Electromagnetic Waves: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Who first produced EM waves?
2. Write the Ampere–Maxwell law.
3. Name the EM waves used in physiotherapy.
4. Range of visible light?
5. Which waves are used in LASIK surgery?
6. Direction of E × B in an EM wave?
7. Name the waves produced by klystrons.
8. Which EM photons have the largest energy?
9. Speed of EM waves in a medium?
10. Why can't we get a tan through glass?

D. Short answer I (2 marks each)

1. Explain the need for displacement current.
2. Write four characteristics of EM waves.
3. $B_z=2\times 10^{-8}\sin (\pi x+3\times 10^8\pi t)$ T. Find λ, ν and E₀.
4. Name two sources and two uses of infrared.
5. Show that the average energy densities of E and B are equal.
6. A radio station broadcasts at 100 MHz. Find λ. In which band?
7. Why is X-ray astronomy done from satellites?
8. Arrange X-rays, microwaves, UV, radio in decreasing wavelength.

Rough work

E. Short answer II (3 marks each)

1. Show how Ampere's law fails for a charging capacitor and how Maxwell corrected it.
2. Draw an EM wave along x with E along y. Write the equations for E and B.
3. Name the waves used for (i) killing germs in water, (ii) mobile phones, (iii) cancer treatment; give their ranges.
4. A capacitor with circular plates of radius 10 cm is charged by 2 A. Find i_d and B at 5 cm from the axis between the plates.
5. Give one method of production and one use of each: microwaves, X-rays, gamma rays.
6. Why does a charge oscillating at frequency ν radiate at the same ν? What is the frequency of the radiated wave if an electron is accelerated in a straight line?

F. Long answer (5 marks each)

1. Explain displacement current and derive i_d = ε₀ dΦ_E/dt. Show that the total current is continuous. Explain how EM waves propagate in free space.
2. Describe the EM spectrum: name the bands in order with their wavelength ranges, sources and one use each.

G. HOTS (higher-order thinking)

1. Why is the sky blue but sunsets red, in terms of the visible part of the spectrum? (Link to scattering, Ch 9.)
2. An astronaut on the Moon cannot hear an explosion but can see it. Why?
3. Can an EM wave be deflected by a magnetic field? Why?
4. Why are greenhouse gases transparent to visible light but trap infrared?

20 MCQs

1. Displacement current is
   (a) ε₀ dΦ_B/dt (b) μ₀ dΦ_E/dt (c) ε₀ dΦ_E/dt (d) dQ/dx
2. EM waves are
   (a) transverse (b) longitudinal (c) both (d) neither
3. c equals
   (a) √(μ₀ε₀) (b) 1/(μ₀ε₀) (c) μ₀/ε₀ (d) 1/√(μ₀ε₀)
4. Phase between E and B
   (a) π/2 (b) zero (c) π (d) π/4
5. EM waves are produced by
   (a) charges at rest (b) accelerated charges (c) steady currents (d) magnets
6. Radar uses
   (a) radio waves (b) UV (c) X-rays (d) microwaves
7. Heat waves are
   (a) infrared (b) UV (c) microwaves (d) γ rays
8. Bone fractures are seen with
   (a) IR (b) γ rays (c) X-rays (d) UV
9. Highest frequency among these
   (a) radio (b) visible (c) UV (d) γ rays
10. E₀ = 600 V/m in vacuum: B₀
    (a) 2 × 10⁻⁸ T (b) 2 × 10⁻⁶ T (c) 1.8 × 10¹¹ T (d) 6 × 10⁻⁶ T
11. Wavelength 500 nm is
    (a) IR (b) UV (c) visible (d) X-ray
12. The ozone layer absorbs
    (a) UV (b) visible (c) IR (d) radio
13. When light enters glass, unchanged is
    (a) speed (b) frequency (c) wavelength (d) direction always
14. FM radio band
    (a) 54–890 MHz (b) 530–1710 kHz (c) 88–108 MHz (d) 2–3 GHz
15. Speed of EM waves in a medium
    (a) c always (b) √(με) (c) με (d) 1/√(με)
16. i_d between capacitor plates at constant voltage
    (a) zero (b) maximum (c) i (d) infinite
17. Largest photon momentum
    (a) X-rays (b) microwaves (c) radio (d) IR
18. Wave along z, E along x: B along
    (a) x (b) −x (c) z (d) y
19. EM waves were first produced by
    (a) Maxwell (b) Hertz (c) Marconi (d) Bose
20. LASIK eye surgery uses
    (a) microwaves (b) X-rays (c) UV (d) radio

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Displacement current exists between the plates of a charging capacitor. R: The electric flux there changes with time.
2. A: EM waves need a medium. R: They carry energy.
3. A: Infrared waves are called heat waves. R: Water molecules readily absorb them.
4. A: X-ray astronomy is done from satellites. R: The atmosphere absorbs X-rays.
5. A: E and B in an EM wave are perpendicular. R: EM waves are longitudinal.
6. A: A steady current produces EM waves. R: A steady current produces a steady magnetic field.

Key: 1 (a) · 2 (d) · 3 (a) · 4 (a) · 5 (c) · 6 (d)

Answers to the practice set