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Published on: 25/10/2025
Download CBSE Class 12th Standard CBSE Physics question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Physics
Questions + Answers key
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1.
Write two uses of microwaves
2.
The current in a circuit containing a capacitor is 0.15A.What is the displacement current and where does it exist?
3.
The magnetic field in a plane electromagnetic wave is given by By = (2 × 10–7) T sin (0.5 x103x + 1.5 x 1011t) .
(a) What is the wavelength and frequency of the wave?
(b) Write an expression for the electric field.
4.
How are microwaves produced?
5.
Give difference between displacement and conduction current
6.
The electric field of a plane e.m.wave in vacuum is represented by; \(\overset { \rightarrow }{ { E }_{ x } } =0\)
\(\overset { \rightarrow }{ { E }_{ y } } =0.5cos\left[ 2\pi \times { 10 }^{ 8 }\left( t-\frac { x }{ c } \right) \right] ;\ \overset { \rightarrow }{ { E }_{ 2 } } =0\)
(a) What is the direction of propagation of electromagnetic waves?
(b) Determine the wavelength of the wave.
(c) Compute the component of the associated magnetic field.
7.
Name the parts of the electromagnetic spectrum which is
(a) suitable for radar system used in aircraft navigation
(b) used to treat muscular strain
(c) use as a diagnostic tool in medicine
Write in brief, how these waves can be produced.
8.
A parallel plate capacitor of plate separation 2mm is connected in an electric circuit having source voltage 400 V. What is the value of the displacement current for \({ 10 }^{ -6 }\) second if the plate area is \(60 \ { cm }^{ 2 }\)?
9.
The correct option if in vacuum the speed of gamma rays, x-rays and microwaves are \({ v }_{ g }, \ { v }_{ x }and \ { v }_{ m }\)
\({ v }_{ g }<{ v }_{ x }<{ v }_{ m }\)
\({ v }_{ g }<{ v }_{ x }>{ v }_{ m }\)
\({ v }_{ g }>{ v }_{ x }>{ v }_{ m }\)
\({ v }_{ g }={ v }_{ x }={ v }_{ m }\)
10.
The refractive index and the permeability of a medium are respectively 1.5 and \(5\times { 10 }^{ -7 }H{ m }^{ -1 }\). The relative permittivity of the medium is nearly
25
15
81
10
6
11.
The waves used in Telecommunications are
infrared
ultraviolet
microwaves
cosmic rays
12.
In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of 2.5 x 1010 Hz and amplitude 480 V/m.The amplitude of oscillating magnetic field will be
1.52 x 10-8 Wb/m2
1.52 x 10-7 Wb/m2
1.6 x 10-6 Wb/m2
1.6 x 10-7 Wb/m2
13.
All the known radiations from a big family of electromagnetic waves which stretch over a large range of wavelengths. Electromagnetic wave include radio waves, microwaves, visible light waves, infrared rays, UV rays, X-rays and gamma rays. The orderly distribution of the electromagnetic waves in accordance with their wavelength or frequency into distinct groups having widely differing properties is electromagnetic spectrum.
(i) Which wavelength of the Sun is used finally as electric energy?
| (a) radio waves | (b) infrared waves |
| (c) visible light | (d) microwaves |
(ii) Which of the following electromagnetic radiations have the longest wavelength?
| (a) X-rays | (b) \(\Upsilon\)-rays |
| (c) microwaves | (d) radiowaves |
(iii) Which one of the following is not electromagnetic in nature?
| (a) X-rays | (b) gamma rays |
| (c) cathode rays | (d) infrared rays |
(iv) Which of the following has minimum wavelength?
| (a) X-rays | (b) ultraviolet rays |
| (c) \(\Upsilon\)-rays | (d) cosmic rays |
(v) The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is
| (a) microwave, infrared, ultraviolet, gamma rays |
| (b) gamma rays, ultraviolet, infrared, microwave |
| (c) microwave, gamma rays, infrared, ultraviolet |
| (d) infrared, microwave, ultraviolet, gamma rays |
14.
15.
Assertion (A) : While applying Ampere's circuital law to given surfaces with same perimeter, the left hand side \(\oint B \cdot d l=\mu_0 i(t)\) has not changed but the right hand side is zero.
Reason (R) : No current passes through the surface.
(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
(b) Both Assertion and Reason are true but Reason in not the correct explanation of Assertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.
16.
Assertion (A) : Velocity of light is constant in all media.
