Electromagnetic Waves Important Questions | Class 12 | Physics | Chapter 8 2024

Last updated on July 14th, 2024 at 04:53 pm

Electromagnetic Waves Questions and Answers

Below are some of the very important NCERT Class 12 Physics Chapter 8 Electromagnetic Waves Questions and Answers. These Class 12 Electromagnetic Waves Questions and Answers have been prepared by expert teachers and subject experts based on the latest syllabus and pattern of term 2 questions with answers to help students understand the concept.

These Questions for Class 12 Physics Electromagnetic Waves Questions and Answers are very important for the latest CBSE term 2 pattern. These Class 12 Q and A are very important for students who want to score high in CBSE Board.

We have put together these NCERT Questions of Class 12 Physics Chapter 8 Electromagnetic Waves Questions and Answers for practice on a regular basis to score high in exams. Refer to these Questions with Answers here along with a detailed explanation.

Short Answer (SA) Type Questions

1. How are electromagnetic waves produced by oscillating charges? What is the source of the energy associated with the EM waves?

2. i) An electromagnetic wave is travelling in a medium with a velocity. Draw a sketch showing the propagation of the electromagnetic wave indicating the direction of the oscillating electric and magnetic fields.

ii) How are the magnitudes of the electric and magnetic fields related to velocity of the electromagnetic wave?

3. A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the direction of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz. What is its wavelength?

4. The electric field of an electromagnetic wave is given by E = (50 NC^-1)sinω(t-x/c). Find the energy contained in a cylinder of cross-section 10 cm² and length 50 cm along the x-axis.

5. An electromagnetic wave is travelling in vacuum with a speed of 3×10⁸ m/s. Find its velocity in a medium having relative electric and magnetic permeabilities 2 and 1, respectively.

6. Even though and electric field E exerts a force qE on a charged particle yet electric field of an electromagnetic wave does not contribute to the radiation pressure (but transfer energy). Explain.

7. Show that the radiation pressure exerted by an EM wave of intensity I on a surface kept in vacuum is I/C.

8. i) Why are infrared waves often called heat waves? Explain.

ii) What do you understand by this statement, “electromagnetic waves transport momentum”?

9. Identify the electromagnetic wave, whose wavelengths vary as

i) 10^-12 m < λ < 10^-8 m
ii) 10^-3 m < λ < 10^-1 m

Write one use of each.

10. i) Arrange the following electromagnetic waves in the descending order of their wavelength.

a) microwaves
b) infrared rays
c) ultraviolet radiation
d) gamma rays

ii) Write one use each of any two of them.

11. Name the constituent radiation of electromagnetic spectrum which is used for

i) aircraft navigation and
ii) studying the crystal structure

Write the frequency range for each.

12. Use the formula λ_mT = 0.29 cm-K to obtain the characteristic temperature ranges for different part of the electromagnetic spectrum. What do the numbers that you obtain tell you?

13. i) Find the energy stored in a 90 cm length of laser beam operating at 6mW.

ii) Find the amplitude of electric field in a parallel beam of light of intensity 17.7 W/m².

14. Name the electromagnetic wave with their frequency range, produced in

a) some radioactive decay
b) sparks during electric welding and
c) TV remote 

Short Answer Type Question Answer

1. An oscillating charge is an example of accelerated charge. It produces an oscillating electric field, which produces an oscillating magnetic field, which in turn produces an oscillating electric field and so on. The oscillating electric and magnetic fields read and write each other as a wave which propagates through space.

Electric and magnetic fields are the source of energy associated with EM waves.

2. (i) Given that, velocity v = v i, i.e. the wave is propagating along X-axis, so electric field is along Y-axis and magnetic field is along Z-axis. The propagation of electromagnetic wave is shown in the figure.

(ii) speed of electromagnetic waves can be given as

where E_o and B_o are peak values of E and B on instantaneous value of E and B.

3. The direction of electromagnetic wave is perpendicular to both electric and magnetic fields. Here, electromagnetic wave is travelling in Z-direction, then electric and magnetic fields are in xy-direction and are perpendicular to each other.

Frequency of waves, f = 30 MHz = 30 x 10⁶ Hz
Speed, c = 3 x 10⁸ m/s

Using the formula, c = f.λ
λ = c/f = (3 x 10⁸) / (30 x 10⁶) = 300 / 30 = 10 m

4. The average value of energy density (energy/volume) is given by

U_av = (ε_oE_o²)/2

Total volume of the cylinder, V = A.l

Total energy contained in the cylinder
U = (U_av)(V) = (ε_oE_o²).(Al)/2

Substituting the values we have
U = ((8.86×10^-12)(50)²(10×10^-4)(50×10^-2))/2 J = 5.5×10^-12 J

5. Given velocity of the electromagnetic wave in vacuum, c = 3×10⁸ m/s

Relative electric permeability, ε_r = 2

Magnetic permeability, μ_r = 1

Since, velocity of electromagnetic wave in a medium can be calculated by

6. Electric field of an electromagnetic wave is an oscillating field which causes force is on the charged particle. This electric force averaged over an integral number of cycles is zero, because its direction changes with every half cycle. So, electric field is not responsible for radiation pressure.

