IA-JGPT-M-QIZ-A I
l'1Oian Forest . u.etmmalI0i. ·2013 PHYSICS
Paper I
(CONVENTIONAL)
ITime allowed: Three Hours I IMaximum Marks: 200 I

Question Paper Specific Instructions
Please read each ofthe following instructions carefully before attempting questions:
There are EIGHT questions in all, out ofwhich FIVE are to be attempted.
Questions no. 1 and 5 are compulsory_ Out of the remaining SIX questions, THREE are to be attempted selecting at least

ONE question from each ofthe two Sections A and B.
All questions carry equal marks. The number ofmarks carried by a part ofa question is indicated against it.
Answers must be written in ENGUSH only.
Unless otherwise mentioned, symbols and notations have their usual standard meanings.
Assume suitable data, ifnecessary and indicate the same clearly.
Neat sketches may be drawn, wherever required.
All parts and sub-parts ofa question are to be attempted together in the answer book.
Anypageorportionofthepageleftblank intheAnswerBookmustbeclearly struckout.
Useful Constants
Electron charge 1·602 x 10-19 C
Electron rest mass 9·109 X 10-31 kg
Proton mass 1·672 x 10-27 kg
Vacuum permittivity eo) 8·854 x 10-12 farad/m
Vacuum permeability 1·257 x 10-6 henry/m
Velocity of light in free space 3 x 108m1s
Boltzmann constant 1·38 x 10-23 JIK
Electron volt 1·602 x 10-19 J
Planck's constant 6·62 x 10-34 J-s
Stefan's constant 5·67 x 10-8 W m­2 K-4
Avogadro's number 6·02 x 1026 kmol-1
Gas constant 8·31 x 103 J kmol-1 K-1
exp 2·7183

A-JGPT-M-QIZ-A)
Pg 1
SECTION A
Q.l.Answer all of the following: 8x5=40
block of mass m is attached to one end of a combination of two massless springs connected in series.
The other end of the combination is fixed to a rigid support. The spring constants of the two springs are k1 and k2,
respectively. The block is free to move on a frictionless horizontal surface. If the block is pulled a little
and then released, calculate the frequency of oscillation of the block. 8
(b)Obtain an expression for the angular speed of the Earth at which Coriolis force makes objects fly from its surface. 8
(c)Consider a zone plate with radii rn=root 0·11 cm illuminated by a monochromatic light of wavelength
589 nm. Calculate the positions of first two foci. 8
satellite revolves in a circular orbit around the Earth at a certain height above it. Calculate the time period
of revolution of the satellite, if the radius of the Earth is significantly higher than the height at which the
satellite revolves. 8

plano-convex lens of radius 1·0m is placed on an optically flat glass plate. The space between the lens and the
glass plate is filled with a liquid. The diameter of the 5th ring changes to 3·0 x 10-3 m. Calculate the refractive
index of the liquid when the ring is bright. The wavelength of light used is 589nm. 8

Q.2(a) Write down the equation of motion of a weakly damped harmonic oscillator driven by a harmonic force.
Obtain an expression for the maximum amplitude of oscillation under steady-state conditions. 20
An oscillator of mass 0·01 kg draws maximum power at a frequency of 96 Hz with half power points
at 93 Hz and 99 Hz. If the amplitude of driving force is 3·88 calculate
(i)quality factor, the dumping factors amplitude of the oscillator at resonance. 20

Q.3(a)In a double slit interference experiment, show that the fringe shape is a hyperbola. 20
a thin sheet of glass of thickness t and refractive index mew is placed in the path of one of the interfering waves,
show that the distance through which a fringe is displaced is a function of t. 10
(c)Discuss spatial evolution of Fresnel diffraction into Fraunhofer diffraction. 10


4.(a)Explain the lasing action of He-Ne laser using energy level diagram and describe the operation
of He-Ne laser experimental set-up. 20
(b)Discuss the applications of lasers in communication and medicine. 10
(c)Classify fibers based on the number of modes that can propagate through them.
Depict step-index and graded-index multi mode fibers pictorially. 10



SECTIONB
Q.5. Answer all of the following: 8x5=40
(a)Using Poisson equation and spherical co-ordinates, calculate the density of continuous charge
distribution that will provide the Yukawa potential pie exp(-alpha ,where alpha is a constant. 8
(b)Sea water has resistivity 0·3ohm and its dielectric constant is 81. Calculate the ratio of the
amplitudes of the conduction and polarisation current intensities when the applied field is oscillating
at 100 MHz. 8
plane electromagnetic wave is given by
E2= acoswx coswt
and Hy -asinwx sinwt.

Evaluate the instantaneous value of the Poynting vector S and show that =0. 8
(d)Show that Wien's law and Stefan-Boltzmann law are limiting cases of Planck's radiation law. 8
(e)Calculate the net change in entropy when 10g water at 60 degree C is mixed with 30g water at 20 degree C. 8

Q.6(a)What do you understand by the term polarization in dielectrics? Obtain Clausius-Mossotti formula
for linear dielectrics. 20
(b)When the current in an R circuit is decaying, what fraction of the original energy stored in the
inductor has been dissipated after 2·3 time constant? 10
(c)With reference to ferromagnetic materials, explain the terms hysteresis and hysteresis loops. 10

Q.7(a)What is transport phenomenon? Obtain the expression for the coefficient of viscosity. Discuss its
temperature dependence. 20
(b)Define a black body. How can we realise a black body in practice? Derive expression for Planck's
radiation law. 15
(c)For a completely degenerate BE gas, discuss the condition for onset of BE condensation. 5
Q.8.(a)Establish Van der Waals equation of state for a real gas. Deduce expressions for critical constants
and show that critical coefficient is independent of the nature of gas. 20
(b)Discuss the significance of Saha's ionisation formula in classification of stars. 10
(c)State Dulong-Petit's law and discuss its limitations in explaining heat capacities of solids. How were
these efficiencies overcome by Debye 10