West Bengal Institute of Technology (WBIT) 2010-1st Year B.Tech Electronics and Communications Engineering WBUT Engineering Physics - Question Paper
CS/B.Tech/SEM-2/PH-201/2010
Engineering Physics
Time Allotted : three Hours
Full Marks : 70
Invigilators Signature:...............
CS / B.Tech/SEM-2 / PH-201 / 2010 2010
Time Allotted : 3 Hours Full Marks : 70
The figures in.the margin indicate full mark Candidates are required to give their answers in their own words
as Jar as practicable. J)
u
( Multiple Choice Type Questions )
1. Choose the correct alternativeCor any ten of the following : 10x1 = 10
i) The average energy (qf a)) free electron in a metal at absolute zero is
P
a) zero
b) mSfppi E
/
2
than or equal to ~Ej
d) equal to Ejwhen Ejis the Fermi level.
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ii) In an interference pattern produced by the identical coherent sources of monochromatic light, the intensity at the site of central maximum is I. If the intensity at the same spot when either of the two slits is closed is I0, we must have
a) I = I0
b) I =3 2I0
c) I = 4I0
d) I and I0 are not related.
iii) Optic axis in a crystal is a
a) particular direction ''sqpendicular to which ordinary and extraordinary rays travel with the same velocity
b) particular plane along which ordinary ray travels faster thaifcgxttaordinary ray
c) particularvgtlrection along which ordinary and extroary rays travel with the same velocity
d) pai ucuiar direction along which extraordinary rays
travel.
iv) Tfte resolving power of a grating having number of rulmgs exposed in the nth order is
a) b) nN N
N N
c) d) n n
v) In a single slit Fraunhofer diffraction, when the slit width is increased the width of the central maximum
a) increases
b) decreases
c) remains unchanged
d) may increase or decrease and that dependsupon the initial value of the slit width.
vi) A black body is something which
a) does not emit any radiation in any temperature
b) in room temperature absorbs more" energy than it radiates
c) it absorbs incident radiations of all frequencies completely
d) absorption coefficient cifes with the third power of frequency.
vii) Work function of a may be obtained by irradiating
the metal surfacvlight and from
a) the x-mtppt of the stopping potential l>s frequerj 'cyta ot
b) tbCgpe of the stopping potential vs frequency plot
c) the y-intercept of the intensity l>s photocurrent plot
d) the slope of the photocurrent vs frequency plot.
3 [ Turn over
viii) Group Veloeity of the wave packet associated with
particle of energy E and momentum P is
a) b)
dE2 dE
dE .. dE2
C) d) . dp dp
ix) If fix) denotes the wave function of particle i dimensional box then the dimesiCof J[x) is of
in a or
a) length 1
b)
-/length
c) none, Jlx) is irnensionless 1
d)
x) If x-comjsoent of the momentum operator, px.
repisnted by -ih, then the commultator of x a
dX
Y a21 , A
'x, px is represented by ( here x is the positi
operator )
f
dX
a) -h2 b) ih
c) 2ft d) zero.
dx
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Example of a boson is
a) neutron
b) deuterium nucleus
c) proton
d) positon.
The coordination number for BCC structure is a) 6 by" 8
c) 12 d) 4.
The activator action in Ruby lase is
a)/ Aluminium
b) Oxygen
c) Gron
d) Chromium.
The angle between th>fcrystal planes (1, 0t 0) and (0, 0, 1) in SC structure is
a) zero **) 90'
c) 45 d) 135..
If the pow&>output of an optical fibre is ~th of the power launched into it then the attenuation loss is
a) 10 dB ' b) 1 dB
c) 01 dB d) 100 dB.
owe>
GROUP -B ( Short Answer Type Questions )
Answer any three of the following. 3 x [
2: a) A film of oil of refractive index 1-70 is placed be plane glass plate and a plan convex lens. The rad curvature of the les is 1 metre. Determine the rad the 10th dark ring formed in the Newton ring { given that the wavelength of lightOOO A
b) State and explain Maluss law o (polarization.
3. a) A light of wavelength 50Qnil incident normally slit of variable width Yarge length. A detec placed at a large distance on the other side at an of 20 with the norgafc For that width of the slit the receiver showintensity ?
b) How are continuous X-rays produced ? Find o relation between the accelerating voltage of the tube aiioS&etninimum wavelength of X ay produ
4. a) is the rest mass of electron and C is the vek
A and the kinetic energy of an electron is equa
pest energy then find its de Broglie wavelength in of Plancks constant m0 and C.
If a system has two eigenstates ipj and ip eigenvalues Ex and E2 ( under what conditio linear combination ( = aipj +fcnp2) be als
eigenstate ?
tc c<
If the wave function (x) of quantum mechanical particle is given by
W (x) = a sin {nx/L) for 0 sxsL = 0 otherwise,
then determine the value of a.
