The electric field of a plane electromagnetic wave in vacuum is E_y = 60 sin(5 × 10² x - 1.5 × 10¹¹ t) V/m. With c = 3 × 10⁸ m/s, which describes the amplitude and direction of its magnetic field?

2 × 10⁻⁷ T, along the z-axis

2 × 10⁻⁷ T, along the y-axis

1.8 × 10¹⁰ T, along the z-axis

2 × 10⁻⁷ T, along the x-axis

Electromagnetic Waves

29 NEET-UG practice questions on Electromagnetic Waves, part of Physics. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Light of wavelength 6.0 × 10⁻⁷ m in vacuum enters glass of refractive index 1.5. With c = 3 × 10⁸ m/s, what are its wavelength and frequency inside the glass?
1. A600 nm and 3.3 × 10¹⁴ Hz
2. B400 nm and 3.3 × 10¹⁴ Hz
3. C400 nm and 5 × 10¹⁴ Hz
4. D900 nm and 5 × 10¹⁴ Hz
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Answer: C · 400 nm and 5 × 10¹⁴ Hz
Frequency is set by the source and stays f = c/λ = (3 × 10⁸)/(6 × 10⁻⁷) = 5 × 10¹⁴ Hz. The speed falls to c/n = 2 × 10⁸ m/s, so the wavelength is 600/1.5 = 400 nm.
- A: This keeps wavelength fixed and lets frequency fall, the reverse of what happens.
- B: The wavelength is right but the frequency was also divided by n.
- D: The wavelength was multiplied by n instead of divided.
Remember this
Passing into a medium, frequency stays fixed while speed and wavelength both fall by the factor n.
Question 2 · Medium
The electric field of a plane electromagnetic wave in vacuum is E_y = 60 sin(5 × 10² x - 1.5 × 10¹¹ t) V/m. With c = 3 × 10⁸ m/s, which describes the amplitude and direction of its magnetic field?
1. A2 × 10⁻⁷ T, along the z-axis
2. B2 × 10⁻⁷ T, along the y-axis
3. C1.8 × 10¹⁰ T, along the z-axis
4. D2 × 10⁻⁷ T, along the x-axis
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Answer: A · 2 × 10⁻⁷ T, along the z-axis
B₀ = E₀/c = 60/(3 × 10⁸) = 2 × 10⁻⁷ T. The form (kx - ωt) gives travel along +x, and E × B must point along +x, so with E along y, B lies along z.
- B: B is perpendicular to E, so it cannot lie along y.
- C: E₀ was multiplied by c instead of divided.
- D: The wave is transverse, so B cannot lie along x, the direction of travel.
Remember this
E, B and the direction of travel are mutually perpendicular, with E₀/B₀ = c and E × B along the motion.
Question 3 · Medium
A plane electromagnetic wave in vacuum has an electric field amplitude of 100 V/m. Taking ε₀ = 9 × 10⁻¹² C² N⁻¹ m⁻² and c = 3 × 10⁸ m/s, find its intensity.
1. A4.5 × 10⁻⁸ W/m²
2. B6.75 W/m²
3. C27 W/m²
4. D13.5 W/m²
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Answer: D · 13.5 W/m²
The electric and magnetic parts share the energy equally, so the total average density is u = (1/2)ε₀E₀² = 0.5 × 9 × 10⁻¹² × 10⁴ = 4.5 × 10⁻⁸ J/m³. Intensity is uc = 4.5 × 10⁻⁸ × 3 × 10⁸ = 13.5 W/m².
- A: The energy density was not multiplied by c.
- B: Only the electric share (1/4)ε₀E₀² was counted.
- C: The factor 1/2 was dropped from the energy density.
Remember this
Average intensity is (1/2)ε₀E₀²c, with the electric and magnetic fields carrying equal shares of the energy.
Question 4 · Easy
Maxwell's displacement current through a surface is given by which expression, where Φ_E is the electric flux through that surface?
1. Aμ₀ dΦ_E/dt
2. Bε₀ Φ_E
3. Cε₀ dΦ_E/dt
4. D(1/ε₀) dΦ_E/dt
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Answer: C · ε₀ dΦ_E/dt
Maxwell added the term I_d = ε₀ dΦ_E/dt to Ampere's law, so a changing electric field produces a magnetic field just as a conduction current does.
- A: μ₀ belongs with the magnetic term of the Ampere-Maxwell law.
- B: A steady flux gives no displacement current, so the rate of change is needed.
- D: The factor ε₀ belongs in the numerator.
Remember this
Displacement current is ε₀ dΦ_E/dt and exists wherever the electric flux changes, even in empty space.
Question 5 · Easy
Klystrons, magnetrons and Gunn diodes are special devices used to generate which radiation?
