Class 12 Physics Chapter 8 Important Questions (2026-27)
Important Questions for Class 12 Physics Chapter 8 – Electromagnetic Waves
Looking for Class 12 Physics Chapter 8 important questions? This page covers Electromagnetic Waves with exam-focused questions and step-by-step answers, arranged by marks: 1 mark, 2–3 marks and 5 marks. Written for the CBSE 2026–27 syllabus, it focuses on displacement current, the electromagnetic spectrum and wave properties. Attempt them after finishing the NCERT chapter.
Very Short Answer Questions (1 Mark)
Electromagnetic Spectrum and Identification
Question 1
Name the part of the electromagnetic spectrum of wavelength \(10^{-2}\ \text{m}\) and mention one application.
Applications: (i) Radar communication, (ii) Microwave ovens, (iii) Analysis of fine details of molecular and atomic structure.
Question 2
Write the following radiations in ascending order of their frequencies: X-rays, Microwaves, UV rays and Radiowaves.
Question 3
Name the electromagnetic radiation to which waves of wavelength in the range of \(10^{-2}\ \text{m}\) belong. Give one use.
Question 4
Name the part of electromagnetic spectrum which is suitable for (i) radar systems used in aircraft navigation, (ii) treatment of cancer tumours.
Question 5
Name the EM waves used for studying crystal structure of solids. What is its frequency range?
Question 6
Which part of the electromagnetic spectrum has the largest penetrating power?
Question 7
Which part of the electromagnetic spectrum is absorbed from sunlight by the ozone layer?
Question 8
Which part of the electromagnetic spectrum is used in radar systems?
Question 9
Name the part of the electromagnetic spectrum whose wavelength lies in the range of \(10^{-10}\ \text{m}\). Give one use.
Question 10
Which of the following has the shortest wavelength: Microwaves, Ultraviolet rays, X-rays?
Question 11
Arrange the following in descending order of wavelength: X-rays, Radio waves, Blue light, Infrared light.
Question 12
To which part of the electromagnetic spectrum does a wave of frequency \(5 \times 10^{19}\ \text{Hz}\) belong?
Question 13
To which part of the electromagnetic spectrum does a wave of frequency \(3 \times 10^{13}\ \text{Hz}\) belong?
Question 14
Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiations. Name the radiations and write the range of their frequency.
Direction of E and B in EM Waves
Question 15
A plane electromagnetic wave travels in vacuum along the z-direction. What can you say about the direction of the electric and magnetic field vectors?
Question 16
A plane electromagnetic wave travels in vacuum along the x-direction. What can you say about the direction of the electric and magnetic field vectors?
Question 17
A plane electromagnetic wave travels in vacuum along the y-direction. What can you say about the direction of the electric and magnetic field vectors?
Question 18
In which directions do the electric and magnetic field vectors oscillate in an electromagnetic wave propagating along the x-axis?
Question 19
What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?
Production of EM Waves
Question 20
How are radio waves produced?
Question 21
How are X-rays produced?
Question 22
How are microwaves produced?
Question 23
Why are microwaves considered suitable for radar systems used in aircraft navigation?
Speed and Properties of EM Waves
Question 24
Name the physical quantity which remains the same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.
Question 25
The speed of an electromagnetic wave in a material medium is given by \(v = \dfrac{1}{\sqrt{\mu\varepsilon}}\), where \(\mu\) is the permeability and \(\varepsilon\) the permittivity of the medium. How does its frequency change?
Question 26
How is the speed of EM waves in vacuum determined by the electric and magnetic fields?
Question 27
Write the relation for the speed of electromagnetic waves in terms of the amplitudes of electric and magnetic fields.
Question 28
Do electromagnetic waves carry energy and momentum?
Displacement Current
Question 29
A capacitor has been charged by a dc source. What are the magnitudes of conduction and displacement currents when it is fully charged?
Short Answer Questions (2–3 Marks)
Mathematical Form of EM Waves
Question 30
The oscillating magnetic field in a plane electromagnetic wave is given by \(B_y = (8 \times 10^{-6})\sin[2 \times 10^{11}t + 300\pi x]\ \text{T}\). (i) Calculate the wavelength of the electromagnetic wave. (ii) Write down the expression for the oscillating electric field.
(i) \(\lambda = \dfrac{2\pi}{k} = \dfrac{2\pi}{300\pi} =\) \(\dfrac{1}{150}\ \text{m} \approx 0.0067\ \text{m}\)
(ii) \(E_0 = cB_0 = 3 \times 10^8 \times 8 \times 10^{-6} = 2400\ \text{V/m}\). Since \(B_y\) is along y and propagation is along x, \(E\) must be along z (to give \(\vec{E} \times \vec{B}\) along x):
\(E_z = 2400\sin[2 \times 10^{11}t + 300\pi x]\ \text{V/m}\)
Question 31
The oscillating electric field of an electromagnetic wave is given by \(E = 30\sin[2 \times 10^{11}t + 300\pi x]\ \text{V/m}\). (a) Obtain the wavelength of the wave. (b) Write down the expression for the oscillating magnetic field.
