Bihar Board - Class 12 Physics - Chapter 8: ELECTROMAGNETIC WAVES Long Answer Question
Long Answer Type Questions
1. A parallel plate capacitor has circular plates, each of radius 5.0 cm it is being charged so that the electric Field in the gap between its plates rises steadily at the rate of 1012 Vm-1s-1. What is the displacement current?
Solution:
Here r = 5 cm = 5 x 10-2 m,
ddt = 1012Vm-1s-1 dt
Displacement current,
Id = ε0 dEdt = ε0 A dEdt
= ε0.πr2 dEdt
= 8.85 x 10-12 x π x(5 x 10-2) x 1012 A
= 0.07 A.
2. Answer the following :
(a) Name the em waves which are suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves.
(b) If the. Earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now? Explain.
(c) An em wave exerts pressure on the surface on which it is incident. Justify.
Answer:
(a) Microwaves are used in radar systems. Its frequency range : 1010 to 1012 Hz
(b) In the absence of earth’s atmosphere, there would be no ozone layer to prevent ultraviolet radiations reaching the earth, the temperature on earth’s surface would have been lower due to greenhouse effect, making it difficult for human survival.
(c) Since an em wave carries both energy and momentum, hence exerts pressure on the surface on which it is incident.
An em wave exerts negligibly very small pressure on the surface on which it is incident.
It is due to the fact that momentum of the photon is extremely small, which can be
calculated by de-Broglie relation (λ=hp)
or p = h = 6.6310-3410-9 =6.6510-25kg ms-1
3. A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?
Answer:
The electromagnetic wave travels in a vacuum along the z-direction. The electric field (E) and the magnetic field (H) are in the x-y plane. They are mutually perpendicular.
Frequency of the wave, ν = 30 MHz = 30 × 106 s-1
Speed of light in a vacuum, c = 3 × 108 m/s
Wavelength of a wave is given as:
λ=hp = 310830106 = 10 m
4. Answer the following :
(a) Name the em waves which are used for the treatment of certain forms of cancer. Write their frequency range.
(b) Thin ozone layer on top of stratosphere is crucial for human survival. Why?
(c) Why is the amount of the momentum transferred by the em waves incident on the surfrace so small?
Answer:
(a) Gamma (γ) rays are used for the treatment of certain forms of cancer. Their frequency range is 1018 Hz to 1022 Hz.
(b) The thin ozone layer on top of the stratosphere absorbs most of the harmful ultraviolet rays coming from the Sun towards the Earth. They include UVA, UVB and UVC radiations, which can destroy the life system on the Earth.
Hence, this layer is crucial for human survival.
(c) Thus, the amount of the momentum transferred by the em waves incident on the surface is very small, because of the small value of planck's constant. For example, an electromagnetic wave of wavelength 1.00 nm will provide momentum (p) according to de-Broglie’s relation,
p = h = 6.6310-34 110-9 = 6.63 10-25 kg ms-1
It is an extremely small value of the momentum.
5. The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B0 = 510 nT. What is the amplitude of the electric field part of the wave?
Answer:
Amplitude of magnetic field of an electromagnetic wave in a vacuum,
B0= 510 nT = 510 × 10-9 T
Speed of light in a vacuum, c = 3 × 108m/s
Amplitude of electric field of the electromagnetic wave is given by the relation,
E = cB0
= 3 × 108 × 510 × 10-9= 153 N/C
Therefore, the electric field part of the wave is 153 N/C.
6. Answer the following questions :
(a) Name the em waves which are produced during radioactive decay of a nucleus. Write their frequency range.
(b) Welders wear special glass goggles while working. Why? Explain.
(c) Why are infrared waves often called heat waves? Give them one application.
Answer:
(a) -rays; Frequency range :1018 Hz to 1022 Hz.
(b) Because to protect eyes from intense ultraviolet radiations produced during welding; and also to protect from glare and flying sparks.
(c) Because infrared waves are em waves of higher wavelength (less frequency) and are produced by highly vibrating molecules of hot bodies.
Applications :
used in the remote switches of household electronic systems.
used for protecting dehydrated fruits.
used in solar water heaters and cookers.
7. Use the formula m T= 0.29 cm K to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?
Answer:
A body at a particular temperature produces a continuous spectrum of wavelengths. In the case of a black body, the wavelength corresponding to maximum intensity of radiation is given according to Planck’s law. It can be given by the relation,
m= 0.29T cm K
Where,
m = maximum wavelength
T = temperature
Thus, the temperature for different wavelengths can be obtained as:
For m = 10-4 cm; T = 0.2910-4 = 2900οK
For m = 5 ×10-5 cm; T = 0.29510-5 = 5800οK
For m = 10-6 cm; T = 0.2910-6 = 290000οK and so on.