Reason (R) : Light is an electromagnetic wave which has constant velocity in all media.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false
1.
(i) In radar system for aircraft navigation.
(ii) In microwave ovens for cooking.
2.
Since, we know, Ic = Id = 0.15 A
3.
(a) Comparing the given equation with
\(B_{y}=B_{0} \sin \left[2 \pi\left(\frac{x}{\lambda}+\frac{t}{T}\right)\right]\)
We get, \(\lambda=\frac{2 \pi}{0.5 \times 10^{3}} \mathrm{~m}=1.26 \mathrm{~cm}\)
and \(\frac{1}{T}=v=\left(1.5 \times 10^{11}\right) / 2 \pi=23.9 \mathrm{GHz}\)
(b) E0 = B0c = 2 x 10–7 T x 3 x 108 m/s = 6 x 101 V/m
The electric field component is perpendicular to the direction of propagation and the direction of magnetic field. Therefore, the electric field component along the z-axis is obtained as
Ez = 60 sin (0.5 x 103x + 1.5 x 1011 t) V/m.
4.
microwaves are produced due to an oscillating current in special vacuum tubes, namely klystrons and magnetrons.
5.
conduction current is due to flow of electrons in the circuit. It exists even if the flow of electrons is at uniform rate. Displacement current is due to the time-varying electric field. It does not exist under steady condition.
6.
(a) Equation second shows that the e.m.wave travels along the positive x-axis
(b) Wavelength of wave, \(\lambda =\frac { c }{ v } =\frac { c }{ (\omega /2\pi ) } =\frac { 2\pi c }{ \omega } =\frac { 2\pi \times (3\times { 10 }^{ 8 }) }{ 2\pi \times { 10 }^{ 8 } } =3.0m\)
(c) since the magnetic field is perpendicular to an electric field as well as the direction of propagation of e.m.wave, hence magnetic field must be varying along a z-axis. Therefore
\(\overset { \rightarrow }{ { B }_{ x } } =0; \ \overset { \rightarrow }{ { B }_{ y } } =0; \ and \ \overset { \rightarrow }{ { B }_{ z } } =\frac { 0.5 }{ 3\times { 10 }^{ 8 } } cos\left[ 2\pi \times { 10 }^{ 8 }(t-x/c) \right] \ \therefore [E/B=c]\)
7.
(i) Microwave
(ii) Infrared waves are used to treat muscular strain.
(iii) X-rays are used as a diagnostic tool in medicine. X-rays are produced by X-ray tubes or inner shell electrons.
8.
\({ I }_{ D }={ \epsilon }_{ 0 }\frac { { d\phi }_{ E } }{ dt } ={ \epsilon }_{ 0 }\frac { EA }{ t } =\frac { { \epsilon }_{ 0 }(V/d)\times A }{ t }\)
\( =\frac { { \epsilon }_{ 0 }VA }{ td } =\frac { 8.85\times { 10 }^{ -12 }\times 400\times (60\times { 10 }^{ -4 } }{ } \)
\( =1.062\times { 10 }^{ -2 }\)
9.
(d)
\({ v }_{ g }={ v }_{ x }={ v }_{ m }\)
10.
(e)
6
11.
(c)
microwaves
12.
(c)
1.6 x 10-6 Wb/m2
13.
(i) (b): Infrared rays can be converted into electric energy as in solar cell.
(ii) (d): Radiowaves have longest wavelength.
(iii) (c) : Cathode rays are invisible fast moving streams of electrons emitted by the cathode of a discharge tube which is maintained at a pressure of about 0.01 mm of mercury.
(iv) (c): \(\Upsilon\)-rays have minimum wavelength
(v) (a): \(\lambda_{\text {micro }}>\lambda_{\text {infra }}>\lambda_{\text {ultra }}>\lambda_{\text {gamma }}\)
14.
15.
(a) Both Assertion and Reason are true and Reaton is the correct explanation of Assertion.
On applying Ampere's circuital law to such sarfaces with the same perimeter, we find that the left hand side of equation \(\oint B \cdot d l=\mu_0 i(t)\)has not changed but the right hand side is zero and not \(\mu_0\)i. Since, no carrent passes through the surface.
16.
(d): Velocity of light has different values in different media. It depends on the refractive index of the medium. Related by formula
\(v_{\text {medium }}=\frac{\text { velocity in vacuum }}{\text { refractive index of medium }}\)
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