Electromagnetic Waves Chapter-Wise Test 11

Correct answer Carries: 4.

Wrong Answer Carries: -1.

Why do infrared lamps find application in physical therapy?

The document mentions that infrared waves are absorbed by materials, increasing thermal motion and heating the surroundings, which is useful for physical therapy.

They emit ultraviolet radiation
They heat tissues by increasing thermal motion
They emit visible light for visibility
They produce high-frequency waves
2

Why can’t one get tanned through ordinary glass windows?

The document explains that UV radiation, which causes tanning, is absorbed by ordinary glass, preventing it from passing through.

Glass emits infrared radiation
Glass reflects visible light
Glass scatters UV radiation
Glass absorbs UV radiation
4

In electromagnetic wave theory, what ensures the symmetry between electric and magnetic fields in wave propagation?

Faraday’s law and the Ampere-Maxwell law together ensure symmetry, as a changing electric field induces a magnetic field and vice versa, sustaining wave propagation.

Gauss’s laws
Static field equations
Constant field amplitudes
Faraday’s and Ampere-Maxwell laws
4

An electromagnetic wave has a wave number \( k = 2 \, \text{rad/m} \). What is its wavelength in vacuum?

The wave number \( k = \frac{2 \pi}{\lambda} \). Given \( k = 2 \, \text{rad/m} \), we have \( \lambda = \frac{2 \pi}{k} = \frac{2 \pi}{2} = \pi \approx 3.14 \, \text{m} \).

1.57 m
3.14 m
4.71 m
6.28 m
2

Why do infrared waves contribute to the greenhouse effect on Earth?

Infrared waves emitted by the Earth's surface are absorbed and trapped by greenhouse gases, leading to an increase in atmospheric temperature, a phenomenon known as the greenhouse effect.

They reflect sunlight
They are trapped by greenhouse gases
They enhance UV radiation
They cool the atmosphere
2

An electromagnetic wave in vacuum has a frequency of \( 15 \, \text{MHz} \). What is its wavelength? (Given \( c = 3 \times 10^8 \, \text{m/s} \))

Using \( v \lambda = c \), we have \( \lambda = \frac{c}{v} = \frac{3 \times 10^8}{15 \times 10^6} = 20 \, \text{m} \).

10 m
20 m
30 m
40 m
2

Which of the following is a common method to detect ultraviolet rays?

The document mentions that ultraviolet rays can be detected using photocells and photographic film.

Receiver's aerials
Thermopiles
Photocells
Bolometers
3

What physical property of electromagnetic waves explains why they can propagate through interstellar space?

Electromagnetic waves require no medium for propagation, as they are self-sustaining oscillations of electric and magnetic fields, allowing them to travel through the vacuum of interstellar space.

High frequency
Particle nature
No medium required
Thermal energy
3

Which of the following is true about the displacement current in a charging capacitor?

Inside a charging capacitor, there is no conduction current (\( i_c = 0 \)), but there is a displacement current due to the changing electric field between the plates, as explained by Maxwell.

It exists outside the plates only
It exists inside the plates and equals the conduction current in the wires
It is zero inside the plates
It is independent of the electric field
2

A radio tunes to stations in the \( 7.5 \, \text{MHz} \) to \( 12 \, \text{MHz} \) band. What is the corresponding wavelength range? (Given \( c = 3 \times 10^8 \, \text{m/s} \))

Using \( \lambda = \frac{c}{v} \), for \( 7.5 \, \text{MHz} \), \( \lambda = \frac{3 \times 10^8}{7.5 \times 10^6} = 40 \, \text{m} \). For \( 12 \, \text{MHz} \), \( \lambda = \frac{3 \times 10^8}{12 \times 10^6} = 25 \, \text{m} \). So the range is \( 25 \, \text{m} \) to \( 40 \, \text{m} \).

\( 15 \, \text{m} \) to \( 20 \, \text{m} \)
\( 25 \, \text{m} \) to \( 40 \, \text{m} \)
\( 50 \, \text{m} \) to \( 60 \, \text{m} \)
\( 10 \, \text{m} \) to \( 15 \, \text{m} \)
2

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