Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves

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Electromagnetic Waves (Emw) Multiple Choice Questions And Answers

Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves

Question 1. In a certain region electric field \(\vec{E}\) and magnetic field \(\vec{B}\) are perpendicular to each other. An electron enters the region perpendicular to the direction of both E and B and moves undeflected. The speed of the electron is _________.

  1. \({\vec{E}} \cdot {\vec{B}}\)
  2. \(|{\vec{E}} \times {\vec{B}}|\)
  3. \(\frac{|{\vec{E}}|}{|{\vec{B}}|}\)
  4. \(\frac{|{\vec{B}}|}{|{\vec{E}}|}\)

Answer: 1. \({\vec{E}} \cdot {\vec{B}}\)

Question 2. Which wave travels with the speed of light _________

  1. Sound wave
  2. Heatwave
  3. Shock wave
  4. Microwave

Answer: 4. Microwave

Question 3. A television network uses _________

  1. Microwaves
  2. High-Frequency Radio waves
  3. Light waves
  4. Sound waves

Answer: 2. High-frequency radio waves

Question 4. Light waves are _______

  1. Longitudinal
  2. Transverse
  3. Both Longitudinal and transverse
  4. Mechanical

Answer: 4. Mechanical

Question 5. An electromagnetic wave passing through the space is given by equations E = E0 sin (ωt – kx), B = B0 sin (ωt- kx) which of the following is true?

  1. E0B0 = ωk
  2. E0ω = B0k
  3. E0k = B0ω
  4. E0ωk = B0

Answer: 3. E0ω = B0k

Question 6. The wavelength range of Heal waves is __________

  1. 400 nm to 1 nm
  2. 1 mm to 700 nm
  3. 0. 1 m to 1 mm
  4. 700 nm to 400 nm

Answer: 2. 1 mm to 700 nm

Question 7. The maximum value of B in an electromagnetic wave is equal to 6 x 10-8T. Thus the maximum value of \(\vec{E}\) is __________.

  1. 2 Vm-1
  2. 18 Vm-1
  3. 2.5 Vm-1
  4. 6 Vm-1

Answer: 2. 18 Vm-1

E = cB

E = 3 x 108 x 6 x 10-8

E = 18 Vm-1

Question 8. Two oppositely charged particles oscillate about their mean equilibrium position in free space, with a frequency of 109 Hz. The wavelength of the corresponding electromagnetic wave produced is _______.

  1. 0.3 m
  2. 3 x 1017 m
  3. 109 m
  4. 3.3 m

Answer: 1. 0.3 m

⇒ \(\lambda=\frac{c}{v}=\frac{3 \times 10^8}{10^9}\)

∴ 0.3 m

Question 9. For a radiation of 6 GHz passing through air, the wave number (number of waves) per 1 m length is _______ (1 GHz = 109 Hz)

  1. 5
  2. 3
  3. 20
  4. 30

Answer: 3. 20

Wave No. \(\bar{v}=\frac{1}{\lambda}\) ∵ \(\left[v=\frac{c}{\lambda}\right]\) \(\left[\frac{c}{v}=\frac{1}{\lambda}\right]\)

∴ \(\bar{v}=\frac{v}{c}=\frac{6 \times 10^9}{3 \times 10^8}=20\)

Question 10. Which one of the following is an equation of magnetic energy density?

  1. \(\frac{B^2}{2 \mu_0}\)
  2. \(\frac{1}{2} \mu_0 B^2\)
  3. \(\frac{2 B^2}{\mu_0}\)
  4. \(\frac{\mathrm{B}^2}{\mu_0}\)

Answer: 1. \(\frac{B^2}{2 \mu_0}\)

Question 11. Dimension of \(\) is same as dimension of ( where μ = magnetic constant, ε = Dielectric constant)

  1. Velocity
  2. Square of velocity
  3. Acceleration
  4. Momentum

Answer: 2. Square of velocity

Electromagnetic Waves Assertion And Reason

For questions numbers 1 to 10 two statements are given labeled Assertion (A) and the other labeled Reason (R). Select the correct answer to these questions from the codes (1), (2), (3), and (4) as given below.