The numbers obtained tell us that temperature ranges are required for obtaining radiations in different parts of an electromagnetic spectrum. As the wavelength decreases, the corresponding temperature increases.
8. Answer the following questions:
(i) Show, by giving a simple example, how em waves carry energy and momentum.
(i) How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of microwaves to match the resonant frequency of water molecules?
(iii) Write two important uses of infrared waves.
Answer:
(i) Consider a plane perpendicular to the direction of propagation of the wave. An electric charge, on the plane, will be set in motion by the electric and magnetic fields of em wave, incident on this plane. This illustrates that em waves carry energy and momentum.
(ii) Microwaves are produced by special vacuum tubes like the Klystron/Magnetron/Gunn diode.
In microwave ovens, the frequency of microwaves is selected to match the resonant frequency of water molecules, so that energy is transferred efficiently to the kinetic energy of the molecules.
(iii) Important uses of infra-red waves :
1. These are associated with the greenhouse effect.
2. These are used in remote switches of household electrical appliances.
9. (a) How are electromagnetic waves produced?
(b) How do you convince yourself that electromagnetic waves carry energy and momentum?
Answer:
(a) Electromagnetic Waves : Accelerating electric charge produces electromagnetic waves.
(b) Einstein’s explanation of the photoelectric effect led de Broglie to the wave-particle duality, i.e., matter exhibits wave as well as particle properties. Electromagnetic waves are characterized by wave properties, such as periodicity in space-time, wavelength, amplitude, frequency, wave velocity etc. It transports energy but no matter.
The term wave-particle duality refers to the behavior where both wave-like and particle-like properties are exhibited under different conditions by the same entity. Hence electromagnetic waves show particle properties such as definite position, size, mass, velocity, momentum, energy etc.
For a photon of momentum (p), an associated wavelength is given by
λ=hp
10. Answer the following questions:
(a) Long distance radio broadcasts use short-wave bands. Why?
(b) It is necessary to use satellites for long distance TV transmission. Why?
(c) Optical and radio telescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?
(d) The small ozone layer on top of the stratosphere is crucial for human survival. Why?
(e) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?
(f) Some scientists have predicted that a global nuclear war on the earth would be followed by a severe ‘nuclear winter’ with a devastating effect on life on earth. What might be the basis of this prediction?
Answer:
(a) Long distance radio broadcasts use shortwave bands because only these bands can be refracted by the ionosphere.
(b) It is necessary to use satellites for long distance TV transmissions because television signals are of high frequencies and high energies. Thus, these signals are not reflected by the ionosphere. Hence, satellites are helpful in reflecting TV signals. Also, they help in long distance TV transmissions.
(c) With reference to X-ray astronomy, X-rays are absorbed by the atmosphere. However, visible and radio waves can penetrate it. Hence, optical and radio telescopes are built on the ground, while X-ray astronomy is possible only with the help of satellites orbiting the Earth.
(d) The small ozone layer on the top of the atmosphere is crucial for human survival because it absorbs harmful ultraviolet radiations present in sunlight and prevents it from reaching the Earth’s surface.
(e) In the absence of an atmosphere, there would be no greenhouse effect on the surface of the Earth. As a result, the temperature of the Earth would decrease rapidly, making it chilly and difficult for human survival.
(f) A global nuclear war on the surface of the Earth would have disastrous consequences. Post-nuclear war, the Earth will experience severe winter as the war will produce clouds of smoke that would cover maximum parts of the sky, thereby preventing solar light form reaching the atmosphere. Also, it will lead to the depletion of the ozone layer.
11. (a) Explain surface wave and sky wave propagation of radio waves. Why is short wave communication over long range not possible by surface wave propagation?
(b) Why is short wave communication over long distances not possible by surface wave propagation?
Answer:
(a) Surface wave or ground wave propagation. Amplitude-modulated radio waves traveling directly from one place to another over the surface of the earth are called surface or ground waves.
Sky wave propagation. The amplitude-modulated radio waves which are received back after being reflected from the ionosphere are called sky waves.
