Electromagnetic Waves Class 12 Assertion and Reason Questions 2026-27 | Chapter 8
Electromagnetic Waves — Assertion and Reason Questions with Answers
Original, syllabus-aligned Class 12 Physics Chapter 8 Assertion–Reason practice with answers. The set covers displacement current, electromagnetic-wave properties, transverse nature, frequency–wavelength relationships, photon energy, spectrum order and elementary applications.
What These Class 12 Physics Assertion–Reason Questions Cover
This Chapter 8 practice set targets the search and exam-intent areas repeatedly associated with Electromagnetic Waves Class 12 Physics Assertion and Reason Questions: displacement current, charging capacitor, transverse nature of EM waves, electric and magnetic fields, speed in vacuum, frequency and wavelength, photon energy, electromagnetic spectrum, and elementary applications of radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.
Important: These are original Learn Revise Hub questions. They are designed for CBSE-style reasoning practice and are not claimed to be official CBSE questions or predictions.
Assertion–Reason Questions
Displacement current during capacitor charging
Assertion (A): A changing electric field between the plates of a charging capacitor is associated with displacement current.
Reason (R): Displacement current in vacuum is proportional to the rate of change of electric flux.
Because Id = ε0 dΦE/dt, a changing electric flux gives rise to displacement current.
Displacement current and a constant electric field
Assertion (A): If the electric flux through a surface is constant with time, the displacement current through that surface is zero.
Reason (R): Displacement current depends on the time rate of change of electric flux.
If dΦE/dt = 0, then Id = ε0 dΦE/dt = 0.
Conduction current and capacitor gap
Assertion (A): During the charging of a capacitor, conduction current does not cross the insulating gap between its plates.
Reason (R): The changing electric field in the gap is associated with displacement current.
Both statements are true, but the reason does not explain why conduction charge does not physically cross the insulating gap. It describes the separate displacement-current concept.
Electromagnetic waves in vacuum
Assertion (A): Electromagnetic waves can propagate through vacuum.
Reason (R): Electromagnetic waves do not require a material medium for propagation.
The absence of a material-medium requirement allows electromagnetic radiation to propagate through vacuum.
Transverse nature
Assertion (A): Electromagnetic waves are transverse in nature.
Reason (R): In a plane electromagnetic wave, the electric field and magnetic field are perpendicular to the direction of propagation.
For a plane EM wave, both fields are transverse to the direction of propagation, so the wave is transverse.
Electric and magnetic fields
Assertion (A): In a plane electromagnetic wave, the electric and magnetic fields are mutually perpendicular.
Reason (R): The electric field, magnetic field and direction of propagation are mutually perpendicular.
If E, B and the propagation direction form mutually perpendicular directions, E and B are necessarily perpendicular.
Direction of propagation
Assertion (A): The direction of propagation of a plane electromagnetic wave is along E × B.
Reason (R): The propagation direction is perpendicular to both the electric and magnetic fields.
E × B gives a direction perpendicular to both E and B and points in the direction of electromagnetic-wave propagation.
Speed of electromagnetic waves
Assertion (A): Electromagnetic waves of different frequencies travel at different speeds in vacuum.
Reason (R): In vacuum, the speed of every electromagnetic wave is c.
Frequency does not determine the vacuum speed. All electromagnetic waves travel at c in vacuum.
Frequency and wavelength in vacuum
Assertion (A): For an electromagnetic wave in vacuum, increasing its frequency decreases its wavelength.
Reason (R): In vacuum, c = νλ and c is constant.
Since c is constant, λ = c/ν; therefore wavelength decreases when frequency increases.
Frequency on entering a medium
Assertion (A): When electromagnetic radiation enters a transparent medium from vacuum, its frequency remains unchanged.
Reason (R): The frequency is determined by the source and the boundary does not change the source's oscillation rate.
The source fixes the frequency. On entering the medium, the speed and wavelength change while frequency remains the same.
Wavelength on entering a medium
Assertion (A): The wavelength of electromagnetic radiation generally decreases when it enters a medium with refractive index greater than one.
Reason (R): In the medium, v = c/n while the frequency remains unchanged.
Because λ = v/ν, a reduced speed at unchanged frequency gives a reduced wavelength.
Electric-field amplitude
Assertion (A): In vacuum, if the magnetic-field amplitude of an electromagnetic wave is doubled, its electric-field amplitude also doubles.
Reason (R): In vacuum, E0 = cB0.
The proportionality E0 = cB0 directly gives the stated result.
Phase relation of E and B
Assertion (A): In a plane electromagnetic wave, the electric and magnetic fields are in phase.
Reason (R): Their maxima and minima occur simultaneously at corresponding positions in the wave.
In a plane EM wave, E and B oscillate in phase, so corresponding maxima and minima occur together.
Longest wavelength in the spectrum
Assertion (A): Radio waves have the longest wavelength among the seven standard regions of the electromagnetic spectrum.
Reason (R): Radio waves occupy the lowest-frequency end of the standard electromagnetic spectrum.
Since c = νλ in vacuum, the lowest frequency corresponds to the longest wavelength.
Gamma rays and wavelength
Assertion (A): Gamma rays have the highest frequency among the seven standard regions of the electromagnetic spectrum.
Reason (R): Gamma rays have the longest wavelength among the seven standard regions.
Gamma rays are at the highest-frequency end and therefore have the shortest wavelength, not the longest.
Photon energy and frequency
Assertion (A): A photon of X-ray radiation can have more energy than a photon of infrared radiation.
