EM Waves — Set 3
Physics · विद्युत चुंबकीय तरंगें · Questions 21–30 of 50
The SI unit of magnetic flux density B is?
Correct Answer: B. Tesla
Magnetic flux density is measured in tesla (T). It describes how strong a magnetic field is in space. In EM waves, B varies with time and position.
If the wavelength of an EM wave in vacuum is doubled, its frequency becomes?
Correct Answer: C. Halved
The correct answer is 'Halved'. In vacuum, c = fλ and c is constant. If λ doubles, f must halve to keep the product the same. This inverse relationship holds for all waves at fixed speed.
Polarization of a wave is possible only if the wave is?
Correct Answer: C. Transverse
Polarization is a property of transverse waves. EM waves are transverse, so they can be polarized. Polarizing filters work by selecting one vibration direction.
When light enters glass from air, which quantity usually remains unchanged?
Correct Answer: B. Frequency
Frequency stays the same when light enters a new medium. Speed changes due to the medium’s refractive index. Wavelength changes because λ = v/f in the medium.
Which electromagnetic waves are commonly used in radar and many satellite links?
Correct Answer: C. Microwaves
Microwaves are widely used in radar and satellite communication. Their wavelengths suit antennas and atmospheric transmission. Many Wi-Fi systems also use microwave frequencies.
For a plane EM wave in vacuum, the magnetic field magnitude B is related to electric field magnitude E by?
Correct Answer: C. B = E/c
In vacuum, E/B = c for a plane electromagnetic wave. Rearranging gives B = E/c. This relation is specific to free-space propagation.
The energy of a photon of frequency 5.0 × 10^14 Hz is closest to?
Correct Answer: D. 3.3 × 10^-19 J
Use E = hf with h ≈ 6.63 × 10^-34 J·s and f = 5.0 × 10^14 Hz. This gives E ≈ 3.3 × 10^-19 J. Higher frequency photons always carry more energy.
The intensity of an electromagnetic wave is proportional to?
Correct Answer: A. The square of its field amplitude
Intensity depends on how much energy passes per unit area per second. For waves, it is proportional to the square of amplitude of the fields. Doubling field amplitude makes intensity four times.
In vacuum, the speed of electromagnetic waves is?
Correct Answer: A. Independent of frequency
The correct answer is 'Independent of frequency'. All electromagnetic waves travel at the same speed c in vacuum. This is true from radio waves up to gamma rays. Different frequencies differ in wavelength, not in vacuum speed.
Which sequence correctly lists EM waves in increasing order of frequency?
Correct Answer: B. Radio < Microwave < Infrared < Visible < Ultraviolet < X-ray < Gamma
Frequency increases from radio to gamma across the spectrum. Microwaves, infrared, visible, ultraviolet, X-rays, and gamma follow in that order. Wavelength decreases as frequency increases when speed is fixed.