Assignment-Physics Name-SUPREET KUMAR SINGH UID-18BPE1031 Question No

Assignment-Physics Name-SUPREET KUMAR SINGH UID-18BPE1031
Question No.1: What is the physical significance of numerical aperture? Derive the relation between numerical aperture and fractional refractive index change.

Answer:
Numerical Aperture (NA) is a number having no dimension or unit which stretches between the range of angles by which system can emit or accept the light. Its significance is that the light which enters the cone having semi-vertical im propagates only through the fiber. As the value of im or NA increases the light collected for propagation also increases in the fiber. Therefore we can say that NA is light gathering power.

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Relation between Numerical Aperture and fractional refractive index change:
?= fractional index change
= n1-n2/n1 <<1
NA=Numerical Aperture
sin?a= n1sin?c
sin?a= n1(1-cos2?c)1/2
= n11-{n2/n1}21/2
= (n12-n22)1/2
?a= sin-1(NA)
NA=(n12-n22)1/2 ~ n1(2?)1/2
Question Number.2: Give the operation of He-Ne laser with its essential components. How stimulated emission takes place with exchange of energy between He and Ne atoms?
Answer:
The essential components of Helium-Neon Laser are:
Pump Source
Gain Medium
Resonating Cavity
Operation:
When the power is switched on, an energetic electron collisionally excites a He atom. A He atom in this excited state is often written He*, where the asterisk means that the He atom is in excited state.

The excited He* atom collides with an unexcited Ne atom collides with an unexcited Ne atom and the atoms exchange internal energy, with an unexcited He atom and excited Ne atom, written Ne*, resulting. This energy exchange process occurs with high probability only because of the accidental near equality of the two excitation energies of the two levels in these atoms. Thus, the purpose of population inversion is fulfilled.

When the excited Ne atom passes from meta-stable state to lower level, it emits photon of 632nm.

This photon travel through the gas mixture parallel to the axis of the tube, it is reflected back and forth by the mirror ends until it stimulates an excited Ne atom and causes it to emit a photon of 632 nm with the stimulating photon.

The stimulated transition from higher energy level to lower energy level is laser transition. This process is continued and when a beam of coherent radiation becomes sufficiently strong, a portion of it escape through partially silvered end. The Ne atom passes to lower energy level emitting spontaneous emission and finally the Ne atom comes to ground state through collision with tube wall and undergoes radiationless transition.

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