RBSE Solutions Class 12 Physics Chapter 17 Electromagnetic Waves Communication and Con

RBSE Class 12 Physics Chapter 17 Multiple Choice Type Questions

Question 1. Average energy density radiated in electromagnetic wave is related to :
(a) Only electric field
(b) Only magnetic field
(c) Both electric and magnetic field
(d) Average energy density is zero
Answer: (c) Both electric and magnetic field
In simple words: The energy density in an electromagnetic wave depends on both the electric field and the magnetic field. Both these fields carry energy as the wave travels.

🎯 Exam Tip: Remember that electromagnetic waves are made of both electric and magnetic fields, so their energy is shared between these two components.

Question 3. Electromagnetic waves does not possess :
(a) energy
(b) charge
(c) momentum
(d) information
Answer: (a) energy
In simple words: Electromagnetic waves always carry energy, momentum, and information. They are made of photons, which have energy and momentum, and can be used to transmit data.

🎯 Exam Tip: Electromagnetic waves are fundamentally energy carriers, which is why we can use them for communication and heating. Always remember they have energy.

Question 4. If \( \vec{E} \) and \( \vec{B} \) are electric and magnetic field vectors of electromagnetic waves, then the propagation of electromagnetic wave is along :
(a) \( \vec{E} \)
(b) \( \vec{B} \)
(c) \( \vec{E} \times \vec{B} \)
(d) \( \vec{E} \cdot \vec{B} \)
Answer: (c) \( \vec{E} \times \vec{B} \)
In simple words: The direction in which an electromagnetic wave travels is found by taking the cross product of its electric field vector and its magnetic field vector. This shows that the wave moves perpendicular to both fields.

🎯 Exam Tip: The cross product \( \vec{E} \times \vec{B} \) gives the direction of wave propagation and also the direction of the Poynting vector, which describes energy flow.

Question 5. Which radiation has least wavelength?
(a) X-ray
(b) y-ray
(c) B-ray
(d) a-ray
Answer: (b) y-ray
In simple words: Among the given options, gamma rays have the shortest wavelength. This means they carry the highest energy and have the highest frequency in the electromagnetic spectrum.

🎯 Exam Tip: Remember the order of electromagnetic spectrum components by wavelength (or frequency) to correctly identify radiations with the shortest or longest wavelengths.

Question 7. For whom the ground waves are possible?
(a) Low radio frequency at low range
(b) High radio frequency at low range
(c) Low radio frequency at high range
(d) Low radio frequency at low range
Answer: (a) Low radio frequency at low range
In simple words: Ground waves work best for transmitting radio signals over short distances when the frequency is low. This type of wave travels along the Earth's surface.

🎯 Exam Tip: Ground wave propagation is effective for AM radio broadcasts over short distances and generally at frequencies below 2 MHz, as higher frequencies tend to be absorbed by the ground.

Question 8. The height of a TV tower is h meter. If radius of the Earth is R, then during TV transmission, the area occupied is (if h < R):
(a) \( \pi R^2 \)
(b) \( \pi h^2 \)
(c) \( 2R \)
(d) \( \pi Rh \)
Answer: (c) \( 2\pi Rh \)
In simple words: The maximum area a TV tower can cover depends on its height and the Earth's radius. The formula \( 2\pi Rh \) calculates this coverage area, which is like a circular patch on the ground.

🎯 Exam Tip: For TV tower transmission range, remember the formula for area covered is \( 2\pi Rh \), assuming the height of the tower is much less than the radius of the Earth.

Question 9. For propagation of radiowaves the mode used is/are :
(a) Ground wave propagation
(b) Sky wave propagation
(c) Space wave propagation
(d) All of the options
Answer: (d) All of the options
In simple words: Radio waves can travel in different ways, like along the ground, bouncing off the sky, or directly through space. Which method is used depends on the frequency and the distance needed.

🎯 Exam Tip: Understanding the different propagation modes is crucial for designing effective radio communication systems. Each mode has its own advantages and limitations.

