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It would decrease.

(a) Perpendicular to each other.

Correct answer is (d) 3 x 10¹⁴ rad s^-1

i. There are two basic modes of communication:

a. point to point communication 

b. broadcast communication

ii. In point to point communication mode, communication takes place over a link between a single transmitter and a receiver e.g. telephony.

iii. In the broadcast mode, there are large number of receivers corresponding to the single transmitter e.g., Radio and Television transmission.

Three basic (essential) elements of every communication system are transmitter, communication channel and receiver.

Correct option is: (B) visible light

(D) message signal generator, modulator and antenna

Correct answer is (c) 30 – 300.

Correct option is: (C) sky wave propagation

Correct answer is (d) Microwave region.

Correct answer is (A) 32000 m

Expression for EM wave travelling along X-axis,

\(\overrightarrow{B}\) = \(\hat{k}\) [4 × 10^-4 sin (ωt – kx)]

Here, B₀ = 4 × 10^-4

Given: q = 5 µC = 5 × 10^-6 C

v = 5 × 10⁵ m/s along Y-axis

∴ E₀ = cB₀ = 3 × 10⁸ × 4 × 10^-4

= 12 × 10⁴ N/C

Maximum electric force = qE₀

= 5 × 10^-6 × 12 × 10⁴

= 0.6 N

For E₀ = 120 N/C, v = 50 MHz = 50 × 10⁶ Hz

i. λ = \(\frac{c}{v}=\frac{3\times10^8}{50\times10^6}\) = 6 m

B₀ = \(\frac{E_0}{v}=\frac{120}{3\times10^8}\) = 4 × 10^-7 T = 400 nT

k = \(\frac{2\pi}{λ}=\frac{2\pi}{6}\) = 1.0472 rad/m

ω = 2πv = 2π × 50 × 10⁶

= 3.14 × 10⁸ rad/s.

ii. Assuming motion of em wave along X-axis, expression for electric field vector may lie along Y-axis,

∴ \(\overset\rightarrow{E}\) = E₀ sin (kx – ωt)

= 120 sin (1.0472 × – 3.14 × 10⁸ t) \(\hat{j}\) N/C.

Also, magnetic field vector will lie along Z-axis, expression for magnetic field vector,

∴ \(\overset\rightarrow{E}\) = B₀ sin (kx – ωt)

= 4 × 10^-7 sin (1.0472 × – 3.14 × 10⁸ t) \(\hat{k}\) T.

Given : B = 5 × 10^-7 T, c = 3 × 10⁸ 

To find : Amplitude of electric field (E₀)

Formula : c = E₀/B₀ 

Calculation : From formula,

E₀ = c × B₀ 

= 3 × 10⁸ × 5 × 10^-7 

= 150 V/m

As electromagnetic waves are plane polarised, so the receiving antenna should be parallel to electric/magnetic part of the wave.

Given: v = 28 MHz, E = 9.6 V/m,

c = 3 × 10⁸ m/s

To find:

i. Amplitude of magnetic field (B)

ii. Direction of B

Formula:

|B| = \(\frac{|E|}{c}\)

Calculation: From formula,

|B| = \(\frac{9.6}{3\times10^8}\) = 3.2 × 10^-8 T

Since that E is along Y-direction and the wave propagates along X-axis. The magnetic induction, B should be in a direction perpendicular to both X and Y axes, i.e., along the Z-direction.

(a), (c), (d)

(a) Radiation pressure is I/c if the wave is totally absorbed. 

(c) Radiation pressure is 2I/c if the wave is totally reflected. 

(d) Radiation pressure is in the range I/c < p < 2I/c for real surfaces.

Electric field of an EM wave is an oscillating field and so is the electric force caused by it on a charged particle. This electric force averaged over an integral number of cycles is zero since its direction changes every half cycle. Hence, electric field is not responsible for radiation pressure.

E₀ = 810V/m, B₀ = ?

We know, B₀ = μ₀∈₀CE₀

Putting the values,

B₀ = 4π x 10^–7 x 8.85 x 10^–12 x 3 x 10⁸ x 810

= 27010.9 x 10^–10 = 2.7 x 10^–6T = 2.7μT.

Correct option is: (A) ionosphere

Given: V₁ = 7.5 MHz = 7.5 × 10⁶ Hz,

V₁ = 12 MHz = 12 × 10⁶ Hz.

To find: Wavelength band

Formula: λ = \(\frac{c}{v}\)

Calculation: From formula,

V₁ = \(\frac{3\times10^8}{7.5\times10^6}\) = 40 m

V₁ = \(\frac{3\times10^8}{12\times10^6}\) = 25 m

∴ Wavelength band = 40 m to 25 m

 X-Rays. Frequency range : 3 x 10¹⁶ Hz ^{– 3} x 10¹⁹ Hz.