Reason (R): Photon energy is E = hν, so higher frequency means higher photon energy.
X-rays have a higher frequency than infrared radiation; therefore their photons have higher energy.
Photon energy and wavelength
Assertion (A): For electromagnetic radiation, photon energy increases when wavelength decreases.
Reason (R): E = hc/λ.
The inverse relationship follows directly from E = hc/λ.
Visible light in the spectrum
Assertion (A): Visible light lies between infrared and ultraviolet in the standard electromagnetic spectrum.
Reason (R): The standard frequency order is radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays.
The stated order places visible light directly between infrared and ultraviolet.
Microwaves and radar
Assertion (A): Microwaves are commonly used in radar systems.
Reason (R): Microwaves are electromagnetic waves and can be used for transmitting and receiving signals in radar applications.
Radar systems commonly use microwave-frequency electromagnetic radiation for transmitting and detecting reflected signals.
Infrared radiation and frequency
Assertion (A): Infrared radiation is associated with thermal effects and is used in thermal sensing.
Reason (R): Infrared radiation has a lower frequency than visible light in the electromagnetic spectrum.
Both statements are correct. However, merely having a lower frequency than visible light does not explain infrared's thermal-sensing applications.
Ultraviolet radiation
Assertion (A): Ultraviolet radiation lies between visible light and X-rays in increasing-frequency order.
Reason (R): Ultraviolet radiation has a lower frequency than visible light.
Ultraviolet is above visible light and below X-rays in frequency; therefore its frequency is higher, not lower, than visible light.
X-rays
Assertion (A): X-rays are used in medical radiography.
Reason (R): X-rays have much higher frequency than visible light.
Both statements are true, but high frequency alone is not a complete explanation of why X-rays are used for radiographic imaging.
Gamma rays
Assertion (A): Gamma rays lie at the highest-frequency end of the standard electromagnetic spectrum.
Reason (R): Gamma rays have the longest wavelength among the seven standard regions.
Gamma rays are at the highest-frequency end and therefore have the shortest, not longest, wavelength.
Electromagnetic spectrum order
Assertion (A): The frequency increases from radio waves to gamma rays.
Reason (R): The wavelength decreases from radio waves to gamma rays.
In vacuum, c = νλ. Thus, moving toward shorter wavelength corresponds to increasing frequency.
Two common spectrum mistakes
Assertion (A): Gamma rays have the longest wavelength among the seven standard regions of the electromagnetic spectrum.
Reason (R): Radio waves have the highest frequency among the seven standard regions.
Gamma rays are at the highest-frequency/shortest-wavelength end, while radio waves are at the lowest-frequency/longest-wavelength end.
Exam-Pattern Note
CBSE has published the Class XII 2026–27 Sample Question Paper and Marking Scheme page, including Physics. Recent official Physics sample papers have used Assertion–Reason questions in Section A; the exact paper structure can change, so students should always check the latest official SQP and marking scheme.
How to Solve Assertion–Reason Questions
- Test A alone: Is the Assertion scientifically correct?
- Test R alone: Is the Reason scientifically correct?
- Test the explanation: If both are true, ask whether R actually explains A. If A is false, check whether R is also false before selecting D.
- Watch for reversed relationships: especially frequency–wavelength, spectrum order and E–B direction.
- Do not infer explanation from association: two true statements can still belong to option B.
High-Yield Chapter 8 Reasoning Bank
| Concept | Exam-ready fact |
|---|---|
| Displacement current | Id = ε0 dΦE/dt in vacuum. |
| Propagation | For a plane EM wave, the propagation direction is along E × B. |
| Transverse nature | E and B are perpendicular to each other and to the propagation direction. |
| Vacuum speed | c = 1/√(μ0ε0). |
| Wave relation | In vacuum, c = νλ. |
| Field amplitudes | E0/B0 = c in vacuum. |
| Photon energy | E = hν = hc/λ. |
| Spectrum order | Radio → microwave → infrared → visible → ultraviolet → X-ray → gamma. |
Frequently Asked Questions
Are these official CBSE questions?
No. These are original Learn Revise Hub practice questions designed in the standard Assertion–Reason format. Use official CBSE sample papers, marking schemes and question papers for official examples. CBSE has published the Class XII 2026–27 SQP and marking scheme page, including Physics.
Which Chapter 8 topics should I revise before attempting these?
Revise displacement current, characteristics of electromagnetic waves, transverse nature, E–B relationships, speed in vacuum, frequency and wavelength, photon energy, spectrum order and elementary uses.
What is the most common Assertion–Reason trap?
Confusing “both statements are true” with “the Reason correctly explains the Assertion.” Always test the logical connection separately.
Continue Chapter 8 Preparation
Source & Content Discipline
- CBSE 2026–27 Physics curriculum: authority for current Chapter 8 scope and assessment structure.
- CBSE Class XII 2026–27 SQP/MS: used for current examination-pattern awareness.
- Competitor and question-bank research was used only to identify recurring search intent and reasoning patterns; it is not treated as syllabus authority. Competitor and question-bank research shows recurring search demand around displacement current, transverse nature, spectrum order, frequency–wavelength relationships and the four-option Assertion–Reason format.
- All 25 questions on this page are original practice material and are not represented as official CBSE questions.
Official CBSE curriculum: Physics 2026–27 Curriculum · Official SQP/MS page: Class XII 2026–27 SQP & MS · NCERT Physics Part-I: NCERT Physics Part-I PDF
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