7. Pressure = Force / Area

Force is the rate of change of momentum
F = dp / dt

Energy in time dt, U = dp . c
dp = U / c

Pressure = U / (A.c.dt) = I / c
(where Intensity(I) = U / (A.dt)

8. (i) Infrared waves have frequency lower than those of visible light, they can vibrate not only the electrons, but also in atom or molecules in the structure of the surface.

This vibration increases the internal energy and hence the temperature of the structure, which is why elected infrared waves are often called heat waves.

(ii) Electromagnetic wave transports linear momentum as it travels through space. If an electromagnetic wave transfers a total energy U to a surface in time t, then total linear momentum delivered to the surface is given as

p = U/c

Were c is the speed of electromagnetic wave

9. (i) 10^-12 m – 10^-8 m = 0.1 A^o = 100 A^o → X-Ray
It’s used in in crystallography.

(ii) 10^-3 m – 10^-1 m = 0.1 cm – 10 cm → Microwaves
It is used in microwave oven for cooking purposes.

10. (i) The decreasing order of wavelength of electromagnetic waves is
Microwave > Infrared waves > Ultraviolet radiation > Gamma rays

(ii) Microwaves are used in RADAR devices.
Gamma rays are used in radio therapy

11. (i) Microwave are used for aircraft navigation, their frequency range is 10⁹ Hz to 10¹² Hz.
(ii) X-Rays are used to study crystal structure, their frequency range is 10¹⁶ Hz to 10²⁰ Hz.

12. Given, λ_mT = 0.29 cm-K
λ_m= 0.29 / (Tx100) m

Let we take, λ_m = 10^-6 m

Required absolute temperature, T = 0.29 / (100 x 10^-6) = 2900 K

Lete we take, λ_m = 5×10^-5 m

Required absolute temperature, T = 0.29 / (100 x 5 x 10^-5) = 6000 K

13. (i) The time taken by wave to move a distance 90 cm,
t = (90 x 10^-2) / (3 x 10⁸) = 3 x 10^-9 s

Energy contained in 90 cm length,
U = Pt
U = 6 x 10^-3 x 3 x 10^-9 = 18 x 10-¹² J

(ii) Intensity of light (I) = (ε_oE_o²c)/2
17.7 = ((8.85 x 10^-12) x E_o² x 3 x 10⁸) / 2
E_o² = (4 x 10⁴) / 3
E_o = 2 x 10² / √3

Therefore, the amplitude of electric field in parallel beam
E_o = (2 x 10²) / √3 V/m

14. (a) Gamma rays – 3 x 10¹⁸ Hz to 5 x 10²² Hz
(b) Ultraviolet rays – 10¹⁴ Hz to 10¹⁶ Hz
(c) Radio waves – 54 MHz to 890 MHz

Long Answer (LA) Type Questions

1. Suppose that, the electric field amplitude of an electromagnetic wave is E_o = 120 N/C and that its frequency is 50 MHz

i) Determine B_o, ω, k and λ
ii) Find expression for E and B

2. Suppose that, electric field part of an electromagnetic wave in vacuum is E=[3.1cos{1.8y+(5.4×10⁶t)}] i

a) what is the direction of propagation?
b) what is the wavelength?
c) what is the frequency
d) what is the amplitude of magnetic field part of the wave?
e) write an expression for magnetic field part of the wave.

3. In a plane electromagnetic wave, the electric field oscillates sinusoidally at frequency of 2.0×10¹⁰ Hz and amplitude 48 V/m.

i) What is the wavelength of the wave?
ii) What is the amplitude of oscillating magnetic field?
iii) Show that, the average energy density of the electric field equals the average energy density of the magnetic field.

4. i) Which segment of electromagnetic wave has highest frequency? How are these waves produced? Give one use of these waves.

ii) Which electromagnetic waves lie near the high frequency and of visible part of electromagnetic spectrum? Give its one use. In what way this component of light has harmful effects on humans?

5. Answer the following questions.

i) Show by giving a simple example how electromagnetic waves carry energy and momentum.
ii) How are microwaves produced? Why is it necessary in microwave oven to select the frequency of microwaves to match the resonant frequency of water molecules?
iii) Write two important uses of infrared waves.