(b) For radio communication of short wavelength (λ less than 200 m or frequency greater than 1500 kHz), sky waves are only used. The ground waves on lending round the corner of the earth lose a good amount of energy due to absorption and their intensity falls to a very small value. Hence short waves are undetectable in surface wave propagation.
12. Identify the electromagnetic waves whose wavelengths lie in the range
(a) 10-11 m < λ < 10-8 m
(b) 10-4 m < λ < 10-6 m Write one use of each.
Answer:
(a) Uses of X-Rays and Gamma rays :
X-rays are used as a diagnostic tool in medicine and as a treatment for certain forms of cancer. Gamma rays are used in medicine to destroy cancer cells.
(b) Uses of Infrared, visible and microwaves :
Infrared waves are widely used in remote switches of household electronic systems such as remotes for TVs, video recorders etc.
Visible rays provide us information about the world.
Microwaves are used in the radar systems in aircraft navigation.
13. State the principle of production of e.m, waves. What is the value of velocity of these waves?
Answer: An electric charge at rest has only an electric field around it and no magnetic field. A moving charge (with uniform velocity) has both electric and magnetic fields around it. But as the current is not changing with time, the magnetic field will also be uniform and hence no electromagnetic wave is produced. If the charge is moving with accelerated motion, both electric and magnetic fields will be changed and hence e.m. waves are produced. Thus, the accelerated charge produces electric and magnetic fields which vary both in space and time. These varying electric and magnetic fields give rise to electromagnetic waves.
Velocity of e.m. waves is 3 x 108 ms-1
14. What is the electromagnetic spectrum? To what uses it is put to?
Answer:
Orderly distribution of electromagnetic radiations according to their frequency or wavelength is called electromagnetic spectrum. The e.m. The spectrum has a much wider range of wavelength, variation from 10-14 m to 6 x 106m and the frequency ranging from 1024 Hz. to 50 Hz. Some overlapping is also observed in some parts of the spectrum showing that the corresponding radiation can be produced by two methods.
Gamma rays have minimum wavelength and are ranging from 10-14 m to 10-20 m. X-rays have wavelengths greater than gamma rays followed by ultraviolet rays and visible light. In visible, violet color has a wavelength of about 3500 Å and red has wavelength of about 7000 Å. Infrared rays come next followed by microwaves, ultra high frequencies, very high frequencies, radio frequencies and power frequencies.
Human eye is sensitive to only a very small part of electromagnetic spectra and is maximum sensitive at 5500 Å.
Uses of e.m. spectra
Study of gamma rays provides useful information about the structure of atomic nuclei.
Study of atomic and crystal structure can be done by X-rays.
X-rays are used in medical diagnosis and as a medicine to kill certain bacteria which cannot be killed by any medicine.
Infrared, visible and ultraviolet radiation help us to know the structure of the molecules and arrangement of electrons in the external shell.
U.V. Radiations are used in the detection of invisible writing, forged documents and in preservation of fingerprints.
Radio and microwaves are used in radio and T.V. communication system.
Infrared rays are used in warfare in looking through fog, mist, clouds etc. It is also used in revealing secret writing on ancient walls and in greenhouses.
15. State and explain electromagnetic waves.
Answer:
Electromagnetic waves: Electromagnetic waves are those waves in which there are sinusoidal variations of electric and magnetic fields at right angle to each other as well as at right angle to the direction of propagation of the waves.
e.g. visible light, U.V. rays, I.R. rays, X-rays, γ-rays microwaves, radio waves etc.
Production of electromagnetic waves
We know that a charge at rest can have only an electric field around it. If the velocity of the charge is uniform, the magnetic field produced by it will also be uniform and hence no electromagnetic wave is produced. But if the charge is moving with accelerated motion both electric and magnetic fields will be changed and hence changing magnetic fields will be produced, hence electromagnetic waves are produced. It means that only an accelerated charge emits electromagnetic waves, e.g. In an L-C circuit, the growth and decay of charge in the capacitor and growth and decay of magnetic field in the inductor L produce electromagnetic waves. In an atom, when an electron jumps from higher energy level to lower energy level, electromagnetic waves are produced.
Characteristics of e.m. waves
When electric and magnetic fields vary along the direction perpendicular to each other, the energy propagates normally to their plane called electromagnetic waves.
No material medium is required for the propagation of electromagnetic waves i.e. they can travel in vacuum.
In free space (vacuum), velocity of electromagnetic waves is 3 x 108 ms-1.
Energy of electromagnetic waves is divided equally between electric and magnetic vectors.