1. AUltraviolet rays
2. BX-rays
3. CGamma rays
4. DMicrowaves
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Answer: D · Microwaves
Microwaves are produced by these special vacuum tubes and diodes, and are used in radar and microwave ovens.
- A: Ultraviolet comes from very hot bodies and special lamps.
- B: X-rays come from electrons striking a metal target.
- C: Gamma rays come from nuclear reactions.
Remember this
Microwaves come from klystrons, magnetrons and Gunn diodes; X-rays come from bombarding a metal target with fast electrons.
Question 6 · Medium
Match List-I with List-II. List-I (radiation) A. Microwaves B. Infrared C. Ultraviolet D. X-rays List-II (use) I. Remote switches of household electronics II. Radar for aircraft navigation III. Medical diagnosis IV. Killing germs in water purifiers
1. AA-II, B-I, C-III, D-IV
2. BA-I, B-II, C-IV, D-III
3. CA-II, B-I, C-IV, D-III
4. DA-II, B-IV, C-I, D-III
Answer and explanation in LearnFree account
Question 7 · Medium
A 2 μF parallel-plate capacitor is being charged, and the voltage across it rises at 5 × 10³ V/s. What is the displacement current between its plates?
1. A10 mA
2. B0.4 nA
3. C10 A
4. DZero
Answer and explanation in LearnFree account
Question 8 · Hard
A light beam of intensity 3.0 W/cm² falls normally on a perfectly absorbing surface of area 2.0 cm² for 100 s. With c = 3 × 10⁸ m/s, what total momentum does it deliver to the surface?
1. A2 × 10⁻⁸ kg m/s
2. B2 × 10⁻⁶ kg m/s
3. C4 × 10⁻⁶ kg m/s
4. D1 × 10⁻⁶ kg m/s
Answer and explanation in LearnFree account
Question 9 · Easy
Which sequence lists these radiations in order of increasing wavelength?
1. AMicrowaves, infrared, ultraviolet, gamma rays
2. BGamma rays, infrared, ultraviolet, microwaves
3. CUltraviolet, gamma rays, infrared, microwaves
4. DGamma rays, ultraviolet, infrared, microwaves
Answer and explanation in LearnFree account
Question 10 · Hard
Examine these statements about a plane electromagnetic wave in vacuum. A. Its electric and magnetic fields oscillate in phase. B. A charge moving with uniform velocity radiates such a wave. C. The average energy density of its electric field equals that of its magnetic field. D. It exerts pressure on a surface it falls on. Which of these are correct?
1. AB, C and D only
2. BA, B and D only
3. CA, C and D only
4. DA and C only
Answer and explanation in LearnFree account
Question 11 · Hard
A capacitor with circular plates of radius 10 cm is being charged by a current of 2 A. Assuming the electric field between the plates is uniform, what is the magnetic field at a point between the plates 5 cm from their common axis? Take μ₀ = 4π × 10⁻⁷ T m/A.
1. A8 × 10⁻⁶ T
2. B4 × 10⁻⁶ T
3. CZero
4. D2 × 10⁻⁶ T
Answer and explanation in LearnFree account
Question 12 · Medium
Given below are two statements: Statement I: While a capacitor is being charged by a steady current, the displacement current between its plates is smaller than the conduction current in the connecting wires. Statement II: Electromagnetic waves carry momentum as well as energy. Choose the correct answer.
1. AStatement I is incorrect but Statement II is correct
2. BStatement I is correct but Statement II is incorrect
3. CBoth Statement I and Statement II are correct
4. DBoth Statement I and Statement II are incorrect
Answer and explanation in LearnFree account
Question 13 · Medium
A plane electromagnetic wave travels along the +x direction in vacuum. At a certain point and instant, its electric field points along +y. Along which direction does its magnetic field point there?
1. A+y
2. B-z
3. C+x
4. D+z
Answer and explanation in LearnFree account
Question 14 · Easy
Electromagnetic waves are radiated by which of these?
1. AAn accelerating charge
2. BA steady current in a straight wire
3. CA charge at rest
