Class 12 Electromagnetic Waves Mock Test (CBSE)

Topics in Electromagnetic Waves

• Displacement current
• Sources of electromagnetic waves
• Nature of electromagnetic waves
• Electromagnetic spectrum

Electromagnetic Waves practice questions with answers

1. Q1Multiple choice·1 mark
   Displacement current comes into existence in a region of space where the electric field is:

   1. (a)Constant with time
   2. (b)Changing with time
   3. (c)Uniform in space
   4. (d)Zero
   Show answer and step marks

   1. Correct option: (b) Displacement current is defined as $I_d = \varepsilon_0 \frac{d\Phi_E}{dt}$, which exists only when the electric flux or electric field changes with time.+1

   Answer: (b)
2. Q2Multiple choice·1 mark
   Which of the following electromagnetic waves is primarily absorbed by the ozone layer in the earth's atmosphere?

   1. (a)Infrared rays
   2. (b)Microwaves
   3. (c)X-rays
   4. (d)Ultraviolet rays
   Show answer and step marks

   1. Correct option: (d) Ultraviolet rays are absorbed by the ozone layer in the stratosphere, preventing harmful radiation from reaching the Earth's surface.+1

   Answer: (d)
3. Q3Short answer·2 marks
   Name the electromagnetic waves which are (i) used in radar systems for aircraft navigation, and (ii) produced by radioactive decay of a nucleus. Write one frequency range for each.
   Show answer and step marks

   1. (i) Microwaves.+0.5
   2. Frequency range of microwaves: about $3 \times 10^9$ Hz to $3 \times 10^{11}$ Hz (wavelength about $0.1$ m to $1$ mm); ranges starting near $10^9$ Hz, as in some standard tables, accepted.+0.5
   3. (ii) Gamma rays.+0.5
   4. Frequency range of gamma rays: above about $10^{18}$ to $10^{19}$ Hz (standard-table values accepted).+0.5

   Answer: (i) Microwaves (about $3 \times 10^9$ Hz to $3 \times 10^{11}$ Hz), (ii) Gamma rays (above about $10^{18}$ to $10^{19}$ Hz)
4. Q4Short answer·3 marks
   (a) Which part of the electromagnetic spectrum is used in radar systems for aircraft navigation? Write its wavelength range. (b) State the source of infrared waves and mention one of their uses. (c) How does displacement current differ from conduction current?
   Show answer and step marks

   1. (a) Microwaves.+0.5
   2. (a) Wavelength range about $0.1\text{ m}$ to $1\text{ mm}$.+0.5
   3. (b) Source: hot bodies and molecules.+0.5
   4. (b) Use, any one of: infrared lamps in physical therapy; remote switches of TVs and other household appliances; infrared detectors in Earth satellites and night-vision devices; or any other valid use.+0.5
   5. (c) Conduction current is due to the actual flow of charge carriers (electrons), whereas displacement current is due to a time-varying electric field or changing electric flux.+1

   Answer: (a) Microwaves, wavelength about $0.1$ m to $1$ mm. (b) Produced by hot bodies and molecules; used in, e.g., physiotherapy lamps or TV remote controls. (c) Conduction current is a flow of charges; displacement current arises from a changing electric flux, $I_d = \varepsilon_0 \frac{d\Phi_E}{dt}$, with no charge flow.

More Class 12 Physics mock tests