  1. Both (A) and (R) are correct, (R) is the correct explanation of (A).
  2. Both (A) and (R) are correct, (R) is not the correct explanation of (A).
  3. (A) is correct; (R) is incorrect.
  4. (A) is incorrect; (R) is incorrect

Question 1. Assertion: Electromagnetic waves do not require a medium for their propagation.

Reason: They cannot travel in a medium.

Answer: 3. (A) is correct; (R) is incorrect.

Question 2. Assertion: A changing electric field produces a magnetic field.

Reason: A changing magnetic field produces an electric field.

Answer: 2. Both (A) and (R) are correct, (R) is not the correct explanation of (A).

Question 3. Assertion: X-rays travel with the speed of light.

Reason: X-rays are electromagnetic rays.

Answer: 1. Both (A) and (R) are correct, (R) is the correct explanation of (A).

Question 4. Assertion: Environmental damage has increased the amount of ozone in the Atmosphere.

Reason: The increase of ozone increases the amount of ultraviolet radiation on the earth.

Answer: 4. (A) is incorrect; (R) is incorrect

Question 5. Assertion: Electromagnetic Radiation exerts pressure.

Reason: Electromagnetic waves carry both momentum and energy.

Answer: 1. Both (A) and (R) are correct, (R) is the correct explanation of (A).

Question 6. Assertion: The EM waves of shorter wavelengths can travel longer distances than those of longer wavelengths.

Reason: The shorter the wavelength, the larger the velocity of propagation.

Answer: 3. (A) is correct; (R) is incorrect.

Question 7. Assertion: EM waves follow the Superposition principle.

Reason: Differential expression of EM wave is linear.

Answer: 1. Both (A) and (R) are correct, (R) is the correct explanation of (A).

Question 8. Assertion: Sound waves cannot travel in a vacuum, but light waves can.

Reason: Light is an electromagnetic wave – but sound is a mechanical wave.

Answer: 1. Both (A) and (R) are correct, (R) is the correct explanation of (A).

Question 9. Assertion: Microwaves are better carriers of signals than radio waves.

Reason: Electromagnetic waves do not require any medium to propagate.

Answer: 2. Both (A) and (R) are correct, (R) is not the correct explanation of (A).

Question 10. Assertion: Transverse waves are not produced in liquids and gases.

Reason: The shorter the wavelength, the larger the velocity of propagation in air.

Answer: 3. (A) is correct; (R) is incorrect.

Electromagnetic Waves Short Questions And Answers

Question 1. The electric field of an electromagnetic wave is represented as Ex = E0 sin (ωt + kz).

  1. In which direction is the wave propagating?
  2. In which direction does the magnetic field oscillate?

Answer:

  1. Negative z direction
  2. y direction

Question 2.

  1. In which situation is there a displacement current but no conduction current?
  2. Why are Microwaves considered suitable for radar systems used in aircraft navigation?

Answer:

  1. In between the plates of a capacitor, during charging and discharging of a capacitor.
  2. Microwaves have energy more than radio waves, so these can travel up to greater distances.

Question 3. Match the column

Electromagnetic Waves Match The Column

Answer: 1-C, 2-D, 3-B, 4-A, 5-E

Question 4.

  1. Suppose that the earth’s atmosphere is absent, will the average temperature on the earth’s surface
    be higher or lower than what it is at present?
  2. What is an electromagnetic constant?

Answer:

  1. The average temperature will be lower due to the absence of the greenhouse effect.
  2. All types of electromagnetic waves move with the same speed c = 3 x 108 m/s in air or vacuum, so ‘c’ is called the electromagnetic constant.

Question 5.

  1. The charging current for a capacitor is 0.25 A. What is the displacement current across its plates?
  2. How are infrared waves produced? Write their one important use.

Answer:

  1. 0.25 A
  2. Infrared waves are produced by hot bodies due to molecular vibrations. These are used to
    treat muscular strain.

Question 6.

  1. Which part of the electromagnetic spectrum is used for eye surgery?
  2. Which part of the electromagnetic spectrum is blocked by protective welding glass?