The broad spectrum of electromagnetic waves, stretching from γ rays (wavelength ~10^–12 m) to long radio waves (wavelength ~10⁶ m). 

James Clerk Maxwell (1831-1879), argued that time-varying electric field generates magnetic field. 

Maxwell formulated a set of equations involving electric and magnetic fields, and their sources, the charge and current densities. These equations are known as Maxwell’s equations. The most important prediction to emerge from Maxwell’s equations is the existence of electromagnetic waves, which are (coupled) time varying electric and magnetic fields that propagate in space. The speed of the waves, according to these equations, turned out to be very close to the speed of light( 3 ×10⁸ m/s), obtained from optical measurements. This led to the remarkable conclusion that light is an electromagnetic wave. Maxwell’s work thus unified the domain of electricity, magnetism and light. 

Hertz, in 1885, experimentally demonstrated the existence of electromagnetic waves. Its technological use by Marconi by inventing Radio Communication. 

(1) Accelerated charges radiate electromagnetic waves. The frequency of the electromagnetic wave naturally equals the frequency of oscillation of the charge. The energy associated with the propagating wave comes at the expense of the energy of the source – the accelerated charge. 

(2) Hertz’s experiment (1887) produced low frequency region (the radio wave region). 

(3) Jagdish Chandra Bose, working at Calcutta, produced electromagnetic waves of much shorter wavelength (25 mm to 5 mm). 

(4) Guglielmo Marconi in Italy succeeded in transmitting electromagnetic waves over distances of many kilometres.

An oscillating charge produces an oscillating electric field in space, which produces an oscillating magnetic field. The oscillating electric and magnetic fields regenerate each other, and this results in the production of em waves in space.

There are different waves in EM spectrum :

(a) Radio waves

(b) Microwaves 

(c) Infrared waves

(d) Visible rays

(e) Ultraviolet rays 

(f) X-rays

(g) Gamma rays

The electric field E₀  is 1/r

Multiple Choice Questions for an important part of exams for class 12 Physics and if practiced properly can help you to get higher marks. Refer to more Chapter-wise MCQ Questions and also learn more latest study material for all subjects. CBSE Class 12 Physics MCQ Questions of Electromagnetic Waves with Answers available here. The MCQ Questions for Class 12 Physics with answers have been prepared as per the latest syllabus and exam pattern. 

Let’s check Multiple Choice Questions with Answers that are given below. These MCQ Questions of Electromagnetic Waves help to understand exact pattern of exam and level of questions.

Practice MCQ Question for Class 12 Physics chapter-wise 

1. Which of the following laws was modified by Maxwell by introducing the displacement current?

(a) Gauss’s law
(b) Ampere’s law
(c) Biot-Savart’s law
(d).None of these

2. Choose the wave relevant to telecommunication :

(a) ultraviolet
(b) infrared
(c) microwave
(d) visible light

3. EMW are produced by:

(a) charge in uniform motion only
(b) charge at rest only
(c) accelerated or decelerated charge only
(d) all of the above

4. Who first demonstrated the existence of em waves?

(a) Hertz
(b) Maxwell
(c) Ampere
(d) Faraday

5. Name the law which states that a varying electric field produces a magnetic field:

(a) Biot-Savart’s law
(b) Faraday’s law
(c) Modified Ampere’s law
(d) None of these

6. Solar radiation is :

(a) transverse EMW
(b) longitudinal EMW
(c) both (a) and (b)
(d) None of these

7. Light Year is the unit of:

(a) distance
(b) time
(c) energy
(d) intensity of light

8. The electromagnetic waves do not transport

(a) energy
(b) charge
(c) momentum
(d) information

9. The path along which electric current flows is called

(a) electric circuit
(b) insulator
(c) conduct
(d) resistor Medium

10. Displacement current goes through the gap between the plates of a capacitor when the charge on the capacitor

(a) is changing with time
(b) decreases
(c) does not change
(d) decreases to zero

11. Which of the following has/have zero average value in a plane electromagnetic wave?

(a) Both magnetic and electric field
(b) Electric field only
(c) Magnetic energy
(d) Electric energy

12. According to Maxwell’s hypothesis, a changing electric field gives rise to

(a) an e.m.f
(b) electric displacement current
(c) magnetic field
(d) pressure gradient

13. An electromagnetic wave propagating along north has its electric field vector upwards. Its magnetic field vector point towards

(a) north
(b) east
(c) west
(d) downwards

14. When radio waves passes through ionosphere, phase difference between space current and capacitive displacement current is

(a) 0 rad
(b) (3π /2) rad
(c) (π/ 2) rad
(d) π rad

15. Electromagnetic wave consists of periodically oscillating electric and magnetic vectors

(a) in mutually perpendicular planes but vibrating with a phase difference of π
(b) in mutually perpendicular planes but vibrating with a phase difference of π/2
(c) in randomly oriented planes but vibrating in phase
(d) in mutually perpendicular planes but vibrating in phase