Question 11. Modulation factor of over modulated wave is
(a) 1
(b) zero
(c) < 1
(d) > 1
Answer: (d) > 1
In simple words: When a wave is "over-modulated," it means the modulation factor is greater than 1. This can cause the signal to be distorted and difficult to receive clearly.

🎯 Exam Tip: Over-modulation leads to signal distortion. A modulation factor less than or equal to 1 ensures proper signal transmission without loss of information.

RBSE Class 12 Physics Chapter 17 Very Short Answer Type Questions

Question 1. What is the speed of the electromagnetic waves in vacuum?
Answer: The speed of electromagnetic waves in a vacuum, which is the speed of light, is \( c = 3 \times 10^8 \) m/s. This speed is a fundamental constant in physics.
In simple words: Electromagnetic waves, like light, travel at a fixed speed in empty space. This speed is \( 3 \times 10^8 \) meters per second.

🎯 Exam Tip: Always remember the speed of light in vacuum (c) as a constant value, \( 3 \times 10^8 \) m/s, as it is frequently used in calculations involving electromagnetic waves.

Question 2. For electromagnetic waves, what is the effect on refractive index of Ionosphere on increasing height from Earth's surface?
Answer: As the height from the Earth's surface increases in the Ionosphere, the refractive index for electromagnetic waves decreases. This happens because the electron density in the ionosphere changes with altitude.
In simple words: The higher you go from Earth into the Ionosphere, the less the electromagnetic waves bend. This means the refractive index gets smaller.

🎯 Exam Tip: The ionosphere's varying refractive index is crucial for skywave propagation, allowing radio signals to bounce off it and travel long distances.

Question 3. The vibration of \( \vec{E} \) of an electromagnetic wave propagating in X-direction are parallel to Y-axis. Then in which axis vibration of \( \vec{B} \) are parallel?
Answer: If the electric field \( \vec{E} \) of an electromagnetic wave vibrates parallel to the Y-axis and the wave propagates along the X-direction, then the magnetic field \( \vec{B} \) will vibrate parallel to the Z-axis. This is because \( \vec{E} \), \( \vec{B} \), and the direction of propagation are mutually perpendicular to each other.
In simple words: If the electric wave wiggles up-down (Y-axis) and the whole wave moves forward (X-axis), then the magnetic wave must wiggle side-to-side (Z-axis). They all point at right angles to each other.

🎯 Exam Tip: Remember that in an electromagnetic wave, the electric field, magnetic field, and direction of propagation are always perpendicular to each other, forming a right-handed system.

Question 5. What are the frequency limits for sky waves in sending signal to remote places?
Answer: For sky waves, which are used to send signals to distant places, the frequency limits for radiowaves typically range from 1.5 MHz to 30 MHz. These waves can reflect off the ionosphere, making long-distance communication possible.
In simple words: Sky waves use radio frequencies between 1.5 MHz and 30 MHz to send signals far away. They bounce off a layer in the sky called the ionosphere.

🎯 Exam Tip: Sky waves are essential for long-distance shortwave radio communication, especially for amateur radio and international broadcasting, due to their ability to reflect off the ionosphere.

Question 6. Name the part of the communication system which converts signal for propagation to send on communication channel to receiver.
Answer: The part of the communication system that converts the signal for propagation to send it over a communication channel to the receiver is called the Transmitter. The transmitter processes the information into a suitable form for transmission.
In simple words: The transmitter is the part that takes the message, changes it so it can travel, and then sends it out through the air or cables to the receiver.

🎯 Exam Tip: Identify the key function of each component in a communication system: transmitter (sends), channel (carries), receiver (gets), noise (interferes).

Question 7. Name the method used to superimpose signal on carrier wave?
Answer: The method used to superimpose a signal onto a carrier wave is called Modulation. Modulation helps in transmitting low-frequency signals over long distances by using a high-frequency carrier wave.
In simple words: The process of adding a message signal onto a carrier wave is called modulation. This helps the message travel farther.

