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UNIT 5 : LASER & INTERFERENCE


anilpangantiwar.tripod.com/physicsinfo.htm



Lasers, introduction, absorption, spontaneous emission & stimulated emission, active medium, population, thermal equilibrium, condition for light amplification, population inversion, pumping, the principal pumping schemes, three level & four level pumping schemes, optical resonator, action of optical resonator, laser beam characteristics, types of lasers, working of ruby, He-Ne & semiconductor lasers, applications.

Interference in plane parallel thin film by reflection, interference in wedge shaped thin film by reflection, Newton's rings, Determination of wavelength, radius of curvature & determination of refractive index of a liquid by Newton's ring method, applications of interference phenomena in antireflection coating.


Edited by:- Dr. A. W. Pangantiwar

What is population inversion ? How is it achieved by optical pumping ?

Explain the terms temporal and spatial coherence.

Explain the reason for monochromaticity of laser beam.

Explain the working of He-Ne laser.

Explain how lasing action is achieved in a semiconductor laser.

What is resonant cavity ? Explain in short its importance in producing a laser beam.

What is population inversion ? Explain why laser action cannot occur without population inversion between atomic levels.

Explain the difference between the laser light and light from an ordinary sourse.

Find the coherence length of white light whose spectrum lies in the region of 4000 Angstrom to 7000 Angstrom.

Obtain an expression for coherence length.

Explain in brief :-
(i) Coherence length, (ii) Spatial coherence and (iii) Temporal coherence.

Explain clearly spontaneous and stimulated emission of radiation with the help of neat diagrams. Why the probability of spontaneous emission is very high compared to stimulated emission at room temperature ? In laser how stimulated emission is made to dominate over spontaneous emission ?

A Sodium atom radiates for 0.04 s. What is the coherence length of light from a Sodium lamp ?

Why is the optical resonator required in lasers ? Illustrate your answer with neat sketches.

Describe the action of a ruby laser.

Explain the lasing action based on cavity resonator.

What are the important characteristics of laser ? Explain one of them.

What is population inversion ? How it is achieved by optical pumping ?

Explain spatial coherence and temporal coherence.

Describe the working of a solid state Ruby laser.

What do you understand by a negative temperature state ? How can it be achieved ?

In He-Ne laser, why it is necessary to use a tube of narrow diameter ?

In a He-Ne laser, what is the function of He atoms ? Explain the answer with the help of energy level diagram.

Explain the terms :-
(i) Stimulated emission, (ii) Population inversion, (iii) Pumping and (iv) Metastable state.
What is their importance in explaining the working of the laser ?

Give in brief any one engineering application.

Explain in brief the following typical characteristics of laser :-
(i) Coherence, (ii) Divergence and (iii) monochromaticity.

What is the role of a resonant cavity length in supporting different frequency modes of the   radiation ?

Explain with a diagram absorption, spontaneous emission and stimulated emission of radiation.

A ruby laser emits light of wavelength 694.4 nm. If a laser pulse is emitted for 0.00000000012 s and the energy release per pulse is 0.15 J, (i) what is the length of the pulse and (ii) how many photons are there in each pulse ?

With the help of energy band diagram discuss the working of a semiconductor laser.

Explain in brief the characteristics of a laser beam.

Discuss in brief He-Ne laser.

Obtain an expression for fringe width in interference pattern obtained with a wedge shaped thin film. How can it be used to test the optical planeness of surface ?

How can Newton's rings experiment be used to determine refractive index of a liquid ?

A soap film 0.00005 cm thick is viewed at an angle of 35 degree to the normal. Find the wavelengths of light in the visible spectrum which will be absent from the reflected light ( R.I. = 1.33 )

The calculated path difference in interference due to reflected light is given by 2 ut cos r, where symbols have their usual significance. What type of fringes do we get if :
(i) t & u are constant and
(ii) r & u are constant ?
Expalin how former are different from later.

Explain how interference phenomena can be used for:
(i) Testing the optical flatness of a surface and
(ii) Comparing the thickness of mechanical gauges.

In a Newton's rings experiment, is the central spot as seen by refraction dark or bright ? Explain.

What do you understand by antireflection coating ? Highlight their important applications.

Why are circular fringes obtained in Newton's ring arrangement ? Why are these fringes called fringes of equal thickness ? Why is the central fringe a dark spot when examined in reflected light

Suppose that in the experiment of Newton's rings, light of red colour is used first and then a blue light. Which set of rings would have the larger diameter and therefore the greater spacing between them ?

A material having an index of refraction of 1.3 is used to coat a piece of glass. What should be the minium thickness of thin film in order to minimize feflected light at wavelength of 500 nm ? What should be the refractive index of the glass to get best effects ? Why ?

Derive an expression for brightness and darkness for a monochromatic light beam reflected from a thin parallel film of transparent material.

We wish to coat a flat slab of glass ( R.I = 1.5 ) with a thinnest possible film of a transparent material so that light of wavelength 600 nm incident normally is not reflected. We have two materials to choose from M1 ( R.I. = 1.25 ) and M2 ( R.I. = 1.6 ). Which one should be appropriate ? What will be the smallest possible thickness of the coating ?

Define optical path difference and distinguish it from geometrical path difference.

What is antireflection coating ?

A glass microscope lense ( R.I. = 1.5 ) is coated with magnesium fluoride ( R.I. = 1.38 ) film to increase the transmission of normally incident light ( wavelength = 5800 Angstrom ). What minimum film thickness should be deposited on the lens ?

Obtain an expression for fringe width in wedge shaped thin film. How it is used for testing the optically flat surface ?

A wedge shaped air film is supported between two microslides. It is illuminated from above by a monochromatic light. What are salient features exhibited by the interference pattern obtained ? Discuss.

What are antireflection coatings ? Explain the principle with a neat sketch.

A thin film of cryolite ( R.I. = 1.35 ) is applied to a common lens. The coating is designed to reflect at blue end of the visible spectrum and transmit wavelength at the near infrared. What should be the minimum thickness of the film to be given to the lens to transmit highly light at 9000 Angstrom ?

Why are circular fringes obtained in Newton's rings arrangement ? Why are these fringes called fringes of equal thickness ? Wht is the central fringe is dark spot when examined in reflected light ?

Obtain condition for maxima and minima due to interference of reflected light in thin films of uniform thickness " t ".

Why are lenses coated with transparent thin films ?

A thin film 40 micro m thick is illuminated by white light normal to its surface. Its index of refraction is 1.5. What wavelength within the visible spectrum will be intensified in the reflected beam ?

Derive the formula for the n th Newton's ring in the following way :-
Consider a lens of radius of curvature R in contact with a plane surface. At a distance r from the point of contact, the thickness of the air wedge is " t ".
(i) From the Pythagorras theorem, show that :
R x R = r x r + (R-t) x (R-t)
(ii) Show that if t is very small compared to R, then t is given, to good approximation, by
t = (r x r) / (2R)
(iii) From this, show that the radius of the n th dark ring is equal to Square root of (n x wavelength x R )

Suppose that in the experiment on Newton's rings, first light of red colour is used and then blue light. Which set of rings would have the larger diameter and therefore the greater spacing between rings ?

Could Newton's rings be observed with transmitted light ? If so, would the centre spot be dark ?

A material having an index of refraction of 1.3 is used to coat a piece of glass. What should be the minium thickness of this film in order to minimize reflected light at a wavelength of 500 nm ?