6. i) Name the electromagnetic waves which are used for the treatment of certain forms of cancer. Write their frequency range.

ii)  Ozone layer on top of status fare is crucial for human survival. Why?

iii)  Why is the amount of momentum transferred by electromagnetic waves incident on the surface so small?

7. State clearly how a microwave oven works to heat up a food item containing water molecules. Why are microwaves found useful for the raw systems in aircraft navigation?

8. Name the parts of electromagnetic spectrum which is

i) suitable for RADAR systems in aircraft navigation
ii) used to treat muscular strain
iii) used as a diagnostic tool in medicine

Write in brief, how these waves can be produced?

9. Given below are some famous members associated with electromagnetic radiation in different context in physics. State the part of the electromagnetic spectrum to which it belongs.

i) 21cm (wavelength emitted by atomic hydrogen in interstellar space)
ii) 1057 MHz (frequency of radiation arising from to close energy levels and hydrogen, known as Lamb shift)
iii) 2.7 K (temperature associated with the isotropic radiation filling all space thought to be a relic of The Big Bang origin of the universe.
iv) 5890 A^o – 5896 A^o (double lines of sodium)
v) 14.4 keV (energy of particular translation in ⁵⁷Fe nucleus associated with the famous high resolutions spectroscopic method)

Long Answer Type Question Answers

1. Given, amplitude of an electromagnetic wave, Eo = 120 N/C
Frequency of wave, f = 50 MHz = 50 x 10⁶ Hz

(i) Speed of light in vacuum, c = E_o/B_o
B_o = E_o / c = 120 / (3 x 10⁸) = 40 x 10^-8
B_o = 400 x 10^-9 T = 400 nT

Angular frequency of wave, ω = 2πf = 2 x 3.14 x 50 x 10⁶
ω = 3.14 x 10⁸ rad/s

Wave number of electromagnetic waves,
k = ω / c = (3.14 x 10⁸) / (3 x 10⁸) = 1.05 rad/m

Wavelength of electromagnetic wave,
λ = c / f = (3 x 10⁸) / (50 x 10⁶) = 6 m

(ii) Expression of electric field, E = E_osin(kx-ωt) = 120sin(1.05x-(3.14×10⁸t))

Expression of magnetic field, B = B_osin(kx-ωt) = 4 x 10^-7sin(1.05x-(3.14×10⁸t))

2. (i) The given equation signifies that, the electromagnetic wave is moving along Y-axis and also in negative direction, so it moves in -j-direction.

(ii) The electric part of electromagnetic wave in vacuum
E = [3.1cos{1.8y+(5.4×10⁶)}] i

Comparing with standard equation,
E = E_ocos(ky+ωt)
Angular frequency, ω = 5.4×10⁶ rad/s

Wave number k = 1.7 rad/m

The amplitude of the electric field part of the wave, Eo = 3.1 N/C
λ = 2π / k = 2π / 1.8 = 3.491 m = 3.5 m

(iii) Angular frequency, ω = 2πf
f = ω/2π = (5.4 x 10⁶ x 7) / (2 x 22) = 0.86 x 10⁶ Hz

(iv) c = E_o/B_o
B_o = E_o/c = 3.1 / (3 x 10⁸) = 1.03 x 10^-8 T

(v) Expression for the magnetic field part of wave
B = B_ocos(ky+ωt) k = 1.03 x 10^-8 cos (1.8y + (5.4×10⁶t)) k

3. Given, frequency of oscillation = 2 x 10¹⁰ Hz
Speed of wave, c = 3 x 10⁸ m/s
Electric field, E_o = 48 V/m

(i) Wavelength of waves, λ = c / f = (3 x 10⁸) / (2 x 10⁸) = 1.5 x 10^-2 m

(ii) Using the formula, B_o = E_o / c = 48 / (3 x 10⁸) = 1.6 x 10^-7 T

(iii) Average energy density of electric field, u_E = (ε_oE_o²) / 4
E_o = B_o x c
u_E = (ε_oB_o²c²) / 4
c = 1 / √(μ_oε_o)
u_E = (ε_oB_o²) / (4μ_oε_o)
u_E = B_o² / (4μ_o)

Average energy density in EM waves
u = (ε_oE_o²) / 2 = B_o² / 2μ_o

Thus, the average energy density of the electric field equals the average energy density of magnetic field.

4. (i) Gamma rays has the highest frequency in the electromagnetic waves. These rays are of the nuclear origin and are produced in the disintegration of radioactive atom and nuclei and the decay of certain sub-atomic particles. They are used in the treatment of cancer and tumors.

(ii) Ultraviolet rays lie near the high frequency and of visible part of electromagnetic spectrum. These rays are used to preserve food stuff. The harmful effects from exposure of ultraviolet (UV) radiation can be life threatening and include premature aging of the skin, suppression of the immune system, damage to the eyes and skin cancer.