4. DA charge moving with constant velocity
Answer and explanation in LearnFree account
Question 15 · Medium
The electric field of a wave in vacuum is E_y = E₀ sin[(2 × 10⁷ m⁻¹)x - (6 × 10¹⁵ s⁻¹)t]. To which part of the electromagnetic spectrum does this wave belong?
1. AVisible light
2. BInfrared
3. CMicrowaves
4. DUltraviolet
Answer and explanation in LearnFree account
Question 16 · Medium
A plane wave travelling along +x in free space has E_z = 90 sin[(π × 10⁷ m⁻¹)x - ωt] V/m. With c = 3 × 10⁸ m/s, what are its wavelength and the amplitude of its magnetic field?
1. A0.2 µm and 2.7 × 10¹⁰ T
2. B3.2 × 10⁻⁸ m and 3 × 10⁻⁷ T
3. C0.4 µm and 3 × 10⁻⁷ T
4. D0.2 µm and 3 × 10⁻⁷ T
Answer and explanation in LearnFree account
Question 17 · Medium
An LC oscillator with L = (10/π) µH and C = (10/π) pF radiates electromagnetic waves at its natural frequency. With c = 3 × 10⁸ m/s, what is the wavelength of these waves?
1. A6 m
2. B1.9 m
3. C0.95 m
4. D12 m
Answer and explanation in LearnFree account
Question 18 · Hard
A small lamp radiates 15 W of electromagnetic power uniformly in all directions. Taking 1/(4πε₀) = 9 × 10⁹ N m²/C² and c = 3 × 10⁸ m/s, what is the amplitude of the electric field 3 m away?
1. A10 V/m
2. B7.1 V/m
3. C20 V/m
4. D17 V/m
Answer and explanation in LearnFree account
Question 19 · Medium
Consider these statements about electromagnetic waves in vacuum: A. They carry momentum as well as energy. B. The ratio of the amplitudes of their electric and magnetic fields, E₀/B₀, equals c. C. Their speed depends on their wavelength. D. Their electric and magnetic fields oscillate a quarter-cycle (π/2) out of phase. Which of these are correct?
1. AB and D only
2. BA, B and D only
3. CA, B and C only
4. DA and B only
Answer and explanation in LearnFree account
Question 20 · Easy
Lamps used in physiotherapy to warm muscles, and the remote controls of television sets, both use which part of the electromagnetic spectrum?
1. AMicrowaves
2. BInfrared
3. CX-rays
4. DUltraviolet
Answer and explanation in LearnFree account
Question 21 · Medium
In a plane electromagnetic wave travelling through vacuum, what is the ratio of the average energy density stored in its magnetic field to that stored in its electric field?
1. Ac²
2. B1
3. C1/c
4. Dc
Answer and explanation in LearnFree account
Question 22 · Easy
Electromagnetic waves travel through a non-magnetic medium (μᵣ = 1) whose relative permittivity is 9. Taking c = 3 × 10⁸ m/s, what is their speed in this medium?
1. A2.7 × 10⁹ m/s
2. B3.3 × 10⁷ m/s
3. C9 × 10⁸ m/s
4. D1 × 10⁸ m/s
Answer and explanation in LearnFree account
Question 23 · Medium
Given below are two statements: Statement I: A displacement current exists in a region only while the electric flux through it changes with time. Statement II: While a parallel-plate capacitor is charging, the displacement current between its plates equals the conduction current in the connecting wires. Choose the correct answer.
1. ABoth Statement I and Statement II are correct
2. BBoth Statement I and Statement II are incorrect
3. CStatement I is incorrect but Statement II is correct
4. DStatement I is correct but Statement II is incorrect
Answer and explanation in LearnFree account
Question 24 · Hard
Light of intensity 1200 W/m² falls normally on a surface that reflects 25% of the incident energy and absorbs the rest. Taking c = 3 × 10⁸ m/s, what radiation pressure does the light exert?
1. A5 × 10⁻⁶ N/m²
2. B4 × 10⁻⁶ N/m²
3. C8 × 10⁻⁶ N/m²
4. D7 × 10⁻⁶ N/m²
Answer and explanation in LearnFree account
Question 25 · Easy
Life on Earth is shielded from much of the Sun's harmful radiation by the ozone layer, which absorbs it strongly. Which radiation is this?
1. ARadio waves
2. BInfrared rays
3. CUltraviolet rays
4. DMicrowaves
Answer and explanation in LearnFree account

