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

10 NEET-UG practice questions on Electromagnetic Waves, part of Physics. The first 10 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

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?