Question 11:
easyAssertion (A): An E.M. wave has constantly interchanging electric and magnetic fields, which are perpendicular to each other.
Reason (R): The direction of propagation is given by \((\vec{B} \times \vec{E})\).
Electromagnetic waves consist of perpendicular oscillating electric and magnetic fields. The direction of propagation is given by \((\vec{E} \times \vec{B})\) (Poynting vector), not \((\vec{B} \times \vec{E})\). Thus, A is true, but R is false.
Question 12:
easyAssertion (A): Average value of electric field and magnetic field in one cycle is same in electromagnetic waves.
Reason (R): The ratio of electric field intensity and magnetic field intensity of EM waves is equal to speed of EM waves.
In electromagnetic waves, the average energy density stored in the electric field is equal to the average energy density stored in the magnetic field over one cycle. Also, the ratio of electric field intensity to magnetic field intensity equals the wave's speed \(v\). Both statements are true, but the second does not explain the first.
Question 13:
easyAssertion (A): In an electromagnetic wave, magnitude of magnetic field vector is much smaller than the magnitude of electric field vector.
Reason (R): Energy of electromagnetic waves is shared equally by the electric and magnetic fields.
In an EM wave, \(E_0 = cB_0\). Since \(c = 3 times 10^8 text{ m/s}\), \(E_0\) is much larger than \(B_0\) in SI units, so A is true. Energy is equally shared between fields, so R is true. However, R does not explain why \(E_0\) is numerically larger than \(B_0\).
Question 14:
easy**Assertion (A):** The gyrating electron can be a source of EMW.
**Reason (R):** The electron in circular motion is accelerated motion.
A gyrating electron undergoes circular motion, which is accelerated motion. Accelerated charges are sources of electromagnetic waves. Therefore, Assertion and Reason are true, and R correctly explains A.
Question 15:
easy**Assertion (A):** When electromagnetic waves are incident on a surface, exert radiation pressure on the surface.
**Reason (R):** Electromagnetic waves carry energy as well as momentum.
EM waves carry momentum. When they strike a surface, their momentum changes, exerting a force and thus pressure. Therefore, Assertion and Reason are true, and R is the correct explanation for A.
Question 16:
easy**Assertion (A):** In an electromagnetic wave, both the electric and magnetic field vibrates with same phase and frequency.
**Reason (R):** Amplitude of vibration of electric and magnetic field are equal in an electromagnetic wave.
In an EM wave, electric and magnetic fields oscillate in phase and with the same frequency. However, their amplitudes are related by \(E_0 = cB_0\) and are generally not equal. Thus, A is true, but R is false.
Question 17:
easy**Assertion (A):** The energy in a small volume through which an EM wave is passing oscillates with double the frequency of wave.
**Reason (R):** The electric field and magnetic field can not have equal average numerical value in each half cycle.
The energy density of an EM wave involves \(E^2\) and \(B^2\), which oscillate at \(2omega\). So, A is true. The average magnitudes of \(E\) and \(B\) over a half-cycle are not equal because \(E_0 = cB_0\). So, R is true. However, R does not explain A.
Question 18:
easyPart of EM spectrum which is used to destroy bacteria and sterilizing the surgical instruments is
Ultraviolet (UV) rays are widely used for germicidal and sterilization applications because their energy is sufficient to disrupt bacterial DNA.
Question 19:
easyThe velocity of an electromagnetic wave is along (where E represents the electric field and B represents the magnetic field)
The direction of propagation of an electromagnetic wave is given by Poynting vector, which is parallel to the cross product of electric and magnetic fields, \(\vec{E} \times \vec{B}\).
Question 20:
easyIn an electromagnetic wave, the oscillating electric and magnetic field vectors are oriented along
In an electromagnetic wave, the electric field \(\vec{E}\), magnetic field \(\vec{B}\), and direction of wave propagation are mutually perpendicular to each other.