Answer:

  1. Ultra-violet rays arc used in Lasik Laser, for eye surgery.
  2. Ultra-violet.

Question 7. Slate two properties of electromagnetic waves.

Answer:

  1. All EM waves travel with the same speed c = 3 x 108 m/s in air or vacuum.
  2. EM waves have energy and momentum and these apply radiation pressure, on the surface on which they are made to fall.

Question 8.

  1. The thin Ozone layer on top of the stratosphere is crucial for human survival. Why?
  2. How can we show that em waves carry momentum?

Answer:

Question 9.

  1. Which component of the electromagnetic wave is responsible for producing an optical effect?
  2. Light can travel in a vacuum whereas sound cannot do so. Why?

Answer:

  1. The Electric vector of the em wave is responsible.
  2. Light is electromagnetic while sound is a mechanical wave.

Question 10.

  1. For which wavelength our eyes are most sensitive?
  2. Which of the electromagnetic waves is capable of penetrating layers of dust?

Answer:

  1. 555nm i.e. yellow colour.
  2. Infra-red.

Question 11. Electromagnetic waves of wavelengths λ1,λ2, and λ3 are used in radar systems, water purifiers, and in remote switches of TV. respectively.

  1. Identify the electromagnetic waves, and
  2. Write one source for each of them.

Answer:

1. In radar systems ⇒ microwaves

In water purifies ⇒ UVrays

In remote switches in TV ⇒ Infrared rays

2. Microwave arc produced by special vacuum lubes (Klystrons, Magnetrons, and Gunn diodes)

  • UV radiation is produced in welding arc and the sun is an important source of ultraviolet light.
  • Infrared waves are produced by hot bodies and molecules.

Electromagnetic Waves Long Questions And Answers

Question 1. Gamma rays are used in radiotherapy to treat cancer. They are used to spot tumors. They kill the living cells and damage malignant tumors.

Electromagnetic Waves Gamma Knife Machine

(1). What is the source of gamma rays?

  1. Radioactive decay of the nucleus
  2. Accelerated motion of charges in conducting wire
  3. Hot bodies and molecule
  4. Klystron valve

Answer: 1. Radioactive decay of the nucleus

(2). How is the wavelength of gamma rays

  1. Low
  2. High
  3. Infinite
  4. Zero

Answer: 1. Low

(3). Choose the one with the correct penetrating power order of radiation.

  1. Alpha > beta > gamma
  2. Beta > alpha > gamma
  3. Gamma > beta > alpha
  4. Gamma > alpha > beta

Answer: 3. Gamma > beta > alpha

(4). What is the other use of gamma rays?

  1. Used to change white topaz to blue topaz
  2. Used in aircraft navigation
  3. Used to kill microbes
  4. Checking fractures of bone

Answer: 1. Used to change white topaz to blue topaz

Question 2. X-rays are a form of electromagnetic radiation, similar to visible light. Unlike light, however, x-rays have higher energy and can pass through most of the objects, including the body. Medical x-rays are used to generate images of tissues and structures inside the body

Electromagnetic Waves Projectional Radiography

(1). What is the most common method of preparation of X-rays?

  1. Magnetron valve
  2. Vibration of atoms and molecules
  3. Bombardment of metal by high-energy electrons
  4. Radioactive decay of the nucleus

Answer: 3. Bombardment of metal by high-energy electrons

(2). Which of the following sets of instruments or equipment can detect X-rays

  1. Photocells, photographic film
  2. Thermopiles, bolometer
  3. Photographic film. Geiger tube
  4. Geiger lube, the human eye

Answer: 3. Photographic film. Geiger tube

(3). Where do X-rays fall on the electromagnetic spectrum?

  1. Between the UV region and infrared region
  2. Between gamma rays and UV region
  3. Between infrared and microwaves
  4. Between microwaves and radio waves

Answer: 2. Between gamma rays and UV region

(4). What is the use of rays lying beyond the X-ray region in the electromagnetic spectrum

  1. Used to kill microbes
  2. Used to detect heat loss in insulated systems
  3. Used in standard broadcast radio and television
  4. Used in oncology, to kill cancerous cells.

Answer: 4. Used in oncology, to kill cancerous cells.

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