Practice Question Paper
Class XII –Physics
(Chapter: Electromagnetic Waves, Dual Nature of Radiation and
matter, Atom & Nuclei)
Time Allowed: 3 Hours
Maximum Marks: 70
-----------------------------------------------------------------------------------------------------------------------General Instructions
1. All questions are compulsory. There are 26 questions in all.
2. This question paper has five sections: Section A, Section B, Section C, Section D and
Section E.
3. Section A contains five questions of one mark each, Section B contains five questions of
two marks each, Section C contains twelve questions of three marks each, Section D contains
one value based question of four marks and Section E contains three questions of five marks
each.
4. There is no overall choice. However, an internal choice has been provided in one question
of two marks, one question of three marks and all the threequestions of five marks weightage.
You have to attempt only one of the choices in such questions.
5. You may use the following values of physical constants wherever necessary.
c = 3 x 108 m/s
h = 6.63 x 10-34Js
e = 1.6 x 10-19 C
μo = 4 x10-7T m A-1
0
= 8.854 x 10-12 C2 N-1m-2
= 9 x 109 N m2 C-2
Mass of electron= 9.1 x 10-31 kg
mass of neutron = 1.675 x 10-27kg
mass of proton = 1.673 x10-27 kg
Avogadro’s number = 6.023 x 1023 per gram mole
Boltzmann constant = 1.38 x10-23 JK-1
Page 1 of 7
Section A
What is average value of electric energy density in Electromagnetic waves?
Why thin gold foil is used in alpha-particle scattering experiment? Give any two reason.
What is an angular momentum of an electron in Bohr’s hydrogen atom whose energy is3.4 eV ?
If radius of the
nucleus is estimated to be 3.6 Fermi, then calculate the radius of
nucleus?
Write the de-Broglie wavelengths corresponding to an electron accelerated by potential
difference of 10 kV?
=3.162].
Section B
Explain Ampere-Maxwell Law?
(i) Write the dynamic mass and momentum of photon?
(ii) The cut-off wavelength for a metal surface is 4950 Å . Give its significance.
Define atomic mass unit of substance and find mass energy equivalence of it?
Figure is the plot of the stopping potential versus the
frequency of the light used in an experiment on
photoelectric effect. Find
(a) the ratio h/e and
(b) the work function.
Figure shows tracks of three charged particles in a
uniform electrostatic field. Give the signs of the three charges. Which particle
has the highest charge to mass ratio?
1
2
+ + + + + + + + + + + + +
– – – – – – – – – – – – – –
3
OR
Explain the following with example
(i) Ionisation and excitation potential
(ii) Ionisation and excitation energy.
Section C
The total energy of an electron in the first excited state of the hydrogen atom is about
-3.4 eV.
(a) What is the kientic energy of the electron in this state ?
(b) What is the potential energy of the electron in this state ?
(c) Which of the answer above would change if the choice of the zero of potential energy
is changed?
Energy levels of A,B,C of a certain atom correspond to ascending values of energy,i.e.
EA < EB < EC . If
and
are the wavelengths of radiation corresponding to the
transition C to B, B to A and C to A respectively ,then prove that
Prove that radii of stable orbits of Hydrogen atom are in the ratio 1:4:9:16-------------------.?
Page 2 of 7
Write nuclear equations for:
(a) the -decay of
(b) the -decay of
(c) the
decay of
Calculate mass defect, Binding energy and Binding Energy per nucleon?
Given: M H 1.007825 M P
(Here M H is Mass of Hydrogen Atom)
Mn
1.008665
and
M He 4
.
4.00260
Draw a graph showing variation of potential energy of a pair of nucleons as a function of
their separation. Indicates the region in which nuclear force is (a) attractive
(b) repulsive. Write two important conclusions which you can draw regarding the nature
of the nuclear forces.
Given below are some famous numbers associated with electromagnetic radiation in
different contexts in physics. State the part of the em spectrum to which each belongs.
(a) 21 cm ( wavelength emitted by atomic hydrogen in intersteller space).
(b) 1057 MHz (frequency of radiation arising from two close energy levels in hydrogen :
known as Lamb shift).