The velocity of electromagnetic waves is independent of amplitude of electric and magnetic vectors and depends only on the electric and magnetic properties of the medium in which e.m. waves travel.
Velocity of electromagnetic waves decreases in a denser medium.
The electric vector is responsible for the optical effect of the wave and hence the electric vector is called the light vector.
Electric and magnetic fields are in the same phase, their values become zero and maximum at the same instant as shown in this figure.
16. Why are infrared radiations referred to as heat waves also? Name the radiations which are next these radiations in electromagnetic spectrum having (a) shorter wavelength, and (b) longer wavelength
Answer: Infrared radiations incident on different materials are readily absorbed by water molecules present in these materials and consequently, their thermal motion increases, ie, water molecules heat up and heat their surroundings too. Due to this reason, infrared radiations are also referred to as heat waves.
(a) Electromagnetic radiation next to infrared radiation but having shorter wavelengths are visible light.
(b) Electromagnetic radiation next to infrared radiation on a longer wavelength are microwaves.
17. What are the uses of electromagnetic waves?
Answer:
Uses of electromagnetic waves:
Following are some important uses of electromagnetic waves:
Radio waves: Radio waves are electromagnetic waves and are used in radio and television communication systems.
Microwaves: Microwaves are used in radar and other communication systems.
Infrared radiations: Infrared rays are used in:
solar water heater and solar cooker.
weather forecasting
taking photographs during fog, smoke etc.
dehydrating fruits.
the treatment of muscular strain.
greenhouse to keep the plants warm.
Ultraviolet rays: Ultraviolet rays are used:
for checking mineral samples by making use of its property of causig fluorescence and also used for the study of molecular structure.
for sterilizing the surgical instruments because U.V.-rays destroy bacteria.
in food preservatives.
in detection of invisible writing.
X- rays: X-rays are used:
in surgery
in radiotherapy.
in medical diagnosis to detect the fracture in bones etc.
in detective departments to detect gold, diamond etc concealed in bags etc without opening them.
in scientific research to study the crystal structure etc.
γ-rays: Gamma rays are used to get information about the structure of atomic nucleus.
18. Identify the following electromagnetic radiations as per the wavelengths given below. Write one application of each.
(a) 10-3 nm
(b) 10-3 m
(d) 1 nm
Answer:
(a) Radiation of wavelength 10-3 nm (or 10-12 m) are γ-rays which are used in cancer therapy and to cause nuclear reactions.
(b) Radiations of wavelength 10-3 m are microwaves which are used in radar system for aircraft navigation and in microwave ovens.
(c) Electromagnetic radiations of wavelength 1 nm (or 10-9 m) are X-rays which are used as a diagnostic tool in medicine and for studying crystal structure of solids.
19. Name the constituent radiation of electromagnetic spectrum which:
(a) is used in satellite communication,
(b) is used for studying crystal structure,
(c) is similar to the radiations emitted during decay of radioactive nuclei
(d) has its wavelength range between 390 nm and 700 nm,
(e)is absorbed from sunlight by ozone layer, and
(f) produces an intense heating effect.
Ans:-
(a) Microwaves are used in satellite communication.
(b) X-rays are used for studying crystal structure.
(c) Gamma rays are similar to the radiations emitted during decay of radioactive nuclei.
(d) Visible light radiation has its wavelength range between 390 nm and 700 nm.
(e) Ultraviolet radiations are mainly absorbed from sunlight by the ozone layer present in the upper part of the atmosphere.
(f) Infrared rays produce intense heating effects when falling on a material.
20. Write the differences between sound waves and electromagnetic waves.
Answer: Differences between sound waves and electromagnetic waves :
Sound waves:
They are mechanical waves.
They are longitudinal waves.
The sound waves do not show the phenomenon of polarization.
For the propagation of sound waves, medium is required.
The velocity of sound waves is near about 332 m/s at 0°C in air.
Electromagnetic waves:
They are not mechanical waves.
They are transverse in nature.
Electromagnetic waves show the phenomenon of polarization.
Electromagnetic waves can travel in vacuum. For propagation of electromagnetic waves, medium is not required.
Its velocity is very high; about 3 x 108 m/s.
21. Describe different components of the electromagnetic spectrum.
Answer: Electromagnetic waves have a large range of wavelength and frequency. When Maxwell predicted about the existence of electromagnetic waves at that time he was aware of visible light only. Gamma rays, X – rays, UV – rays and infrared rays were discovered at the end of the 19th century. Later on radio waves and microwaves came into existence. So, electromagnetic waves have a large frequency or wavelength range.
