losses due to, straying away of photons from the laser medium, diffraction losses due to definite sizes of the mirrors, radiation losses inside the active medium due to absorption and scattering etc. In order to sustain laser action, one has to confine the laser medium and the pumping mechanism in a special way that should promote stimulated emission rather than spontaneous emission. In practice, photons need to be confined in the system to allow the number of photons created by stimulated emission to exceed all other mechanisms. This is achieved by bounding the laser medium between two mirrors . On one end of the active medium is the high reflectance mirror (100% reflecting) or the rear mirror and on the other end is the partially reflecting or transmissive mirror or the output coupler. The laser emanates from the output coupler, as it is partially transmissive. Stimulated photons can bounce back and forward along the cavity, creating more stimulated emission as they go. In the process, any photons which are either not of the correct frequency or do not travel along the optical axis are lost.

Laser action

Interaction of electromagnetic radiation with matter produces absorption and spontaneous emission. Absorption and spontaneous emission are natural processes. For the generation of laser, stimulated emission is essential.  The population inversion exists between upper and lower levels among atomic systems, it is possible to realize amplified stimulated emission and the stimulated emission has the same frequency and phase as the incident radiation?. In electronic, atomic, molecular or ionic systems the upper energy levels are less populated than the lower energy levels under equilibrium conditions. Pumping mechanism excites say, atoms to a higher energy level by absorption .
The atom stays at the higher level for a certain duration and decays to the lower stable ground level spontaneously, emitting a photon, with a wavelength decided by the difference between the upper and the lower energy levels. This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. The spontaneous emission or fluorescence has no preferred direction and the photons emitted have no phase relations with each other, thus generating an incoherent light output (Fig.4). But it is not necessary that the atom is always de-excited to ground state. It can go to an intermediate state, called metastable state with a radiation less transition, where it stays for a much longer period than the upper level and comes down to lower level or to the ground state. Since period of stay of atoms in the metastable state is large, it is possible to have a much larger number of atoms in metastable level in comparison to the lower level so that the population of metastable state and the lower or ground state is reversed. i.e. there are more atoms in the upper metastable level than the lower level. This condition is referred to as population inversion. Once this is achieved, laser action is initiated in the following fashion. The atom in the metastable state comes down to the ground state emitting a photon. This photon can stimulate an atom in the metastable state to release its photon in phase with it. The photon thus released is called stimulated photon. It moves in the same direction as the initiating photon, has the same wavelength and polarization and is in phase with it, thus producing amplification. Since there are a large number of initiating photons, it forms an initiating electromagnetic radiation field. An avalanche of stimulated photons is generated, as the photons traveling along the length of the active medium stimulates a number of excited atoms in the metastable state to release their photons. This is referred to as the stimulated emission. These photons are fully reflected by the rear reflector (100% reflective) and the number and consequently the intensity of stimulated photons increases as they traverse through the active medium, thus increasing the intensity of radiation field of stimulated emission. At the output coupler, a part of these photons are reflected and the rest is transmitted as the laser output. This action is repeated and the reflected photons after striking the rear mirror, reach the output coupler in the return path. The intensity of the laser output increases as the pumping continues. When the input pumping energy reduces, the available initiating and subsequently the stimulated photons decrease considerably and the gain of the system is not able to overcome the losses, thus laser output ceases. Since the stimulation process was started by the initiating photons, the emitted photons can combine coherently, as all of them are in phase with each other, unlike in the case of spontaneous emission and coherent laser light is emitted (Fig.5). Though the laser action will continue as long as the energy is given to the active medium, it may be stated that pulsed laser is obtained if the population inversion is available in a transient fashion and continuous wave (CW) laser is possible if the population inversion is maintained in a steady-state basis. If the input energy is given by say a flash lamp, the output will be a pulsed output and the laser is called a pulsed laser. If equilibrium can be achieved between the number of photons emitted and the number of atoms in the metastable level by pumping with a continuous arc lamp instead of a flash lamp, then it is possible to achieve a continuous laser output, which is called continuous wave laser.