Also determine the value of x where probability of finding the particle is maximum. 2 + 2
Mention one application of graded index optical fibre. 1
b)
6. a)
A system has non-degenerate single particle states with energy levels 0, E, 2E, 3E. Three /iicles are to be distributed in these states such that the total energy is 6E. Write down in each case (pill possible microstates assuming that the particles jefctfroM-B statistics or F-D stat,!tlcs'
What is acceptance angle <sFan optical fibre ? Explain with a diagram. 1
b)
P - C Type Questions )
AnswejRmTiree of the following. 3 x 15 = 45
Fraunherluble slit diffraction pattern is observed in the foM)lane of a lens of focal length 50 cm. The distance between two maxima adjacent to principal maxima is 5 mm and the fourth order is missing. Find the width of each slit and the distance between their
7. a)
o
centres. Given, X = 5000 A. 5
b) Write down the expression for intensity distribution of single slit diffraction pattern and explain the symbol used. Sketch how the intensity varies with angle of diffraction, 0. Find the values of the angles at which minima are located. Also find the condition that 0 satisfies in order to be the position of a secondary maximum. 2 + 2 + 4
c) In a Youngs double slit interference experiment distance between the coherent sources is 1*15. mm. Calculate the fringe width that would be observed on a screen placed at a distance of 85<xcm from the source.
Xx oO
The wavelength of light used is 58S3 A. 2
8, aL Calculate the minimum thickness of a calcite plate
which would convert plane polai'feed light into circularly polarized light. ue = 1 '486 and \xQ =1-658 and
X = 5890 A. , 2
What is Brewsters nragle ? What is the value of this angle for glass of refractive index 1-5 immersed in water of refractive indel33 ? 1+2
p) What do youomsan by temporal and spatial coherence ? Define coheFmylength. 3
d] Find tkS<P*tton between lattice constant and density of the crystalline material for a cubic cell. 3
e) Inertain crystal, the crystal plane of Miller indices
(110) has a separation 1*12 A. If these planes show
0
X-ray diffraction with wavelength 1-54 A, calculate up to which order, Braggs reflection can be observed. 3
f\y What is a Bravais lattice ? 1
CS/B.Tech/SEM-2 /PH-201 /2010 What is Compton shift ? 2
Find the change in wavelength of an X-ray photon when it is scattered through an angle of 135 C by a free electron. Use approximate values of the constants. 2
Why in case of macroscopic bodies uncertainty principle is not relevant ?
o
Describe photoelectric effect. How does classical theory
functions of an atom are ijjand respectively, the
V. )i
corresponding energy E0 and E1. If the probability
of finding the atom in fhe ground state is 90% and that for the excij %ate is 10%, then find the average
energy of the/atom. Also determine the normalized wave function ol the atom. 2 + 2
Show that the momentum of a photon of frequency v is
hv
given by p = , where h is Plancks constant and C is c
the velocity of light in vacuum. 2
9. a) b)
c)
d) e)
fail to explain this effect ? 2 + 2
The ground state and the exitea>state normalized wave
Q
10. a) A particle, of mass m, moving in three dimensions is confined within a box 0 < x < a, 0 < y < b, 0 < z < c. ( The potential is zero inside and infinite outside.) Write down Schrodinger equation for the particle. By considering a stationary state wave function of the form X(x, y, z) = /1(x)/2(y)/3(z), show that the allowed
energies are =n
4- X 2 a -| 4
jz n n t rio n?
--L j--v a. _
2m + +
degeneracy of the first excited energy level when a = b = c ? Explain, What is the ground state energy ?
1 + 4 + 2 + 1
What is the
b) Show that the groip velocity of a wave packet associated with particle is same as the particle
velocity.
c) Write down Plancks law of frequency distribution of Black body ldiation. Convert the expression into the wavelength distribution and find out th limiting form of this distribution at very small wavelengths. How does this result compare with the predictions from Rayleigh -J e aix for mu la ? 1+2+1
11. a) Write down the time-dependent Schrodinger equation explaining the symbols used. Derive time-independent Schrodinger equation assuming that the potential is time independent. 1+2
b) What are Fermions ? Give two examples. 1 + 1
c) Write the expression of B-E distribution law and derive Plancks back body radiation law from B-E statistics.
1+6
Sketch the Fermi distribution for T = 0 and T > 0 and
CS/B.Tech/SEM-2/PH-20-l /201C Define the following : 3
i) Microstate.
ii) Macrostate.
iii) Thermodynamic probability.
What do you mean by spontaneous qjA stimulated
b)
emission of radiation ? Why i&opulation inversion
2 + 2
necessary in a laser ?
Describe He-Ne laser sour diagram.
c)
relevant energy level
6
Find the atomic pckilfactor for body centred cubic and face centredeiikic crystals. 2
d)
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