(c) 2.7 K [ temperature associated with the isotropic radiation filling all space-thought to
be a relic of sodium]
(d) 5890 Å - 5896 Å [ double lines of sodium]
(e) 14.4 keV [ energy of a particular transition in 57 Fe nucleus associated with a famous
high resolution spectroscopic method (Mossbauer spectroscopy].
(f) 0.1 Å -10 Å ( associated with Coolidge tube).
Answer the following questions:
(a) Optical and radio-telescopes are built on the ground but X -ray astronomy is
possible only from satellites only from satellites of orbiting the earth. Why?
(b) The small ozone layer on top of the stratosphere is crucial for human survival .why?
(c) If the earth did not have an atmosphere, would its average surface temperature
higher or lower than what it is now?
(d) Some scientists have predicted that a global nuclear war on the earth would be
followed by a severe ‘nuclear winter’ with a devastating effect on life on earth. What
might be basis of this prediction ?
(a) Write the de - Broglie's hypothesis realeted to the dual nature of matter.
(b) Calculate the ratio of de-broglie wavelengths of proton and a -particle accelerated
by
(i) Same potential
(ii) Same kinetic energy
Why the explanation of photoelectric effect is not possible on the basis of classical wave
theory ? Explain
OR
The rate of disintegration of a radioactive substance falls from 800 decay/min
to 100 decay/min in 6 hours. Then calculate half-life of the radioactive
substance?
(a) The rest mass of photon is zero. Explain?
(b) Draw a curve for dependence of stopping potential on
(i) Frequency of incident light and (ii) Intensity of incident light
Describe in brief the Devision and Germer experiment for experimental verification of
de-Broglie hypothesis
OR
How the Bohr's stationary orbits are described by then principle of matter waves ?
Explain. Also draw n = 3 orbit as per de-Broglie hypothesis.
Page 3 of 7
Section D
Chirag was at the restaurant chatting with his cousins. The restaurant was clean and
free of flies and insects to his relief. His cousin was curious to know about the uv lamp
in the corner and asked Chirag about it. Chirag explained that inside the fluorescent
lamp, the electrical energy is converted into uv radiation. The inside of the tube is
coated with a fluorescent powder which absorbs the UV and emits violet light in the
visible region. These attract the insects which are electrocuted by high voltage wires
near the lamp, so that they don’t fall on the food and contaminate them.
i) Name the main source of uv rays?
ii) Why are they considered harmful to us?
iii) Why are they considered beneficial to us?
iv) What impressed you about chirag?
Section E
A sinusoidal electromagnetic plane wave travel through empty space in the x-direction.
The electric wave is polarized in the y-direction. Frequency of wave is 1.5 GHz. At a certain
point P in space, the oscillating electric field Ey attains a maximum magnitude Eo = 63V/m.
(a)What is the wave length of the wave?
(b)What is the maximum magnitude Bo of the magnetic field at the same point? What is the
dirction of B?
(c) Express the electric and magnetic fields as functions of x and t in the vicinity of P.
(d) Draw a diagram showing the propagation of wave, indicating clearly the direction of
field associated with it?
Describe in detail the experimental observations related to photoelectric effect.
OR
What do you mean by photoelectric effect and photo sensitive materials?
Define stopping potential and write Einstein's photoelectric equation in terms of stopping
potential. Calculate stopping potential, If energy of incident photon is 40 x 10-19 J and
work function is 7eV?
State the law of radioactive decay and deduce the expression
. Plot a graph
showing the number (N) of undecayed nuclei as a function of time for a given radioactive
sample.
OR
For scattering by an ‘inverse square’ field (such as that produced by charged nucleus in
Rutherford’s model) the relation between impact parameter ‘b’ and scattering angle is is
given by
(a) What is the scattering angle for b = 0 ?
(b) For a given impact parameter b, does the angle of deflection increase or decrease
with increasing energy ?
(c) What is the impact parameter at which the scattering angle is 90 for Z= 79 and
initial energy of 10 MeV ?
(d) Why is it the mass of the nucleus does not enter the formula above but charge does ?