5. (i) Consider a plane perpendicular to the direction of propagation of the wave. An electric charge, on the plane will be set in motion by an electric and magnetic field of electromagnetic wave, incident on this plane. This is only possible, if electromagnetic wave constitutes momentum and energy. Thus, this illustrates that electromagnetic waves carry energy and momentum.

(ii) Microwaves are produced by special vacuum tube like the klystron, magnetron and Gunn diode. The frequency of microwaves is selected to match the resonant frequency of water molecules, so that energy is transformed efficiently to increase the kinetic energy of molecules. Thus, facilitating the food cook properly.

(iii) Uses of infrared waves :-
(a) to know the molecular structure and therapy to heal muscular pain
(b) In remote control of TV, VCR, etc.

6. (i) Gamma Rays are used for the treatment of certain forms of cancer. Its frequency range is 3×10¹⁹ Hz to 5×10²² Hz.

(ii) The thin ozone layer on top of stratosphere absorbs most of the harmful ultraviolet rays coming from sun to earth the earth. They include UVA, UVB and UVC radiations, which can destroy the life system on the earth. Therefore, this layer is crucial for human survival.

(iii) An electromagnetic wave transports linear momentum as it travels through space. If an electromagnetic wave transport transfers a total energy U to a totally absorbing surface in time ‘t’, then total linear momentum delivered to the surface,
p = U / c = hf / c
This means, the momentum range of electromagnetic waves is 10^-19 to 10^-41. Thus, the amount of momentum transferred by the electromagnetic waves incident on the surface is very small.

7. In microwave oven, the frequency of the microwave is selected to match the resonant frequency of water molecules. This leads to the vibrations of these molecules. As these vibrations increase with time, the temperature increases leading to production of heat and this is the heat which is responsible for the cooking of food in the oven.

As microwaves are short wavelength radio waves, with frequency of order of GHz. Due to short wavelength, they have high penetrating power with respect to atmosphere layers. So, these waves are suitable for the RADAR systems used in aircraft navigation.

8. (i) Microwaves are suitable for RADAR system that are used in aircraft navigation. These rays are produced by special vacuum tubes, namely klystrons, magnetrons and Gunn diode.

(ii) Infrared rays are used to treat muscular strain. These rays are produced by hot bodies and molecules.

(iii) X-rays are used as a diagnostic tool in medicine. These rays are produced, when high energy electrons are stopped suddenly on a metal of high atomic number.

9. (i) The wavelength (21 cm) corresponds to the radio waves.

(ii) This frequency (1057 MHz) also corresponds to the radio waves (short wavelength).

(iii) T = 2.7 K
λ_mT = b = 0.29 cm-K
λ_m = 0.29 / 2.7 = 0.11 cm

This wavelength corresponds to the microwave region of the electromagnetic spectrum.

(iv) This wavelength lie in the visible region of the electromagnetic spectrum.

(v) Energy, E = 14.4 keV = 14.4 x 10³ x 1.6 x 10^-19 J
f = E / h = (14.4 x 10³ x 1.6 x 10^-19) / (6.6 x 10^-34) = 3.5 x 10¹⁵ Hz

This frequency lies in the X-ray region of the electomagnetic spectrum.

Case Study Based Question

1. X-ray is a type of radiation known as electromagnetic waves. It helps to create pictures of the inside of human body. These images show the different part of the body in various shades of black and white. It is due to the difference in amount of absorption by various tissues in the body.

As calcium in bones absorb most of the X-rays, so bone look white in colour. Fat and other tissue absorb less and defects grey colour.

i) To which part of the electromagnetic spectrum does a wave of frequency 2×1018 Hzz belong?
ii) What is the range of wavelength for X-rays?
iii) How are the X-rays produced?
iv) What are the techniques by which X rays can be detected?
v) Mention any two uses of X-rays.

Case Study Based Questions Answers

1. (i) A wave of frequency 2×10¹⁸ belong to X-rays of electromagnetic spectrum

(ii) The range of wavelength for X-rays is around 1 nm to 10^-3 nm.

(iii) X-rays are produced due to change in speed of fast moving electrons, when they collide and interact with the target anode.

(iv) X-rays can be detected with the help of Geiger-Muller tube or GM counter and also with ionization chamber.

(v) 2 application or uses of X-rays :-
(a) X rays are used in medical diagnosis and to cure malignant growth
(b) These rays are used in detecting faults, cracks etc. in metal products

Final Words

From the above article, you have practiced Class 12 Physics Chapter 8 Electromagnetic Waves Questions and Answers. We hope that the above-mentioned notes and Q & A for term 2 will surely help you in your exam. 

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