All these electromagnetic waves have general properties. But they do have some individual features. So, the classification on the basis of frequency or wavelength is called “Electromagnetic spectrum”. “An orderly arrangement of radiations according to wavelength (or frequency) is called the electromagnetic spectrum”. Names of rays in decreasing frequency and increasing wavelength are as follows :
Gamma rays (y rays)
X – rays (X – rays)
UV rays
Visible rays
IR rays
Microwaves
Radio waves
22. Describe Gamma rays.
Answer: Gamma rays:
These rays were discovered by Henery Becqurel in 1896. These rays have frequency from 1018 Hz to 1022 Hz (i.e., from wavelength 10-14 m to 10-10 m).
Production / generation:
The rays are produced due to nuclear reactions, radioactive elements like uranium and radium emit these rays by themselves.
Properties:
These rays affect photographic plates.
These rays ionize gasses.
These rays produce luminescence.
These rays have high penetration power.
Uses:
Making polythene from Enelene.
To detect the nuclear structure.
In destroying unwanted celis.
In treatment of cancer.
23. Describe X – rays.
Answer: X – rays:
These rays were discovered by Roentgen in 1895. These rays have frequency from 1016 Hz to 1020Hz (or wavelength from 10-13 m to 102 m).
Production:
These rays are produced by bombarding electrons on heavy metals.
Properties:
These rays affect photographic plates.
These rays ionize gasses.
These rays produce luminescence.
These rays have less penetration power as compared to Gamma rays.
X – rays can penetrate the human body, but can be obstructed by bones and metallic materials.
Uses:
In medical investigation, like in the detection of fractured bone, the presence of metallic things in the body such as coins, bullets etc.
In the study of crystalline structure.
In the treatment of cancer and tumor.
In the engineering field, in detecting cracks and defects etc.
They are used in metal detectors in restricted areas to find the prohibited things.
24. Answer the following questions:
(a) Long-distance radio broadcasts use short-wave bands. Why?
(b) It is necessary to use satellites for long-distance TV transmission. Why?
(c) Optical and radio telescopes are built on the ground, but X-ray astronomy is possible only from satellites orbiting the earth. Why?
(d) The small ozone layer on top of the stratosphere is crucial for human survival. Why?
(e) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?
(f) Some scientists have predicted that a global nuclear war on the earth would be followed by a severe ‘nuclear winter’ with a devastating effect on life on earth. What might be the basis of this prediction?
Answer:
(a) The shortwaves (wavelength less than 200m or frequencies greater than 1,500 kHz) are absorbed by the earth due to their high frequency but are effectively reflected by Flayer in the ionosphere. After reflection from the ionosphere, the short waves reach the surface of the earth back only at a large distance from the transmitter. For this reason, short waves are used in long-distance transmission.
(b) Television signals are not reflected by the ionosphere. The TV signals from an earth station are reflected back to the earth by making use of artificial satellites.
(c) The earth’s atmosphere is transparent to visible light and radio waves but absorbs X-rays. Therefore, X-ray astronomy is possible only from the satellites orbiting the earth.
(d) The ultraviolet radiation from the sun is harmful to live cells and plants. The ozone layer absorbs ultraviolet radiation and prevents it from reaching the earth. It also keeps the earth warm by trapping infrared radiation.
(e) The infra-red radiation emitted by the earth is retained by the earth’s atmosphere due to the greenhouse effect and this keeps the earth warm. If the earth didn’t have an atmosphere, its average temperature would have been low.
(f) Scientists estimate that in case of a global nuclear war, the clouds produced will cover probably the whole of the sky. In that case, solar radiation would be prevented from reaching the earth and it will result in, what they call nuclear winter on the earth.
25. Give uses each of (i) IR radiation, (ii) Microwaves and (iii) UV radiation.
Answer: i) Uses of IR radiation:
It provides electrical energy to satellites by means of solar cells.
It is used to produce dehydrated fruits.
Heat therapy for muscular pain or sprain.
In TV remote as a signal carrier
To look through haze, fog or mist.
In night vision or infrared photography
ii) Uses of Microwaves:
In the RADAR system for aircraft navigation.
To find the speed of the vehicle.
In a microwave oven for cooking.
In very long distance wireless communication through satellites.
iii) Uses of UV radiation:
To destroy bacteria by sterilizing the surgical instruments.
In burglar alarm
To detect the invisible writing, finger prints
In the study of atomic structure.
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