We may conclude that, laser action is preceded by three processes, namely, absorption, spontaneous emission and stimulated emission - absorption of energy to populate upper levels, spontaneous emission to produce the initial photons for stimulation and finally, stimulated emission for generation of coherent output or laser.

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WHAT ARE THE APPLICATIONS OF SUPERCONDUCTORS

There are a lot of applications of superconductors -

• The superconductors are used to perform logic and storage functions in computers .
• Superconducting magnets are some of the most powerful electromagnets known. They are used in MRI/NMR machines, mass spectrometers, and the beam-steering magnets used in particle accelerators. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in the pigment industries.
• Electrical machines and transformers developed using superconductors will have small size and high efficiency .
• Superconductors are used to build Josephson junctions which are the building blocks of SQUIDs (superconducting quantum interference devices), the most sensitive magnetometers known. SQUIDs are used in scanning SQUID microscopes and magnetoencephalography. Series of Josephson devices are used to realize the SI volt. Depending on the particular mode of operation, a superconductor-insulator-superconductor Josephson junction can be used as a photon detector or as a mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic micro-calorimeter photon detectors.
• As there is no loss of heat in a superconductor due to their zero resistance , so power can be transmitted through the superconducting cables without any losses .

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WHAT ARE SUPERCONDUCTORS

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a particular temperature is called CRITICAL TEMPERATURE .

The electrical resistivity of most metals and many alloys drops suddenly to zero at very low temperature .This phenomena is called  SUPERCONDUCTIVITY .

Superconductivity was discovered by Dutch physicist Heike Kamerlingh Onnes .

The critical temperature for mercury is 4.2 K . And the critical temperature for other superconducting elements varies from less than 0.1 K to nearly10 K .The elements which are good conductors such as copper and silver , do not become superconducting when cooled .

types of superconductors

There are many criteria by which superconductors are classified. The most common are:

• Response to a magnetic field: A superconductor can be Type I, meaning it has a single critical field, above which all superconductivity is lost; or Type II, meaning it has two critical fields, between which it allows partial penetration of the magnetic field.
• By theory of operation: It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional, otherwise.
• By critical temperature: A superconductor is generally considered high temperature if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only T_c > 77 K) – or low temperature if more aggressive cooling techniques are required to reach its critical temperature.
• By material: Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium, germanium-niobium, and niobium nitride),  organic superconductors (fullerenes and carbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon).

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WHAT IS LASER .

 LASER -Light amplification by stimulated emission of radiation

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation .

When an electron absorbs energy either from light (photons) or heat (phonons), it receives that incident quantum of energy. But transitions are only allowed in between discrete energy levels . This leads to emission lines and absorption lines.
When an electron is excited from a lower to a higher energy level, it will not stay that way forever. An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, that is called "spontaneous emission". .
 . Thus, the rate of transitions between two stationary states is enhanced beyond that due to spontaneous emission. Such a transition to the higher state is called absorption, and it destroys an incident photon (the photon's energy goes into powering the increased energy of the higher state). A transition from the higher to a lower energy state, however, produces an additional photon; this is the process of stimulated emission.

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THE ELECTRIC LINES OF FORCE

Definition

The path traced by a unit positive charge free to move under the effect of an electric field is called electric line of force .

or                   Electric lines of force can be defined as a way or path, it may be straight or curved, so that the tangent at any point to it gives the direction of the electric field intensity at that point.

 

Electric lines of force due to a positive charge are directed outwards

But in case of negative charge instead of going away from the charge the lines of force are directed inwards.


these lines are directed outward

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WHAT ARE THE PROPERTIES OF ELECTRIC CHARGE

 As electric charge is fundamental property  .It displays following  properties -

• Electric charge is quantised .
• Electric charge is conserved .
• Electric charge is a scalar quantity.
• This is associated with mass always .
• This is additive i.e. its sum total can be found by an algebraic sum .
• Electric charge remains unaffected by the motion .

  Like mass, electric charge in a closed system is conserved. As long as a system is impermeable, the amount of charge inside it will neither increase nor decrease; it can only be transferred. However, electric charge differs from other properties—like mass—in that it is a relativistic invariant. That is, charge is independent of speed. The mass of a particle will rise exponentially  as its speed approaches that of light, its charge, however, will remain constant.