16. We consider the radiation emitted by the human body. Which one of the following statements is true?

(a) The radiation emitted is in the infrared region
(b) The radiation is emitted only during the day
(c) The radiation is emitted during the summers and absorbed during winters
(d) The radiation is emitted lies in the ultraviolet region and hence is not visible

17. X-rays, gamma rays and microwaves travelling in vacuum have

(a) same wavelength but different velocities
(b) same frequency but different velocities
(c) same velocity but different wavelength
(d) same velocity and same frequency

18. Radio waves diffract around building, although light waves do not. The reason is that radio waves

(a) travel with speed larger than c
(b) have much larger wavelength than light
(c) are not electromagnetic waves
(d) None of these

19. Microwaves are detected by

(a) bolometer 
(b) point contact diodes
(c) thermopiles 
(d) the eye

20. An accelerated electron would produce

(a) \(\gamma\)-rays 
(b) \(\beta\)-rays
(c) \(\alpha\)-rays 
(d) e.m. waves

21. Radio waves of constant amplitude can be generated with

(a) rectifier 
(b) filter
(c) F.E.T. 
(d) oscillator

22. When electromagnetic waves enter the ionised layer of ionosphere, then the relative permittivity i.e. dielectric constant of the ionised layer

(a) does not change
(b) appears to increase
(c) appears to decrease
(d) sometimes appears to increase and sometimes to decrease

23. Which colour of the light has the longest wavelength?

(a) violet
(b) red
(c) blue
(d) green

24. X-rays are not used for radar purposes, because they are not 

(a) reflected by target
(b) partly absorbed by the target
(c) electromagnetic waves
(4) completely absorbed by the target

25. The frequency 1057 MHz of radiation arising from two close energy levels in hydrogen belongs to

(a) radio wave
(b) infrared waves
(c) micro wave
(d) \(\gamma\)-rays

Answer:

1. Answer : (b) Ampere’s law

Explanation: Maxwell found that Ampere circutal law is logically inconsistent. To make this law logically consistant, Maxwell introduce the concept of displacement current, which is known as Ampere Maxwell's law.

2. Answer : (c) microwave

Explanation: Electromagnetic radiation is used for communications and transmission of information. The waves that are used in communication are radio waves, microwaves, infrared radiation and light. But in telecommunication microwaves is used.

3. Answer : (c) accelerated or decelerated charge only

Explanation: Any charged particle can generate an electromagnetic wave. All it has to do is accelerate or decelerate. Using electrons to generate electromagnetic waves is really standard procedure.

4. Answer : (a) Hertz

Explanation: Heinrich Rudolf Hertz was a German physicist, who first conclusively proved the existence of Electromagnetic waves then theorised by James clerk Maxwell. So Hertz is the one who proved the existence of electromagnetic waves.

5. Answer : (c) Modified Ampere’s law

Explanation: Modified Ampere's law states that the variations of electric field causes magnetic field.

6. Answer : (a) transverse EMW

Explanation: Solar radiations are transverse electromagnetic waves. The cont7. ral care of the sun emits a continuous electromagnetic spectrum.

7. Answer : (a) distance

Explanation: A light-year is a measurement of distance and not time (as the name might suggest). A light-year is the distance a beam of light travels in a single Earth year, or 6 trillion miles (9.7 trillion kilometers).

8. Answer : (b) charge

Explanation: EM waves carry energy (light ), momentum (can be seen by photoelectric experiment) and information (like radiowaves ) but doesn't carry charge.

9. Answer : (a) electric circuit

Explanation: An electric circuit is a path in which electrons from a voltage or current source flow. The point where those electrons enter an electrical circuit is called the "source" of electrons. The point where the electrons leave an electrical circuit is called the "return" or "earth ground". 

10. Answer : (a) is changing with time

Explanation: Displacement current arises when electric field in a region is changing with time. It will be so if the charge on a capacitor is changing with time.

11. Answer : (a) Both magnetic and electric field

Explanation: Both magnetic as well as electric field in EM waves are sinusoidal in nature and we know that the average value of sinusoidal wave is zero. Hence, both magnetic and electric fields have zero average value in a plane EM wave.

12. Answer : (c) magnetic field

Explanation: According to the Maxwell's EM theory, the EM waves propagation contains electric and magnetic field vibration in mutually perpendicular direction. Thus the changing of electric field five rise to magnetic field.

13. Answer : (b) east

Explanation: As the electromagnetic wave is the crossed field of electric and magnetic waves. So, the direction of propagation of EM wave is the direction of vector E \(\times\) B. Here E is upward and (E \(\times\) B) is towards north. So, from right hand thumb rule B will be along east.