(b) \(B_0 = \dfrac{E_0}{c} = \dfrac{30}{3 \times 10^8} = 10^{-7}\ \text{T}\).
Expression: \(B_y = 10^{-7}\sin[2 \times 10^{11}t + 300\pi x]\ \text{T}\) (perpendicular to E and to propagation direction).
Question 32
For a plane electromagnetic wave propagating along the z-axis, write two possible pairs of expressions for its oscillating electric and magnetic fields. How are the peak values of these fields related?
Pair 1: \(E_x = E_0\sin(\omega t - kz)\), \(B_y = B_0\sin(\omega t - kz)\)
Pair 2: \(E_y = E_0\sin(\omega t - kz)\), \(B_x = -B_0\sin(\omega t - kz)\)
Peak values are related by: \(\dfrac{E_0}{B_0} = c\).
Question 33
An EM wave is travelling in a medium with velocity \(\vec{v} = v\hat{i}\). Draw a sketch showing the propagation of the EM wave, indicating the direction of the oscillating electric and magnetic fields. How are the magnitudes of E and B related to the velocity of the EM wave?
Relation: \(v = \dfrac{E_0}{B_0}\) (peak values).
Question 34
Draw a sketch of a plane electromagnetic wave propagating along the z-direction. Depict clearly the directions of the electric and magnetic fields varying sinusoidally with z.
Production of EM Waves
Question 35
How does a charge q oscillating at a certain frequency produce electromagnetic waves? Sketch a schematic diagram depicting electric and magnetic fields for an EM wave propagating along the z-direction.
Sketch: E along x, B along y, propagation along z.
Question 36
How are electromagnetic waves produced? How do you convince yourself that electromagnetic waves carry energy and momentum?
Energy and momentum: Electromagnetic waves transport energy (as seen in wave properties — wavelength, frequency, amplitude) but no matter. They also carry momentum — a photon of momentum p has wavelength \(\lambda = \dfrac{h}{p}\), and via the photoelectric effect, EM waves show particle-like behaviour too.
Question 37
How is an electromagnetic wave produced? Draw a sketch of a plane EM wave propagating along the X-axis, depicting the directions of the oscillating electric and magnetic fields.
Sketch: E along y, B along z, propagation along x.
Question 38
How are EM waves produced by oscillating charges? Draw a sketch of linearly polarized EM waves propagating in the Z-direction. Indicate the directions of the oscillating electric and magnetic fields.
Displacement Current and Ampere-Maxwell Law
Question 39
A parallel plate capacitor is being charged by a time-varying current. Explain briefly how Ampere's circuital law is generalized to incorporate the effect due to the displacement current.
For a capacitor being charged: a loop enclosing just the plate (not the wire) has no conduction current through it, giving zero magnetic field; but a loop enclosing the wire gives a nonzero B — a contradiction.
Maxwell resolved this by introducing the displacement current: \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\). The generalized Ampere-Maxwell law is:
\(\oint \vec{B}\cdot d\vec{l} = \mu_0(I_c + I_d) = \mu_0 I_c + \mu_0\varepsilon_0\dfrac{d\phi_E}{dt}\)
Question 40
When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used, the current flows continuously. How does one explain this, based on the concept of displacement current?
With an ac source, the charge on the plates — and hence the electric field — keeps changing. So a displacement current \(I_d \propto \dfrac{d\phi_E}{dt}\) keeps flowing, sustaining the circuit current.
Question 41
A capacitor of capacitance C is being charged by connecting it across a dc source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuity of current in the circuit? Write the expression for the current inside the capacitor.
During charging, the electric field between the plates changes, producing a displacement current \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\) in the gap. This maintains continuity of current across the gap.
Question 42
A capacitor, made of two parallel plates each of area A and separation d, is being charged by an external ac source. Show that the displacement current inside the capacitor is the same as the current charging the capacitor.
Electric field between plates: \(E = \dfrac{q}{\varepsilon_0 A}\). Flux: \(\phi_E = EA = \dfrac{q}{\varepsilon_0}\).
Displacement current: \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt} = \varepsilon_0 \cdot \dfrac{1}{\varepsilon_0}\cdot \dfrac{dq}{dt} = \dfrac{dq}{dt} = I\).
Hence, \(I_d = I\) — the displacement current equals the charging current.
Question 43
Considering the case of a parallel plate capacitor being charged, show how one is required to generalize Ampere's circuital law to include the term due to displacement current.
