LASER
THE PRINCIPLE OF LASER:
The laser light is a source that produces a beam of highly coherent and a very monochromatic light as a result of a co – operative emission from many atoms.
Principle of laser: Let is consider an assembly of atoms of some kind ‘hf’. If we somehow raise the atoms of the metastable level and let a light of frequency ‘f’ fall upon them, there will be more induced emission from the metastable level than that of induced absorption by the lower level. As a result an amplification of the original light is obtained. This is the principle of laser.
Ruby laser:
The ruby is a crystal of Aluminum oxide (Al2O3) mixed with it 0.05% of chromium oxide (Cr2O3).
Construction: A ruby laser consists of a cylindrical ruby rod whose ends are optically flat. One end is fully silvered and the other end is partially silvered. The ruby rod is enclosed in a glass tube which is surrounded by a xenon flash tube which acts as an optical pumping device in figure.
Working: The chromium atoms are excited from E1 state to E3 state by absorption of light of wavelength 350 nm produced by xenon flash tube. The excited chromium atoms then undergo non radioactive transition to E2 state which is metastable state having means time about 10-3 sec. When population inversion (N2> N1) is achieved, some of the atoms undergo spontaneous transition from E2 to E1, thus emitting photons of frequency f = E3 – E1/h. The photons moving not parallel to the ruby rod escapes out and those moving not parallel to the ruby rod escape out and those moving parallel to the rod gets deflected back by the silvered surface. The photons which are moving parallel to the ruby rod can cause the stimulated emission. As a result, a beam of coherent photons moving in phase and parallel to the ruby rod comes out of the partially silvered end of the rod.
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Population inversion:
Let N1 and N2be the number of atoms lying in the ground state E1 and E2 respectively. Under ordinary condition of thermal equilibrium the number of atoms in the higher energy state is considerably smaller than that of the number of atoms in the lower energy state is N2< N1. In such a situation of a light of frequency f = E2−E1h is incident on a large collection of such atoms, the atoms are excited due to absorption of photons and raise to the excited state E2than that in the lower energy state E1 i.e. N2> N1. This phenomenon of having more number of atoms in the excited state than in the ground state is called population inversion.
The process by which population inversion is carried out is called optical pumping.
Optical pumping is a method of achieving population inversion. For this atom which have three energy states E1, E2 and E3 are taken when E3> E2> E1. E1 is a ground state, E2in metastable state and E3 is short lived state. The atoms in the E1 state are pumped into E3stated by photon of energy hf = E3 – E1 by stimulated absorption. E3 is shortly lived state and E3 to E1 transition is prohibited. Since, N3> N2 cannot be achieved, so simulated emission results from E3 to E2 transition. Thus, the atoms in the atoms in the E3 state then go to the E2 state either by simultaneous emission or by non radioactive transition in which the energy E3 – E2 is converted into vibration energy of the atom forming the substance. Since E2 is metastable state, atoms can remain in this state for comparatively longer time. As a result, population inversion (i.e. N2> N1) takes place. Now, the atoms in E2 state are bombarded by a photon of energy hf = E2 – E1to cause them to a stimulated emission of radiation of energy hf in the direction of incident photon. As a result a highly intense coherent and unidirectional beam of radiation comes. The beam is called laser beam.
Construction of He-Ne laser:
It consist of a long and narrow discharge tube about 8ocm long and 1cm in diameter, fill with the mixture of Heat a pressure of 1mm and Ne at a pressure 0.1mm of Hg, which forms the leaser medium. Two electrodes P and Q fitted to the discharge tube as shown in figure M and N are the two mirrors which form a resonant cavity. The mirror M is fully reflecting whereas N is partially reflecting so, allow the laser beam to pass out of it.
Working:
As soon as the electric discharge passes through the mixture of He and Ne gases electron in the tube are accelerated. Theses collide with helium atom and excited them to higher energy level S2 and S3.as shown in figure. These levels of the helium atoms are metastable and the excited helium atom s remains in these for a long time before being de-excited. On the other hand, some of the excited energy state, say E4and E6 of neon correspond very approximately to the energy state S2 and S3.
Because of this when helium atoms in the energy state S2and S3 collide with neon atoms in the ground state E1, Ne atom absorb energy and are excited to the energy state E4and E6whereas He atom lose energy and are de-excited to the ground state S1. This process is continuously transfer more and more neon atom from the ground state to the excited energy sate E4 and E6
There are to more energy E3 and E5 slightly below the sates E4and E6 and another energy state E2 in Ne. Since the energy state E4and E6are highly populated, there is a population inversion between the state E4and E6and the lower energy sate E3and E5. And the emission of the radiation are transition between E6and E5,E4 and E3, and E6and E2 respectively lead to emission of wavelength 3.539μm, 1.15μm and 6328A0. The first two lies on the infra red region and the last cross ponds to the red light from He-Ne laser in the visible region.
In an improved of the He-Ne laser, the resonator mirror, P and Q are used externally to the lase cavity because the mirror sealed inside the discharge tube are eroded by gas discharge and have to replaced.
Further for the minimization of the loss due to reflection at the ends of the discharge tube, Brewster windows W1 and W2 are used as shown in figure these are fitted at an angle called Brewster angle and given by tanɵB=μ
Where μ is the refractive index of the material of which the window are made.
Uses:
It is used in holography
It is used in scientific research.
It is used in medical treatment.
Heisenberg’s uncertainty principle:
In classical mechanics position co-ordinates, component of momentum, components of angular momentum are measured in an arbiter precision but in quantum mechanics particle is considered as wave and components of position and momentum are measured with highly improved precision as compared to classical mechanics.
The amplitude of the wave remaining almost zero in wide range of wave but specific value in narrow region of wave indicates high precision in position of the particle measured as momentum is conjugate quantity with position, high precision in position reflects worst precision in momentum. Hence, Heisenberg states that ‘it is impossible to determine position and momentum simultaneously with 100% accuracy.
These quantities are related as:
Δx. Δp ∼ h
Where Δx is uncertainty in position
Δp is uncertainty in momentum
Planks constant
This equation was further improved by Heisenberg which can be expressed as
Δx.Δp≥h2π
Uncertainty principle proposed by Heisenberg was further modified by Kennard and obtained the result as:
Δx.Δp≥h4π
The most important thing is that uncertainty principle holds true for both microscopic and microscopic particle and it is independent with the method of measurement.
Electron cannot be constituents of nucleus:
Let electron lie inside the nucleus. Then its uncertainty in position is equal to diameter (size) of nucleus which is about 2 * 10-14m.
So, uncertainty in position of electron,
Or, Δx = 2 * 10-14m
Using, Heisenberg’s uncertainty principle the uncertainty in momentum of electron is given by:
Or, ΔP = h2πΔn
So, Uncertainty in speed of electron is:
Or, ΔV=ΔPme
= h2πΔn.me
= 6.62∗10−342π∗2∗10−14∗9.1∗10−31
= …… ms-1 >>>> C
Where, C is the speed of light. Since, speed of electron cannot be greater than speed of light, electron cannot be constituents of nucleus.
THE END