🎯 Exam Tip: Modulation is a core concept in communication, allowing efficient transmission of information by shifting its frequency range to a higher, more suitable one.

Question 8. Which types of matter are studied in Nanotechnology?
Answer: In Nanotechnology, the types of matter studied are those with dimensions typically below 100 nm (nanometers). This includes materials at the atomic and molecular scale, where unique properties emerge due to their tiny size.
In simple words: Nanotechnology studies very tiny things, like materials smaller than 100 nanometers. This means looking at stuff at the level of atoms and molecules.

🎯 Exam Tip: The 'nano' scale (1-100 nanometers) is where many materials exhibit novel and useful properties different from their bulk counterparts.

RBSE Class 12 Physics Chapter 17 Short Answer Type Questions

Question 1. Name the components of electromagnetic spectrum in decreasing order of wavelength.
Answer: The electromagnetic spectrum is typically divided into seven main regions. In decreasing order of wavelength, these components are:
1. Radiowaves (\( 10^3 \) m)
2. Microwaves (\( 10^{-2} \) m)
3. Infrared rays (\( 10^{-5} \) m)
4. Visible rays (\( 0.5 \times 10^{-6} \) m)
5. Ultraviolet rays (~ \( 10^{-8} \) m)
These waves all travel at the speed of light but differ in their energy and frequency.
In simple words: The electromagnetic spectrum includes different types of waves, from long radio waves to very short gamma rays. They are listed from the longest wavelength to the shortest.

🎯 Exam Tip: Memorize the order of the electromagnetic spectrum (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma ray) by wavelength or frequency. This helps in understanding their properties and applications.

Question 3. Explain ground waves and sky waves.
Answer: Ground waves are radio waves that travel along the Earth's surface. They are suitable for low-frequency transmissions over short distances, as they tend to be absorbed by the ground at higher frequencies. Sky waves, on the other hand, are radio waves that are reflected back to Earth by the ionosphere, allowing for long-distance communication. This reflection enables signals to reach receivers far beyond the horizon.
In simple words: Ground waves travel along the ground for short distances. Sky waves bounce off a layer in the sky (ionosphere) to travel much farther.

🎯 Exam Tip: Differentiate between ground and sky waves based on their propagation path, frequency range, and typical communication distances to understand their applications.

Question 4. What is communication system?
Answer: A communication system is a way to transfer information from one source to another. The signals used are often electrical. The basic parts of such a system are the transmitter (which sends the signal), the carrier (which carries the signal), and the receiver (which gets the signal). This system ensures information can be shared across distances effectively.
In simple words: A communication system helps send information from one place to another. It has a sender, a way to carry the message, and a receiver.

🎯 Exam Tip: Clearly define the purpose of a communication system and list its three fundamental components: transmitter, channel (or carrier), and receiver.

Question 5. Name the parts of the communication system.
Answer: There are three essential parts of any communication system:
(i) Transmitter
(ii) Transmission channel
(iii) Receiver
These components work together to ensure that information is successfully encoded, transmitted, and decoded. The transmitter converts the message into a signal, the transmission channel carries it, and the receiver converts it back into a usable form.

🎯 Exam Tip: When describing a communication system, clearly list and define the three main components and illustrate their interaction, perhaps with a simple block diagram.

Question 6. Explain modulation.
Answer: Modulation is a process where information, like audio, video, or text, is added to a carrier signal. This carrier signal, which can be electrical or optical, then transmits the information over a medium. Modulation makes it possible to send information over long distances and allows multiple signals to share a communication channel without interfering with each other. The receiving device then "demodulates" the signal to get the original information back.
In simple words: Modulation is like putting a small message onto a bigger, faster signal (carrier wave) so it can travel far. Then, the receiver takes the message off the big signal.

🎯 Exam Tip: Focus on modulation's purpose: to enable efficient transmission of information over long distances and to allow multiple channels to coexist without interference.

