Every Book Back multiple-choice question from Electromagnetic waves (12th Standard Physics, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
The dimension of \(\frac{1}{\mu_o \epsilon_o}\) is
- A. \([L T^{-1}]\)
- B. \([L^2 T^{-2}]\)Correct
- C. \([L^{-1} T]\)
- D. \([L^{-2} T^2]\)
Explanation. The speed of light in a vacuum is given by \(c = 1/\sqrt{\mu_0 \epsilon_0}\), which means \(c^2 = 1/\mu_0 \epsilon_0\). Since velocity has dimensions of \([LT^{-1}]\), its square results in the dimension \([L^2 T^{-2}]\).
Q2
If the amplitude of the magnetic field is \(3 \times 10^{-6}\) T, then the amplitude of the electric field for an electromagnetic wave is
- A. 100 V m\(^{-1}\)
- B. 300 V m\(^{-1}\)
- C. 600 V m\(^{-1}\)
- D. 900 V m\(^{-1}\)Correct
Explanation. In an electromagnetic wave, the amplitudes are related by the equation \(E_0 = c B_0\). Using the speed of light \(c = 3 \times 10^8\) m/s and the given magnetic field, the electric field amplitude is \(900\) V/m.
Q3
Which of the following electromagnetic radiations is used for viewing objects through fog?
- A. microwave
- B. gamma rays
- C. X-rays
- D. infraredCorrect
Explanation. Infrared radiation is effective for viewing through fog because it has longer wavelengths that scatter less than visible light. This property allows it to penetrate atmospheric obstacles like haze and mist more effectively.
Q4
Which of the following is false for electromagnetic waves?
- A. transverse
- B. non-mechanical waves
- C. longitudinalCorrect
- D. produced by accelerating charges
Explanation. Electromagnetic waves are transverse waves and do not require a medium for propagation. Longitudinal waves require a medium to travel through, whereas electromagnetic waves are non-mechanical and travel through a vacuum.
Q5
Consider an oscillator which has a charged particle oscillating about its mean position with a frequency of 300 MHz. The wavelength of electromagnetic waves produced by this oscillator is
- A. 1 mCorrect
- B. 10 m
- C. 100 m
- D. 1000 m
Explanation. The wavelength is found using \(\lambda = c/f\). With \(c = 3 \times 10^8\) m/s and a frequency of \(300 \times 10^6\) Hz, the resulting wavelength is 1 meter.
Q6
The electric and the magnetic fields, associated with an electromagnetic wave, propagating along negative X axis can be represented by
- A. \(\vec{E} = E_o \hat{i}, \vec{B} = B_o \hat{k}\)
- B. \(\vec{E} = E_o \hat{k}, \vec{B} = B_o \hat{j}\)Correct
- C. \(\vec{E} = E_o \hat{i}, \vec{B} = B_o \hat{j}\)
- D. \(\vec{E} = E_o \hat{j}, \vec{B} = B_o \hat{i}\)
Explanation. The direction of propagation is determined by the cross product \(\vec{E} \times \vec{B}\). For propagation along the negative X axis, the electric field vector along \(\hat{k}\) and the magnetic field vector along \(\hat{j}\) give \(\hat{k} \times \hat{j} = -\hat{i}\).
Q7
In an electromagnetic wave travelling in free space the rms value of the electric field is 3 V m\(^{-1}\). The peak value of the magnetic field is
- A. \(1.414 \times 10^{-8}\) TCorrect
- B. \(1.0 \times 10^{-8}\) T
- C. \(2.828 \times 10^{-8}\) T
- D. \(2.0 \times 10^{-8}\) T
Explanation. The peak electric field is \(E_0 = \sqrt{2} E_{rms} = 3\sqrt{2}\). The peak magnetic field \(B_0\) is \(E_0/c\). Substituting \(c = 3 \times 10^8\), we get \(B_0 = \sqrt{2} \times 10^{-8}\), which is approximately \(1.414 \times 10^{-8}\) T.