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DEFINITION OF ELECTRIC CHARGE

The electric charge may be defined as that property of matter due to which the matter is able to exhibit electric , magnetic ,and electromagnetic effects .

A basic property of some elementary particle of matter  by virtue of which they develop electrical influence is called electric charge .

A charge can be positive or negative  . Electric charge can not exist on its own , it always requires a mass to sit on . But electric monopole is possible i.e. positive and negative charge can exist independently e.g. an electron carries only negative charge on it .

Like charge repel each other whereas unlike charges attract each other .

Rubbing a glass rod with silk gives positive charge on the rod .

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TYPES OF CLASSICAL OPTICS

Classical optics is divided into two main branches: geometrical optics and physical optics. In geometrical, or ray optics, light is considered to travel in straight lines, and in physical, or wave optics, light is considered to be an electromagnetic wave.
Geometrical optics can be viewed as an approximation of physical optics which can be applied when the wavelength of the light used is much smaller than the size of the optical elements or system being modelled.

Geometrical optics

Main article: Geometrical optics

Geometry of reflection and refraction of light rays

Geometrical optics, or ray optics, describes the propagation of light in terms of "rays" which travel in straight lines, and whose paths are governed by the laws of reflection and refraction at interfaces between different media.
When a ray of light hits the boundary between two transparent materials, it is divided into a reflected and a refracted ray.

The law of reflection says that the reflected ray lies in the plane of incidence, and the angle of reflection equals the angle of incidence.

The law of refraction says that the refracted ray lies in the plane of incidence, and the sine of the angle of refraction divided by the sine of the angle of incidence is a constant.

where n is a constant for any two materials and a given colour of light. It is known as the refractive index.
The laws of reflection and refraction can be derived from Fermat's principle which states that the path taken between two points by a ray of light is the path that can be traversed in the least time.

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WHAT IS OPTICS

Definition :

The branch of physics which deals with light is known as optics .

Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

  

^{Most optical phenomena can be accounted for using the classical electromagnetic description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice.}  

Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, and medicine (particularly ophthalmology and optometry). Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics.

There are three types of optics :

• Wave optics
• Ray optics
• Quantum optics

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WHAT ARE LIGHT WAVES

Light waves are electromagnetic waves . They do not need a material medium for their propagation . Light waves are transverse waves by nature .They show the phenomena of reflection , refraction , dispersion , diffraction , interferences ,and polarization  . 

Light waves move fastest in vacuum and their velocity decreases in denser medium .Light waves  travel faster in air as compared to solid and liquid .there is no effect of change in temperature or humidity on velocity of light .

Light is radiant energy, usually referring to electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10⁻⁹ m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths. These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances. Various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm, children and young adults ultraviolet down to about 310 to 313 nm.

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WHAT ARE LONGITUDINAL AND TRANSVERSE WAVES

Longitudinal and transverse waves

 LONGITUDINAL WAVES :  If the direction of the wave and its constituent particles is same i.e they are alligned in same direction then the wave is known as longitudinal wave .

TRANSVERSE WAVE :  If the direction of wave and its constituent particles is perpendicular to each  other  ,then the wave is known as transverse wave .

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation. Additionally, sound waves may be viewed simply by parabolic mirrors and objects that produce sound.
The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.

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PROPAGATION OF SOUND WAVES

Sound propagates through compressible media such as air, water and solids as longitudinal waves and also as a transverse waves in solids (see Longitudinal and transverse waves, below). The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change). During propagation, waves can be reflected, refracted, or attenuated by the medium.
The behavior of sound propagation is generally affected by three things:

• A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.
• The propagation is also affected by the motion of the medium itself. For example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
• The viscosity of the medium also affects the motion of sound waves. It determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