RBSE Class 12 Physics Chapter 17 Long Answer Type Questions

Question 1. What is the nature of electromagnetic wave? Explain the Hertz's experiment related to electromagnetic wave.
Answer: Electromagnetic waves are made of electric and magnetic fields that vibrate perpendicular to each other and to the direction the wave travels. They carry energy and momentum. For example, if the wave moves in the X-direction and the electric field vibrates along the Y-axis, then the magnetic field vibrates along the Z-axis. This relationship ensures the wave can propagate without a physical medium. We can represent these oscillating fields mathematically as:
\( E_y = E_0 \sin(kx - \omega t) \)
\( B_z = B_0 \sin(kx - \omega t) \)
Here, \( k = 2\pi/\lambda \) is the wave number, and \( \omega \) is the angular frequency. The speed of light \( c \) relates the magnitudes of the electric and magnetic fields, \( B_0 = E_0/c \). In any medium, the speed \( v = c/\sqrt{\mu_r \varepsilon_r} \), where \( \mu_r \) is relative permeability and \( \varepsilon_r \) is relative permittivity. The refractive index \( n = \sqrt{\mu_r \varepsilon_r} \).

Hertz's Experiment of Electromagnetic Waves:

Heinrich Hertz confirmed the existence of electromagnetic waves in 1888. His experiment was based on the idea that an oscillating electric charge produces electromagnetic waves. Hertz used a spark-gap transmitter to generate these waves and a simple wire loop with another spark gap to detect them. The transmitter consisted of two metal plates (A and B) connected to polished metal spheres \( S_1 \) and \( S_2 \). A high voltage from an induction coil caused sparks to jump across the small gap between the spheres, generating high-frequency oscillations. These oscillations created electromagnetic waves that traveled through space. A circular wire loop acted as a receiver; when the electromagnetic waves hit it, they induced tiny sparks in its own small gap, proving the waves' existence. Hertz's work showed that light, electricity, and magnetism are all related and part of the same electromagnetic phenomenon.

🎯 Exam Tip: When explaining Hertz's experiment, focus on the generation of sparks creating oscillating fields and the detection of these fields inducing sparks in the receiving loop, confirming wave propagation.

Question 2. Describe various components of electromagnetic wave and explain their characteristics.
Answer: The electromagnetic spectrum is a classification of electromagnetic waves based on their frequency or wavelength. All these waves travel at the speed of light in a vacuum, but they have different energies and are produced or detected in distinct ways. The spectrum generally includes seven main regions, listed here in decreasing order of wavelength:

Radiowaves: These are produced by the accelerated motion of charges in conducting wires. They have the longest wavelengths (around \( 10^3 \) m) and lowest frequencies. Radiowaves are used in radio and television communication, including AM and FM broadcasts, as well as cellular phones (UHF band).

Microwaves: These have shorter wavelengths (around \( 10^{-2} \) m) than radiowaves. They are generated by special vacuum tubes like klystrons and magnetrons. Microwaves are used in radar systems, microwave ovens (heating water molecules), and satellite communication. They efficiently transfer energy to water molecules, making them useful for cooking.

Infrared Rays: With wavelengths around \( 10^{-5} \) m, infrared rays are produced by hot bodies and molecules. They are commonly known as heat waves, as they cause objects to warm up upon absorption. Applications include physical therapy, remote controls, night vision devices, and maintaining Earth's warmth through the greenhouse effect.

Visible Rays: This is the most familiar part of the spectrum, detected by the human eye. It ranges from about 400 nm (violet) to 700 nm (red). Visible light provides us with information about our surroundings and is crucial for vision. Different animals can perceive different ranges of wavelengths.

Ultraviolet Rays: These rays have wavelengths around \( 10^{-8} \) m, shorter than visible light. They are produced by special lamps and very hot bodies like the Sun. The ozone layer protects Earth from most harmful UV radiation. UV light can cause skin tanning and is used in laser eye surgery (LASIK) and water purifiers to kill germs.

