PHYSICS HOMEWORK: ELECTROMAGNETIC WAVES
CLASS XII
1. Which of the following has the lowest frequency?
(a) Infrared rays (b) X-rays (c) UV rays (d) γ-rays
2. The frequency of visible light is of the order of.
(a) 1012 Hz (b) 1015 Hz (c) 108 Hz (d) 1018 Hz
3. Which of the following waves are used in tele-communication?
(a) Visible light (b) microwaves (c) ultraviolet (d) infrared
4. The displacement current flows in the dielectric of the capacitor, when the potential difference across its
plates:
(a) changes (increases/decreases) with time. (b) changes with distance.
(c) attains a constant value. (d) None of these.
5. Normal light is due to:
(a) electric field oscillations. (b) magnetic field oscillations.
(c) oscillations of electric and magnetic fields in mutually perpendicular directions. (d) None
6. Which of the following statement is incorrect.
(a) X-rays have minimum ionising power. (b) α-rays have maximum ionizing power.
(c) ß-rays have maximum penetrating power. (d) α-rays haye more penetrating power than a-rays.
7. Displacement current is-the same as:
(a) r.m.s. current (b) conduction current (c) peak current (d) None of these.
8. The direction of E × B in an electromagnetic wave is
(a) same as that of E (b) same as that of B
(c) same as the direction of propagation of electromagnetic wave. (d) None of these.
9. The correct option, if speeds of gamma rays, X-rays and microwave are Vg, Vx and Vm respectively will
be.
(a) Vg > Vx > Vm (b) Vg < Vx < Vm (c) Vg > Vx > Vm (d) Vg = Vx = Vm
10. Electromagnetic waves travelling in a medium having relative permeability μr = 1.3 and relative
permittivity Er = 2.14. The speed of electromagnetic waves in medium must be
(a) 1.8 × 108 ms-1 (b) 1.8 × 104ms1 (c) 1.8 × 106ms1 (d) 1.8 × 10² ms-1
11. The condition under which a microwave over heats up a food item containing water molecules most
efficiently is
(a) The frequency of the microwaves must match the resonant frequency of the water molecules.
(b) The frequency of the microwaves has no relation with natural frequency of the water molecules.
(c) Microwaves are heat waves, so always produce heating.
(d) Infrared waves produce heating in a microwave oven.
12. The electric field associated with an e.m wave in vacuum is given by 𝐸 =40 cos (kz – 6 × 108t) 𝑖 ,
where E, Z and t are in volt/m, metre and seconds respectively. The value of wave vector K is
(a) 2 m-1 (b) 0.5 m-1 (c) 6 m-1 (d) 3 m-1
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Directions: Questions 13 – 16 consist of two statements, each printed as Assertion and Reason. While
answering these questions, you are required to choose any one of the following four responses.
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.
13. Assertion: Electromagnetic wave are transverse in nature.
Reason: The electric and magnetic fields in electromagnetic waves are perpendicular to each other
and the direction of propagation.
14. Assertion: Electromagnetic waves interact with matter and set up oscillations.
Reason: Interaction is independent of the wavelength of the electromagnetic wave.
15. Assertion: Electromagnetic waves exert radiation pressure.
Reason: Electromagnetic waves carry energy.
16. Assertion: The basic difference between various types of electromagnetic waves lies in their wavelength
or frequencies.
Reason: Electromagnetic waves travel through vacuum with the same speed.
17. The oscillating magnetic field in a plane electromagnetic wave is given by,
By = (8 × 10-6) sin [2 × 10-11 t + 300 π x] T
(i) Calculate the wavelength of the electromagnetic wave.
(ii) Write down the expression for the oscillating electric field.
18. A capacitor, made of two parallel plates each of plate 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.
19. (a) An em wave is travelling in a medium with a velocity = v ̂. Draw a sketch showing the
propagation of the em wave, indicating the direction of the oscillating electric and magnetic fields.
(b) How are the magnitudes of the electric and magnetic fields related to the velocity of the em wave?
20. The oscillating electric field of an electromagnetic wave is given by:
E = 30 sin [2 × 1011 t + 300 π x] Vm-1
(a) Obtain the value of the wavelength of the electromagnetic wave.
(b) Write down the expression for the oscillating magnetic field.