Question 44
A capacitor is connected in series to an ammeter across a dc source. Why does the ammeter show a momentary deflection during the charging of the capacitor? What would be the deflection when it is fully charged?
Question 45
How does Ampere-Maxwell law explain the flow of current through a capacitor when it is being charged by a battery? Write the expression for the displacement current in terms of the rate of change of electric flux.
Expression: \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\)
Question 46
Write Maxwell's generalization of Ampere's circuital law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\), where \(\phi_E\) is the electric flux.
During charging, between the plates of the capacitor (where \(I_c = 0\)), the changing electric flux produces the displacement current \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\) — this is the only current in the gap.
Question 47
How is the generalized form of Ampere's circuital law obtained to include the term due to displacement current?
Question 48
(a) When the oscillating electric and magnetic fields are along the x- and y-direction respectively, point out the direction of propagation of the electromagnetic wave. (b) Express the velocity of propagation in terms of the amplitudes of the oscillating electric and magnetic fields.
(b) \(v = \dfrac{E_0}{B_0}\).
Question 49
For a plane electromagnetic wave propagating along the z-direction, write the two possible pairs of expressions for its oscillating electric and magnetic fields. How are the peak values related?
Applications and Uses of EM Waves
Question 50
How are infrared waves produced? Why are these referred to as 'heat waves'? Write their one important use.
They are called heat waves because they lie adjacent to the low-frequency (long-wavelength) end of the visible spectrum and are readily absorbed by matter, producing heat.
Use: (i) Taking photographs in darkness, (ii) Remote switches of household electronic systems, (iii) Physical therapy.
Question 51
Name the types of EM radiations which (i) are used in destroying cancer cells, (ii) cause tanning of the skin, and (iii) maintain the earth's warmth. Write briefly a method of producing any one of these waves.
Mode of production: γ-rays — radioactive decay of nuclei; UV rays — electron transitions between energy levels in atoms; IR rays — vibration of atoms/molecules.
Question 52
An EM wave Y₁ has a wavelength of 1 cm while another EM wave Y₂ has a frequency of \(10^{15}\ \text{Hz}\). Name these two types of waves and write one useful application for each.
Y₂ — Ultraviolet waves. Applications: Sterilizing surgical instruments, food preservation.
Question 53
Identify the electromagnetic waves whose wavelengths vary as (a) \(10^{-12} < \lambda < 10^{-8}\ \text{m}\) and (b) \(10^{-3}\ \text{m} < \lambda < 10^{-1}\ \text{m}\). Write one use for each.
Question 54
Identify the electromagnetic waves whose wavelengths lie in the range (a) \(10^{-11}\ \text{m} < \lambda < 10^{-8}\ \text{m}\) and (b) \(10^{-4}\ \text{m} < \lambda < 10^{-6}\ \text{m}\). Write one use of each.
Question 55
Identify the following electromagnetic radiations as per the wavelengths given below. Write one application of each. (a) \(10^{-3}\ \text{nm}\) (b) \(10^{-3}\ \text{m}\) (c) 1 nm
(b) \(10^{-3}\ \text{m}\) — Microwaves. Used in radar systems for aircraft navigation.
(c) 1 nm — X-rays. Diagnostic tool for fractures.
Question 56
Identify the following electromagnetic radiations as per the wavelengths given below. Write one application of each. (a) 1 mm (b) \(10^{-12}\ \text{m}\) (c) \(10^{-8}\ \text{m}\)
(b) \(10^{-12}\ \text{m}\) — γ-rays. Medicine to destroy cancer cells.
(c) \(10^{-8}\ \text{m}\) — Ultraviolet rays. LASIK eye surgery.
Question 57
Name the parts of the electromagnetic spectrum which are (a) suitable for radar systems used in aircraft navigation, (b) used to treat muscular strain, (c) used as a diagnostic tool in medicine. Write in brief how these waves can be produced.
(b) Infrared radiations — produced by hot bodies / vibration of atoms and molecules.
(c) X-rays — produced by bombarding high-energy electrons on a metal target.
Question 58
Name the EM waves in the wavelength range 10 nm to \(10^{-3}\ \text{nm}\). How are these waves generated? Write their two uses.
Uses: (i) Diagnosis of bone fractures, (ii) Treatment of some forms of cancer.
Question 59
Name the type of EM waves having wavelength range of 0.1 m to 1 mm. How are these waves generated? Write their two uses.
Uses: (i) Radar systems in aircraft navigation, (ii) Ovens for heating and cooking.
Question 60
Name the type of EM waves having wavelength range \(10^{-7}\ \text{m}\) to \(10^{-9}\ \text{m}\). How are these waves generated? Write their two uses.