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VARIOUS PHYSICS SYMBOLS , MEANINGS AND UNITS

Symbol	Meaning	SI unit of measure
	area	meter squared (m2)
	magnetic vector potential
	amplitude
	acceleration	meter per second squared (m/s2) or (m s−2)
	magnetic flux intensity also called the magnetic field density or magnetic induction	tesla (T), or equivalently,   weber per square meter (Wb/m2)
	capacitance	farad (F)
	heat capacity	joule per kelvin (J K−1), or equivalently, joule per degree Celsius (J °C−1)
	constant of integration	varied depending on context
	speed of light (in vacuum)	299,792,458 meter per second (m/s)
	speed of sound	340.29 meter per second (m/s)
	specific heat capacity	joule per kilogram per kelvin (J kg−1 K−1), or equivalently, joule per kilogram per degree Celsius (J kg−1 °C−1)
	viscous damping coefficient	kilogram per second (kg/s)
	electric displacement field also called the electric flux density	coulomb per square meter (C/m2)
	distance	meter (m)
	impact parameter	meter (m)
	diameter	meter (m)
	differential (e.g. )
	differential vector element of surface area A, with infinitesimally small magnitude and direction normal to surface S	square meter (m2)
	differential element of volume V enclosed by surface S	cubic meter (m3)
	electric field	newton per coulomb (N C−1), or equivalently, volt per meter (V m−1)
	energy	joule (J)
	eccentricity	unitless
	2.71828... (base of the natural logarithm), electron, elementary charge
	force	newton (N)
	frequency	hertz (Hz)
	function
	friction	newton (N)
	the gravitational constant	newton meter squared per kilogram squared (N m2/kg2)
	acceleration due to gravity	meter per second squared (m/s2)
	magnetic field strength also called just magnetic field	ampere per meter (A/m)
	Hamiltonian	joule (J)
	height	meter (m)
	Planck's constant	joule second (J s)
	reduced Planck's constant	joule second (J s)
	action	joule second (J s)
	intensity	watt per square meter (W/m2)
	sound intensity	watt per square meter (W/m2)
	electric current	ampere (A)
	moment of inertia	kilogram meter squared (kg m2)
	intensity	watt per square meter (W/m2)
	imaginary unit
	Cartesian x-axis basis unit vector	unitless
	free current density, not including polarization or magnetization currents bound in a material	ampere per square meter (A/m2)
	impulse	kilogram meter per second (kg m/s)
	Cartesian y-axis basis unit vector	unitless
	kinetic energy	joule (J)
	Boltzmann constant	joule per kelvin (J/K)
	wave number	radians per meter (m−1)
	Cartesian z-axis basis unit vector	unitless
	inductance	henry (H)
	luminosity	watt (W)
	angular momentum	newton meter second (N m s or kg m2 s−1)
	length	meter (m)
	magnetization	ampere per meter (A/m)
	moment of force often simply called moment or torque	newton meter (N m)
	mass	kilogram (kg)
	normal vector	unit varies depending on context
	atomic number	unitless
	refractive index	unitless
	principal quantum number	unitless
	power	watt (W)
	momentum	kilogram meter per second (kg m/s)
	pressure	pascal (Pa)
	electric charge	coulomb (C)
	Heat	joule (J)
	electric charge	coulomb (C)
	electrical resistance	ohm (Ω)
	Ricci tensor	unitless
	radiancy
	radius vector (position)	meter (m)
	radius of rotation or distance between two things such as the masses in Newton's law of universal gravitation	meter (m)
	surface area	m2
	entropy	joule per kelvin (J/K)
	action
	arc length	meter (m)
	displacement
	period	second (s)
	thermodynamic temperature also called absolute temperature	kelvin (K)
	time	second (s)
	four-velocity	meter per second (m/s)
	potential energy	joule (J)
	internal energy	joule (J)
	relativistic mass	kilogram (kg)
	energy density	joule per cubic meter (J/m3) or joule per kilogram (J/kg) depending on the context
	voltage also called electric potential difference	volt (V)
	volume	cubic meter (m3)
	shear force
	velocity	meter per second (m/s)
	mechanical work	joule (J)
	width	meter (m)
	a generic unknown	varied depending on context
	displacement	meter (m)
	electrical impedance	ohm (Ω)

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