X-rays: With wavelengths from \( 10^{-8} \) m down to \( 10^{-13} \) m, X-rays are produced when high-energy electrons bombard a metal target. They are widely used in medical diagnostics (imaging bones) and cancer treatment. X-rays can damage living tissues, so exposure must be carefully managed.

Gamma Rays: These are at the highest frequency and shortest wavelength end of the spectrum (less than \( 10^{-14} \) m). Gamma rays are produced during nuclear reactions and radioactive decay. They are highly energetic and are used in medical treatments (like radiation therapy) and sterilization, but prolonged exposure can be very dangerous to living organisms.

🎯 Exam Tip: For this type of question, ensure you list all major parts of the EM spectrum, provide a typical wavelength/frequency range, and mention at least one common application or characteristic for each.

Question 3. Explain the process of modulation and demodulation. How they are used in message signal propagation?
Answer:

Modulation: This is the process of changing one or more properties of a high-frequency carrier wave (like its amplitude, frequency, or phase) according to the information contained in a lower-frequency message signal. Modulation is essential because it allows low-frequency signals to be transmitted efficiently over long distances. Without modulation, low-frequency signals would be heavily absorbed and would require very long antennas. It also enables multiple signals to share the same transmission medium without interfering with each other.

Demodulation: Also known as detection, demodulation is the reverse process of modulation. It involves extracting the original information-bearing signal from the modulated carrier wave at the receiver. A demodulator circuit recovers the original message, whether it's audio, video, or data. This process effectively removes the high-frequency carrier, leaving only the desired information.

Usage in Message Signal Propagation: Modulation prepares the message for transmission, making it suitable for traveling across communication channels. For example, radio systems use frequency modulation (FM) or amplitude modulation (AM) to carry radio broadcasts. Once the modulated signal reaches the receiver, demodulation is used to convert it back into the original sound, image, or data that can be understood by the user. Together, modulation and demodulation form the complete cycle of communication, allowing information to be sent and received over vast distances.

🎯 Exam Tip: Clearly define modulation and demodulation as inverse processes. Explain their role in enabling long-distance, interference-free communication for various types of information.

Question 4. Explain with diagram amplitude modulation, frequency modulation and phase modulation.
Answer:

(A) Amplitude Modulation (AM): In amplitude modulation, the amplitude (strength) of the carrier signal changes in proportion to the instantaneous amplitude of the modulating (message) signal. The frequency and phase of the carrier wave remain constant. When the message signal is strong, the carrier wave becomes taller; when it's weak, the carrier wave becomes shorter. The "envelope" of the modulated wave recreates the shape of the original message signal. AM is often used in shortwave and radio wave broadcasting.

(B) Frequency Modulation (FM): In frequency modulation, the frequency of the carrier wave varies in accordance with the instantaneous amplitude of the modulating signal, while its amplitude remains constant. When the message signal's amplitude is high, the carrier's frequency increases; when the message signal's amplitude is low, the carrier's frequency decreases. FM is widely used in high-quality radio broadcasting because it is less affected by noise compared to AM. Higher frequencies in FM tend to pass through the ionosphere without reflection, limiting their long-distance reach via skywaves.

(C) Phase Modulation (PM): Phase modulation is a form of modulation where the instantaneous phase of the carrier wave is varied in proportion to the instantaneous amplitude of the modulating signal. In PM, the amplitude and frequency of the carrier remain constant, but changes in the message signal's amplitude cause corresponding changes in the carrier's phase. PM is an integral part of many digital communication technologies like Wi-Fi, GSM, and satellite TV transmission, as it is effective for transmitting radiowaves.

In simple words: Amplitude modulation (AM) changes the height of the signal wave. Frequency modulation (FM) changes how quickly the wave wiggles. Phase modulation (PM) shifts the wave's starting point. All these methods help put information onto a main wave for sending.

🎯 Exam Tip: When explaining modulation types, clearly describe what parameter of the carrier wave is varied (amplitude, frequency, or phase) and illustrate with simple diagrams showing the message, carrier, and modulated signals.