14. Answer : (a) 0 rad

Explanation: The phase difference between space current and capacitive displacement current is zero.

15. Answer : (d) in mutually perpendicular planes but vibrating in phase

Explanation: Electromagnetic wave consists of periodically oscillating electric and magnetic vectors. in mutually perpendicular planes but vibrating in phase. Electromagnetic wave consists of periodically oscillating electric and magnetic vectors in mutually perpendicular planes but vibrating in phase.

16. Answer : (a) The radiation emitted is in the infrared region

Explanation: The heat radiation emitted by the human body is the infrared radiation. Their wavelength is of the order of 7.9×10⁻⁷m to 10⁻³m which is of cource the range of infrared region. Hence, human body emits radiation in infrared region.

17. Answer : (c) same velocity but different wavelength

Explanation: In a vacuum, all light travels the same speed irrespective of the wavelength or frequency of the wave. Gamma radiation has a smaller wavelength and larger frequency, radio waves a smaller frequency and larger wavelength. So, x−rays, gamma rays and microwaves traveling in vacuum have the same velocity but different wavelengths.

18. Answer : (b) have much larger wavelength than light

Explanation: The wavelength of radiowaves being much larger than light, has a size comparable to those of buildings, hence diffract from them.

19. Answer : (b) point contact diodes

Explanation: Point contact diodes, commonly called crystals, are the oldest microwave semiconductor devices. Unlike the pn-junction diode, the point-contact diode depends on the pressure of contact between a point and a semiconductor crystal for its operation.One section of the diode consists of a small rectangular crystal of n-type silicon. A fine beryllium-copper, bronze-phosphor, or tungsten wire presses against the crystal and forms the other part of the diode During the manufacture of the point contact diode, a relatively large current is passed from the the wire to the silicon crystal. The result of this large current is the formation of a small region of p-type material around the crystal in the vicinity of the point contact. Thus, a pn-junction is formed which behaves in the same way as a normal pn-junction. The pointed wire is used instead of a flat metal plate to produce a high-intensity electric field at the point contact without using a large external source voltage.

20. Answer : (d) e.m. waves

Explanation: An electron moving with a changing velocity that is, when it is accelerating uniformly, however, generates a changing magnetic field, produces an electric field which is changing, and this produces a changing magnetic field. An accelerating charged particle hence produces electromagnetic waves.

21. Answer : (d) oscillator

Explanation: Radio waves of constant amplitude can be produced by using oscillator with proper feedback.

22. Answer : (c) appears to decrease

Explanation: When electromagnetic waves enter the ionised layer, the relative permitting of the ionised layer appears to decrease.

23. Answer : (b) red

Explanation: On one end of the spectrum is red light, with the longest wavelength. Blue or violet light has the shortest wavelength. White light is a combination of all colors in the color spectrum. It has all the colors of the rainbow.

24. Answer : (a) reflected by target

Explanation: X-rays being of high energy radiations, penetrate the target and hence are not reflected back.

25. Answer : (a) radio wave

Explanation: The frequency 1057 MHz of radiation arising from two close energy levels in hydrogen belongs to radio waves.

Click here to practice  MCQ Question for Electromagnetic Waves Class 12

Structure of atoms can be studied with X-rays.

Given: h = 500 m, R = 6.4 × 10⁶ m

To find: Maximum distance or range (d)

Formula: d = \(\sqrt{2Rh}\)

Calculation: From formula,

d = \(\sqrt{2Rh}\) = \(\sqrt{2\times64\times10^6\times500}\)

= 8 × 10⁴

= 80 km

Yes, ordinary electric lamp emits EM waves.

No. 

In vacuum, an electric field cannot directly induce another electric field so a “pure” electric field wave cannot exist and same can be said for a “pure” magnetic wave.

(c) Dimensions of 1/(μ₀ε₀) is L²/T² 

(d) deflects and gradually comes to the original position in a time which is large compared'to the time constant.

Radio frequency wave have wavelength ranging from 10 m to 10⁴ m.

A. radiation

Microwaves heat food like the sun heats your face—by radiation. Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food.

Correct answer is (c) γ – rays.

Correct answer  is  (b) 2 \(\frac{E}{C}\)

a. Transverse wave : A wave in which particles of the medium vibrate in a direction perpendicular to the direction of propagation of wave is called transverse wave. 

b. Longitudinal wave : A wave in which particles of the medium vibrate in a direction parallel to the direction of propagation of wave is called longitudinal wave.

On decreasing the frequency, reactance Xc=1/ωC will increase which will lead to decrease in conduction current. In this case i_D = i_C; hence displacement current will decrease.

The answer is (C) Intensity

(b) 30m

Wave length, λ = \(\frac{C}{v}\) = \(\frac{3\ times 10^8}{10 \times 10^6}\)= 0.3 x 10² = 30m.

(i – a, b), (ii – d), (iii – c)