15 extra multiple-choice questions for Electromagnetic waves (12th Standard Physics, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which term did James Clerk Maxwell introduce to Ampere's circuital law to explain the production of magnetic fields by time-varying electric fields?
- A. Conduction current
- B. Displacement currentCorrect
- C. Eddy current
- D. Static current
Explanation. Maxwell introduced displacement current to resolve inconsistencies in Ampere's law. This term shows that a changing electric flux creates a magnetic field, even in a vacuum.
Q2
What physical reality is implied by the Maxwell's second equation (Gauss's law for magnetism), which states that the surface integral of a magnetic field over a closed surface is zero?
- A. Magnetic monopoles exist in pairs
- B. Isolated magnetic monopoles do not existCorrect
- C. Magnetic field lines are discontinuous
- D. Magnetic field depends on time
Explanation. This law implies that magnetic field lines form continuous closed loops. Consequently, there are no starting or ending points for these lines, confirming that isolated magnetic poles cannot be found.
Q3
In a vacuum, what is the mathematical relationship between the speed of light \(c\), the permittivity of free space \(\epsilon_0\), and the permeability of free space \(\mu_0\)?
- A. \(c = \sqrt{\mu_0 \epsilon_0}\)
- B. \(c = \frac{1}{\mu_0 \epsilon_0}\)
- C. \(c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}\)Correct
- D. \(c = \mu_0 \epsilon_0\)
Explanation. Maxwell's equations predict that electromagnetic waves travel at a speed determined by the fundamental constants of space. This theoretical speed matches the measured speed of light.
Q4
If an electromagnetic wave is propagating along the positive z-direction and its electric field vector points along the x-axis, in which direction does the magnetic field vector oscillate?
- A. Along the x-axis
- B. Along the y-axisCorrect
- C. Along the z-axis
- D. Opposite to the x-axis
Explanation. Electromagnetic waves are transverse. The electric field, magnetic field, and direction of propagation are all mutually perpendicular. Thus, if propagation is along z and E is along x, B must be along y.
Q5
What was the significant conclusion drawn from Heinrich Hertz's 1888 experiment regarding the receiver's orientation?
- A. EM waves are longitudinal
- B. EM waves require a medium
- C. EM waves are transverseCorrect
- D. EM waves have infinite speed
Explanation. Hertz observed that rotating the receiver by 90 degrees stopped the spark detection. This orientation dependence confirmed that the detected electromagnetic waves were transverse, as Maxwell had mathematically predicted.
Q6
Radio waves are primarily produced by which of the following physical processes?
- A. Sudden stopping of high-speed electrons
- B. Accelerated motion of charges in conducting wiresCorrect
- C. Transitions of radioactive nuclei
- D. Rotational transitions of molecules
Explanation. Radio waves are generated by oscillating charges in antennas. These accelerated charges create the time-varying electric and magnetic fields necessary for wave propagation in communication systems.
Q7
Which type of electromagnetic radiation is extensively used in radar systems for aircraft navigation and speed detection of vehicles?
- A. Infrared radiation
- B. Visible light
- C. MicrowavesCorrect
- D. X-rays
Explanation. Microwaves are utilized in radar due to their ability to undergo reflection and polarization. They are suitable for detecting the position and speed of objects over long distances.
Q8
Why are infrared radiations often referred to as 'heat waves'?
- A. They have the highest frequency
- B. They are produced by hot bodies and moleculesCorrect
- C. They are visible to the human eye
- D. They ionize atmospheric gases
Explanation. Infrared radiation is emitted by warm objects and atoms undergoing vibrational transitions. When absorbed by other materials, these waves increase thermal energy, creating a heating effect.
Q9
Which constituent of the Earth's atmosphere is primarily responsible for absorbing harmful ultraviolet (UV) radiation from the Sun?
- A. Nitrogen
- B. Oxygen
- C. OzoneCorrect
- D. Carbon dioxide
Explanation. The atmospheric ozone layer acts as a protective shield by absorbing most of the high-energy UV radiation, preventing it from reaching the surface where it could harm living organisms.
Q10
How are X-rays produced in a laboratory setting for medical applications?
- A. By radioactive decay of nuclei
- B. By sudden stopping of high-speed electrons at a targetCorrect
- C. By heating metals to high temperatures
- D. By passing current through a coil
Explanation. X-rays are generated when fast-moving electrons are rapidly decelerated upon hitting a high-atomic-number metal target. This sudden loss of kinetic energy is released as high-frequency electromagnetic radiation.
Q11
Gamma rays are distinct from other electromagnetic waves because they originate from which source?
- A. Transitions in the outermost electron shells
- B. Oscillating molecular dipoles
- C. Transitions of radioactive nucleiCorrect
- D. Vibrating hot solid bodies
Explanation. Gamma rays are high-energy photons emitted during nuclear reactions and radioactive decay. Unlike other waves that come from electronic or molecular changes, gamma rays provide insight into nuclear structure.
Q12
What type of emission spectrum is obtained from incandescent solids and the light from a carbon arc?
- A. Line emission spectrum
- B. Continuous emission spectrumCorrect
- C. Band emission spectrum
- D. Line absorption spectrum
Explanation. Incandescent solids emit radiation across all visible wavelengths because their particles are densely packed. When passed through a prism, this light forms a continuous rainbow without gaps.
Q13
The discrete dark lines observed in the solar spectrum, known as Fraunhofer lines, are examples of which type of spectrum?
- A. Line emission spectrum
- B. Line absorption spectrumCorrect
- C. Band emission spectrum
- D. Continuous absorption spectrum
Explanation. Fraunhofer lines appear when cooler gases in the Sun's atmosphere absorb specific characteristic wavelengths from the brighter, continuous light of the Sun's interior. This makes them a line absorption spectrum.
Q14
Calculate the refractive index of a medium if its relative permeability is 1.0 and its relative electrical permittivity (dielectric constant) is 2.25.
- A. 1.25
- B. 1.50Correct
- C. 2.25
- D. 3.00
Explanation. The refractive index \(n\) is calculated using \(n = \sqrt{\epsilon_r \mu_r}\). Substituting the given values, \(n = \sqrt{2.25 \times 1.0} = 1.5\). This index indicates how much light slows down in the medium.
Q15
If an electromagnetic wave of energy \(U\) is incident on a surface and is completely absorbed, what is the magnitude of the linear momentum \(p\) imparted to the surface?
- A. \(p = U \cdot c\)
- B. \(p = \frac{U}{c}\)Correct
- C. \(p = \frac{2U}{c}\)
- D. \(p = \sqrt{U \cdot c}\)
Explanation. Electromagnetic waves carry momentum as well as energy. For total absorption, the momentum transferred is equal to the total energy divided by the speed of light, as derived from electrodynamics.