Question 5. Describe some examples observed in nature of Nanotechnique.
Answer: The ideas for nanotechnology first came from physicist Richard Feynman in 1959. He talked about how we could build things by moving individual atoms. This revolutionary concept opened the door for many future advancements in science.
In simple words: Richard Feynman first shared the idea of nanotechnology in 1959, suggesting we could make things by moving atoms.

🎯 Exam Tip: Remember Richard Feynman's pioneering vision of manipulating matter at the atomic level, which laid the foundation for modern nanotechnology.

RBSE Class 12 Physics Chapter 17 Numerical Questions

Question 1. A plane electromagnetic wave travels in free space along the A-direction. At a particular point in space the maximum value of \( \vec{E} \) is 600 volt/meter. What is \( \vec{B} \) at this point? (c = 3 × 10⁸ m/s)
Answer: In an electromagnetic wave, the speed of light (\(c\)) is equal to the maximum electric field strength (\(E_m\)) divided by the maximum magnetic field strength (\(B_m\)). The speed of light is a universal constant.
We are given:
Electric field maximum, \( E_m = 600 \) volt/meter
Speed of light, \( c = 3 \times 10^8 \) m/s
We need to find the magnetic field maximum, \( B_m \).
Using the formula:
\( c = \frac{E_m}{B_m} \)
Rearranging to find \( B_m \):
\( B_m = \frac{E_m}{c} \)
Now, we put in the numbers:
\( B_m = \frac{600}{3 \times 10^8} \)
\( B_m = 2 \times 10^{-6} \) Wb/m²
In simple words: The maximum magnetic field strength is found by dividing the maximum electric field strength by the speed of light. This relationship helps us understand how these fields balance in a wave.

🎯 Exam Tip: Always remember the fundamental relationship \( c = E_m / B_m \) for electromagnetic waves in a vacuum, as it's key for solving problems involving electric and magnetic field strengths.

Question 2. A TV transmitting antenna is 75 m tall. How much maximum distance and area cover by it? Radius of Earth = 6.4 x 10⁶ m.
Answer: We need to find the maximum distance an antenna can send a signal and the total area it covers.
Given:
Height of antenna \( h = 75 \) m
Radius of the Earth \( R = 6.4 \times 10^6 \) m
First, let's find the maximum distance \( d \). We use the formula for line-of-sight distance from an antenna:
\( d = \sqrt{2Rh} \)
Putting in the values:
\( d = \sqrt{2 \times 6.4 \times 10^6 \text{ m} \times 75 \text{ m}} \)
\( d = \sqrt{960 \times 10^6 \text{ m}^2} \)
\( d = \sqrt{960} \times \sqrt{10^6} \text{ m} \)
\( d \approx 30.98 \times 10^3 \text{ m} \)
So, the maximum distance covered is approximately \( 31 \) km. This calculation shows how the Earth's curvature limits signal reach.
Next, let's find the maximum area \( A \) covered by the antenna.
The area covered is a circle with radius \( d \):
\( A = \pi d^2 \)
We can also use the derived formula for area directly:
\( A = \pi (2Rh) \)
Putting in the values:
\( A = 3.14 \times (2 \times 6.4 \times 10^6 \text{ m} \times 75 \text{ m}) \)
\( A = 3.14 \times (960 \times 10^6 \text{ m}^2) \)
\( A = 3014.4 \times 10^6 \text{ m}^2 \)
Converting to square kilometers:
\( A \approx 3014 \) km²
In simple words: We calculate the farthest a signal can go from the antenna and the total circular area it can reach. The antenna height and the Earth's round shape determine these limits.

🎯 Exam Tip: When solving antenna problems, remember to correctly apply the formulas \( d = \sqrt{2Rh} \) for distance and \( A = \pi d^2 \) or \( A = \pi (2Rh) \) for the coverage area, paying attention to unit conversions (meters to kilometers).