(e) For a given energy of the projectile, does the scattering angle increase or decrease
with decrease in impact parameter ?
Page 4 of 7
Q.No
ANSWER KEY
Marks
1
e= 0E2/4
(i) Nuclei of gold is heavy ,so alpha particle are more deflected
(ii) It is malleable, so it can used to make thin foil.
½ +½
E=-13.6/n2
L=nh/2 = h/
R = R0 (A)1/3
½ +½
n=2
RTe = 5/3 × 3.6
½ +½
RTe = 6 Fermi
½ +½
“The total current passing through any surface of which closed loop
is perimeter” is the sum of the conduction current and displacement
current.
The generalised law is
1
½ +½
(i)
(ii) Photoelectric effect does’t occur if wave length of incident light is
greater than this and or
W0 = 2.5 eV
Definition, 1amu.=1.6605 x 10-27Kg
E
1
mc 2
m 1a.m.u. 1.66 10
27
kg
1
1
c 3 108 m / s
1.66 10 27 3 108 1.4924 10 10 Joule
1.4924 10 10
E
MeV
1.6 10 13
1a.m.u. 931.5MeV / c 2
E 931.5 931MeV or
(a) V=h /e –W/e
h/e= slope of graph=4 x10-15
(b) W= h 0=0.8eV
1
E
Charges 1 and 2 are negative, charge 3 is positive. Particle 3 has the
highest charge to mass ratio.
Defination eg E1=-13.6eV then V=13.6eV
1
1
1+1
1+1
(a)K.E.=-(-3.4)=+3.4 ev
1
(b)P.E.=-2K.E=-2(3.4)=-6.8 ev
1
(c) P.E. & Total energy
1
3
3
(a)
(b)
1
+
1
+ e– +
Page 5 of 7
(c)
B He
1
+ e+ + n
4
2M H
2M n
M He
4
1
931.5 MeV
2 1.007825 1.008665 4.00260 931.5 MeV
0.03038 931.5MeV
B He
4
28.3MeV
E
E 28.3
B or E
7.0MeV
A
4
Ncert page 445 fig 13.2
; gka 4m
0.03038a.m.u
1
1
1+1+1
(a) Radio (short wavelength end) (b) Radio (short wavelength end)
(c) Microwave
(d) Visibloe (Yellow)
(e) X-rays (or soft -rays) region. (f) X-rays
1/2x6=3
(a) Atmosphere absorbs X-rays, while visible and radiowaves can
penetrate it.
¾
(b) It absorbs ultraviolet radiations from the sun and prevents it
from reaching the earth’s surface and causing damage to life.
¾
(c) The temperature of the earth would be lower because the
Greenhouse effect of the atmosphere would be absent.
¾
(d) The clouds produced by global nuclear war would perhaps cover
substantial parts of the sky preventing solar light from reaching
many parts of the globe. This would cause a ‘winter’
(a) Hypothesis
¾
(b)
2
using this relation
Shortcomings wave theory 3 statement
OR A = A e – t
0
1+1+1
1
100 = 800 e – (6 × 60)
e –360 = 1/8
– 360 = – ln8
T 1/2 = ln2/ = 120 min. or T 1/2 = 2 hrs.
1
= ln2/120
OR
1
1
1+1+1
2+1
i) Sun is the main source of uv rays.
1
ii)They can cause skin cancer when exposed for a longer time
1
iii) UV induces production of vitamin D in the skin at rates of up to
1,000 IUs per minute. This vitamin helps to regulate calcium
metabolism (vital for the nervous system and bone health),
immunity, cell proliferation, insulin secretion, and blood pressure.
1
iv) clarity in explaining, health awareness, knowledge
1
(a) =0.20m
1
Page 6 of 7
1
(b)Bo = 2.1x10-7T, z-direction
(c)
,
(d) Diagram, indicating clearly the direction of field associated with it?
OR
V0= 18eV
1+1
1
1+1+1+1+1
OR (a) =1800
1
(b) decreases
1
(c) b= 1.137x10-14m
1
(d) proper explanation
1
(e) increases
1
Page 7 of 7