Electromagnetic Waves

In an electromagnetic wave, the electric field vector ( ) and the magnetic field vector ( ) oscillate in mutually perpendicular directions and are in phase with each other. This means that the electric and magnetic fields reach their maximum and minimum values simultaneously, without any phase difference between them.

The general characteristics of an electromagnetic wave are:

• The electric field ( ) and magnetic field ( ) are mutually perpendicular to each other.
• The electric and magnetic fields are also perpendicular to the direction of propagation of the wave.
• The electric and magnetic fields oscillate in phase, meaning they reach their maximum and minimum values at the same time.

Thus, the correct description of the orientation and phase of the electric and magnetic field vectors in an electromagnetic wave is:

Correct Answer:

Correct Answer: (C) Mutually perpendicular directions and are in phase

Electromagnetic waves are detected using different devices depending on their properties. Let's go through the matching of each electromagnetic wave to its detection device:

• Gamma rays : Ionization chamber - Gamma rays are high-energy electromagnetic waves that can ionize atoms. An ionization chamber is used to detect them by measuring the ionization produced by the gamma rays.
• Microwaves : Point contact diode - Microwaves are detected using a point contact diode, which can measure the change in resistance when exposed to microwave radiation.
• X-rays : Photographic film - X-rays are commonly detected by exposing photographic film. The X-rays cause a change in the film's properties, which can then be analyzed.
• Ultraviolet rays : Thermopiles - Thermopiles can detect ultraviolet (UV) rays by measuring the heat generated by the absorption of UV radiation.
• Infrared rays : Bolometer - Infrared rays are detected by a bolometer, which measures the heat produced by the absorption of infrared radiation. The bolometer works by detecting changes in resistance as it absorbs infrared energy.

The mismatch in the provided options is:

• Infrared rays : Bolometer - This is the correct match, as a bolometer is designed to detect infrared radiation by measuring the temperature change caused by it. Therefore, there is no mismatch here. The other devices in the list are appropriately matched to the corresponding waves.

Correct Answer:

Correct Answer: (E) Infrared rays : Bolometer

The minimum length of a dipole antenna is typically half of the wavelength of the carrier wave. The wavelength of a wave is related to the frequency by the equation: where is the speed of light ( m/s) and is the frequency of the wave. Given that the frequency of the carrier wave is 200 MHz Hz, we can calculate the wavelength: The minimum length of the dipole antenna is half the wavelength:

The correct option is (E) :

The greenhouse effect is caused by the trapping of infrared radiation by the Earth's atmosphere. Infrared rays have longer wavelengths than visible light and ultraviolet (UV) rays. These rays are absorbed by gases such as carbon dioxide and water vapor in the atmosphere, contributing to the warming of the Earth's surface.
(A) X-rays
(B) Cathode rays
(C) UV rays
(D) Gamma rays are not responsible for the greenhouse effect.

(E) Infrared rays are the primary electromagnetic waves responsible for the greenhouse effect, as they are absorbed by the greenhouse gases and trapped within the Earth's atmosphere, leading to warming.

The correct option is (E) : Infrared rays

Radio waves are electromagnetic waves that have frequencies in the range of to . They are widely used for various communication systems, including AM/FM radio, TV signals, and cellular phones. In cellular phones, radio waves are used to transmit voice communication by modulating the voice signal onto a carrier wave.
Thus, the correct answer is that radio waves are used in cellular phones to transmit voice communication.

The correct option is (D) : used in cellular phones to transmit voice communication

Device characteristics:

Klystrons and magnetrons are specialized vacuum tubes

Designed to generate and amplify high-frequency electromagnetic waves

Operate in the microwave frequency range (typically 1-300 GHz)

Key applications:

1. Used in radar systems
2. Essential for microwave ovens
3. Employed in satellite communications

Wave production mechanism:

• Klystrons use velocity modulation of electron beams
• Magnetrons utilize crossed electric and magnetic fields
• Both produce coherent microwave radiation

Thus, the correct option is (B): Microwave.