Q8
An e.m. wave is propagating in a medium with a velocity \(\vec{v} = v\hat{i}\). The instantaneous oscillating electric field of this e.m. wave is along +y-axis, then the direction of oscillating magnetic field of the e.m. wave will be along:
- A. –y direction
- B. –x direction
- C. +z directionCorrect
- D. –z direction
Explanation. Since the wave propagates along the positive X axis and the electric field oscillates along the Y axis, the magnetic field must oscillate along the Z axis to satisfy the condition that \(\vec{E}\), \(\vec{B}\), and \(\vec{v}\) are mutually perpendicular.
Q9
If the magnetic monopole exists, then which of the Maxwell’s equation to be modified?
- A. \(\oint \vec{E} \cdot d\vec{A} = \frac{Q_{enclosed}}{\epsilon_o}\)
- B. \(\oint \vec{B} \cdot d\vec{A} = 0\)Correct
- C. \(\oint \vec{B} \cdot d\vec{l} = \mu_o i_c + \mu_o \epsilon_o \frac{d\Phi_E}{dt}\)
- D. \(\oint \vec{E} \cdot d\vec{l} = -\frac{d\Phi_B}{dt}\)
Explanation. Gauss's law for magnetism states that magnetic field lines form continuous closed loops, meaning isolated magnetic monopoles do not exist. If a monopole were found, the surface integral of the magnetic field would no longer equal zero.
Q10
Fraunhofer lines are an example of _______ spectrum.
- A. line emission
- B. line absorptionCorrect
- C. band emission
- D. band absorption
Explanation. Fraunhofer lines are dark lines in the Sun's spectrum caused by the absorption of specific light wavelengths by elements in the Sun's cooler outer atmosphere. This makes them a characteristic example of a line absorption spectrum.
Q11
Which of the following is an electromagnetic wave?
- A. \(\alpha\) - rays
- B. \(\beta\) - rays
- C. \(\gamma\) - raysCorrect
- D. all of them
Explanation. Gamma rays are high-frequency electromagnetic radiation. In contrast, alpha and beta rays are composed of charged particles (helium nuclei and electrons/positrons, respectively) rather than being part of the electromagnetic spectrum.
Q12
Which one of them is used to produce a propagating electromagnetic wave?
- A. an accelerating chargeCorrect
- B. a charge moving with constant velocity
- C. a stationary charge
- D. an uncharged particle
Explanation. Accelerating charges are the primary source of electromagnetic waves. A stationary charge only produces a static electric field, and a charge moving with constant velocity creates a steady magnetic field but does not radiate energy.
Q13
If \(E = E_o \sin[10^6 x - \omega t]\) be the electric field of a plane electromagnetic wave, the value of \(\omega\) is
- A. \(0.3 \times 10^{-14}\) rad s\(^{-1}\)
- B. \(3 \times 10^{-14}\) rad s\(^{-1}\)
- C. \(0.3 \times 10^{14}\) rad s\(^{-1}\)
- D. \(3 \times 10^{14}\) rad s\(^{-1}\)Correct
Explanation. The relationship between angular frequency and wave number is given by \(\omega = ck\). With \(c = 3 \times 10^8\) m/s and the wave number \(k = 10^6\) m\(^{-1}\) from the wave equation, \(\omega\) calculates to \(3 \times 10^{14}\) rad/s.
Q14
Which of the following is NOT true for electromagnetic waves?
- A. it transports energy
- B. it transports momentum
- C. it transports angular momentum
- D. in vacuum, it travels with different speeds which depend on their frequencyCorrect
Explanation. In a vacuum, all electromagnetic waves travel at the same speed, \(c \approx 3 \times 10^8\) m/s, regardless of their wavelength or frequency. They only exhibit different speeds when traveling through material media.
Q15
The electric and magnetic fields of an electromagnetic wave are
- A. in phase and perpendicular to each otherCorrect
- B. out of phase and not perpendicular to each other
- C. in phase and not perpendicular to each other
- D. out of phase and perpendicular to each other
Explanation. Electromagnetic waves consist of oscillating electric and magnetic field vectors that are mutually perpendicular and oscillate in phase, meaning they reach their maximum and minimum values at the same time.
