LASER

Light Amplification by Stimulated Emission of Radiation(LASER) is a device that emits light. It is done by a process known as optical amplification where there is stimulated emission of photons.

The gain medium is a material with properties that allow it to amplify light by stimulated emission.A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium.  In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.

The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

History of LASER: MASER

In 1953, Charles Hard Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave adiation rather than infrared or visible radiation. Townes's maser was incapable of continuous output. Meanwhile, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov were independently working on the quantum oscillator and solved the problem of continuous-output systems by using more than two energy levels. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion. In 1955, Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, later a main method of laser pumping.

Townes reports that several eminent physicists — among them Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch, and Llewellyn Thomas — argued the maser violated Heisenberg's uncertainty principle and hence could not work. In 1964 Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics, “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser–laser principle”.

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Plasma - The 4th state of Matter

Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. After sufficient heating a gas dissociates its molecular bonds, rendering it into constituent atoms. Further heating leads to ionization (a loss or gain of electrons), thus turning it into a plasma, containing charged particles: positive ions also called cations or negative ions also called anions and electrons.


Plasma is found on Earth also. The lightning that we see, is made up of plasma. The Aura Borealis and Aura Australis are also made up of plasma. Stars are mainly made up of plasma. Hydrogen and Helium ions and electrons make up the plasma state. 

The number of electrically charged particles is non-negligible. Therefore, Plasma is affected easily by electrostatic fields. Plasma, therefore, has properties quite unlike those of solids, liquids, or gases and is considered a distinct state of matter. Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container; unlike gas, under the influence of a magnetic field, it may form structures such as filaments, beams and double layers. Some common plasmas are stars and neon signs. Like how Hydrogen(H) and Helium(He)  fill up most of the Universe, plasma state is most ordinary state of matter.

The uniquely different plasma

 Plasma is widely used in laboratories to perform tests and experiments. The main world wide use is its use in TVs, mobile phones, LED displays etc. Plasma screens are used in these products. Due better flow of charge, the efficiency of the item increases. 

Plasma Tubes

Check out this video for better understanding of the Plasma

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Special article: Time Dialation

Time dilation is an observed difference of elapsed time between two observers which are moving relative to each other, or being differently situated from nearby gravitational masses. An observer will see the other observer's clock ticking at a slower rate than his/hers. This effect doesn't arise from technical aspects of the clock or the fact that any signal needs time to propagate, but from the nature of space-time described by the theory of relativity.

Relative velocity time dilation

When two observers are in relative uniform motion and uninfluenced by any gravitational mass, the point of view of each will be that the other's (moving) clock is ticking at a slower rate than the local clock. The faster the relative velocity, the greater the magnitude of time dilation. This case is sometimes called special relativistic time dilation. It is often interpreted as time "slowing down" for the other (moving) clock. But that is only true from the physical point of view of the local observer, and of others at relative rest (i.e. in the local observer's frame of reference). The point of view of the other observer will be that again the local clock (this time the other clock) is correct and it is the distant moving one that is slow. From a local perspective, time registered by clocks that are at rest with respect to the local frame of reference (and far from any gravitational mass) always appears to pass at the same rate.

Simple inference of time dilation due to relative velocity

Time dilation can be inferred from the observed fact of the constancy of the speed of light in all reference frames.

This constancy of the speed of light means, counter to intuition, that speeds of material objects and light are not additive. It is not possible to make the speed of light appear faster by approaching at speed towards the material source that is emitting light. It is not possible to make the speed of light appear slower by receding from the source at speed. From one point of view, it is the implications of this unexpected constancy that take away from constancies expected elsewhere.

Consider a simple clock consisting of two mirrors A and B, between which a light pulse is bouncing. The separation of the mirrors is L and the clock ticks once each time it hits a given mirror.

In the frame where the clock is at rest (diagram at right), the light pulse traces out a path of length 2Land the period of the clock is 2L divided by the speed of light:

From the frame of reference of a moving observer traveling at the speed v (diagram at lower right), the light pulse traces out a longer, angled path. The second postulate of special relativity states that the speed of light is constant in all frames, which implies a lengthening of the period of this clock from the moving observer's perspective. That is to say, in a frame moving relative to the clock, the clock appears to be running more slowly. Straightforward application of the Pythagorean theorem leads to the well-known prediction of special relativity:

The total time for the light pulse to trace its path is given by

The length of the half path can be calculated as a function of known quantities as

Substituting D from this equation into the previous and solving for Δt' gives: 

and thus, with the definition of Δt:

which expresses the fact that for the moving observer the period of the clock is longer than in the frame of the clock itself.

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