Uses: (i) LASIK eye surgery, (ii) Water purifiers (to kill germs).
Question 61
Name the EM waves which are (a) suitable for radar systems used in aircraft navigation and write the range of frequency, (b) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now? Explain. (c) An EM wave exerts pressure on the surface on which it is incident. Justify.
(b) In the absence of atmosphere, there would be no ozone layer to prevent UV radiation from reaching the Earth, and no greenhouse effect. The Earth's average surface temperature would be lower — making human survival difficult.
(c) EM waves carry energy and momentum, so when incident on a surface they transfer momentum, producing pressure.
Question 62
(a) Name the EM waves which are used for the treatment of certain forms of cancer. Write their frequency range. (b) The thin ozone layer on top of the stratosphere is crucial for human survival. Why? (c) Why is the amount of momentum transferred by EM waves incident on the surface so small?
(b) The ozone layer absorbs most harmful UV (UVA, UVB, UVC) radiation from the Sun, which can destroy life on Earth.
(c) The momentum transferred is small because it is on the order of \(p = h/\lambda\), and Planck's constant h is extremely small. For a typical wavelength, the momentum carried by a single photon (or even the total beam) is very small.
Question 63
(a) Name the EM waves which are produced during radioactive decay of a nucleus. Write their frequency range. (b) Welders wear special glass goggles while working. Why? Explain. (c) Why are infrared waves often called heat waves? Give their one application.
(b) To protect eyes from intense ultraviolet radiation produced during welding, and from glare and flying sparks.
(c) Infrared waves are called heat waves because they are produced by hot bodies and are readily absorbed by matter to increase its thermal energy. Applications: (i) Remote controls, (ii) Solar water heaters.
Question 64
(i) Show, by giving a simple example, how EM waves carry energy and momentum. (ii) How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of microwaves to match the resonant frequency of water molecules? (iii) Write two important uses of infrared waves.
(ii) Microwaves are produced by special vacuum tubes — klystron, magnetron, or Gunn diode. In microwave ovens, frequency is matched to the resonant frequency of water molecules so energy transfers efficiently to the water.
(iii) Infrared uses: (i) Greenhouse effect (studying it), (ii) Remote switches of household appliances.
Key Formulas and Facts to Remember
| Concept | Formula / Fact |
|---|---|
| Speed of EM wave in vacuum | \(c = \dfrac{1}{\sqrt{\mu_0\varepsilon_0}} = 3 \times 10^8\ \text{m/s}\) |
| Speed in a medium | \(v = \dfrac{1}{\sqrt{\mu\varepsilon}}\) |
| Relation between E₀ and B₀ | \(c = \dfrac{E_0}{B_0}\) |
| Displacement current | \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\) |
| Ampere-Maxwell law | \(\oint \vec{B}\cdot d\vec{l} = \mu_0 I_c + \mu_0\varepsilon_0\dfrac{d\phi_E}{dt}\) |
| Direction of propagation | \(\vec{E} \times \vec{B}\) direction |
| Energy and momentum | EM waves carry both; photon momentum \(p = h/\lambda\) |
The Electromagnetic Spectrum (in order of increasing frequency)
Radio waves < Microwaves < Infrared < Visible light < Ultraviolet < X-rays < Gamma rays
Production of each: Radio — accelerated charges in conducting wires; Microwaves — klystron/magnetron; IR — hot bodies, molecular vibrations; Visible — electron transitions in atoms; UV — Sun, special lamps; X-rays — decelerating electrons in an X-ray tube; γ-rays — radioactive decay of nuclei.
FAQs
Are these questions enough for CBSE Class 12 Physics Chapter 8?
Yes. These questions cover every major topic — electromagnetic spectrum, production and properties of EM waves, displacement current, Ampere-Maxwell law, and applications of each region of the spectrum — that CBSE has asked in previous board exams. Combine them with NCERT exercises and you'll be well prepared.
Which topics are most important for board exams?
Identification of EM waves by wavelength/frequency, production of each type of EM wave, properties of EM waves (E ⊥ B ⊥ propagation direction, \(c = E_0/B_0\), energy and momentum), displacement current (definition and its role in a charging capacitor), and Ampere-Maxwell law (statement and significance) are consistently high-weightage topics.
How should I prepare for numericals in this chapter?
Practise problems on displacement current using \(I_d = \varepsilon_0\dfrac{d\phi_E}{dt}\), the relation \(c = \dfrac{E_0}{B_0}\) to find E or B from one another, wavelength-frequency conversions \(\lambda = c/f\) to identify which part of the spectrum a wave belongs to, and the direction of propagation using \(\vec{E}\times\vec{B}\). Focus on understanding the conceptual reasoning — why displacement current is needed — as it appears frequently in short answer questions.