4 more Electromagnetic Waves questions

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Electromagnetic Waves

Light of wavelength 6.0 × 10⁻⁷ m in vacuum enters glass of refractive index 1.5. With c = 3 × 10⁸ m/s, what are its wavelength and frequency inside the glass?

The electric field of a plane electromagnetic wave in vacuum is E_y = 60 sin(5 × 10² x - 1.5 × 10¹¹ t) V/m. With c = 3 × 10⁸ m/s, which describes the amplitude and direction of its magnetic field?

A plane electromagnetic wave in vacuum has an electric field amplitude of 100 V/m. Taking ε₀ = 9 × 10⁻¹² C² N⁻¹ m⁻² and c = 3 × 10⁸ m/s, find its intensity.

Maxwell's displacement current through a surface is given by which expression, where Φ_E is the electric flux through that surface?

Klystrons, magnetrons and Gunn diodes are special devices used to generate which radiation?

Match List-I with List-II. List-I (radiation) A. Microwaves B. Infrared C. Ultraviolet D. X-rays List-II (use) I. Remote switches of household electronics II. Radar for aircraft navigation III. Medical diagnosis IV. Killing germs in water purifiers

A 2 μF parallel-plate capacitor is being charged, and the voltage across it rises at 5 × 10³ V/s. What is the displacement current between its plates?

A light beam of intensity 3.0 W/cm² falls normally on a perfectly absorbing surface of area 2.0 cm² for 100 s. With c = 3 × 10⁸ m/s, what total momentum does it deliver to the surface?

Which sequence lists these radiations in order of increasing wavelength?

A capacitor with circular plates of radius 10 cm is being charged by a current of 2 A. Assuming the electric field between the plates is uniform, what is the magnetic field at a point between the plates 5 cm from their common axis? Take μ₀ = 4π × 10⁻⁷ T m/A.

A plane electromagnetic wave travels along the +x direction in vacuum. At a certain point and instant, its electric field points along +y. Along which direction does its magnetic field point there?

Electromagnetic waves are radiated by which of these?

The electric field of a wave in vacuum is E_y = E₀ sin[(2 × 10⁷ m⁻¹)x - (6 × 10¹⁵ s⁻¹)t]. To which part of the electromagnetic spectrum does this wave belong?

A plane wave travelling along +x in free space has E_z = 90 sin[(π × 10⁷ m⁻¹)x - ωt] V/m. With c = 3 × 10⁸ m/s, what are its wavelength and the amplitude of its magnetic field?

An LC oscillator with L = (10/π) µH and C = (10/π) pF radiates electromagnetic waves at its natural frequency. With c = 3 × 10⁸ m/s, what is the wavelength of these waves?

A small lamp radiates 15 W of electromagnetic power uniformly in all directions. Taking 1/(4πε₀) = 9 × 10⁹ N m²/C² and c = 3 × 10⁸ m/s, what is the amplitude of the electric field 3 m away?

Lamps used in physiotherapy to warm muscles, and the remote controls of television sets, both use which part of the electromagnetic spectrum?

In a plane electromagnetic wave travelling through vacuum, what is the ratio of the average energy density stored in its magnetic field to that stored in its electric field?

Electromagnetic waves travel through a non-magnetic medium (μᵣ = 1) whose relative permittivity is 9. Taking c = 3 × 10⁸ m/s, what is their speed in this medium?

Light of intensity 1200 W/m² falls normally on a surface that reflects 25% of the incident energy and absorbs the rest. Taking c = 3 × 10⁸ m/s, what radiation pressure does the light exert?

Life on Earth is shielded from much of the Sun's harmful radiation by the ozone layer, which absorbs it strongly. Which radiation is this?