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Physics M.C.Q (1 Marks)

Electromagnetic Waves · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 179 questions.

Questions · Page 3 of 4

M.C.Q (1 Marks)

Question 1011 Mark
When an electromagnetic wave enters an ionised layer of earth’s atmosphere present in ionosphere:
Answer
  1. The electron cloud will oscillate in the electric field of wave with a phase retardation of 90° for a sinusoidal electromagnetic wave.
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Question 1041 Mark
If the wavelength of red light in air is $7500 A,$ then the frequency of light in air is.
Answer
Velocity of wave $=$ wavelength $\times$ frequency
$\text{v}=3\times10^8\frac{\text{m}}{\text{s}}$
$\lambda=7500\text{A}$
$=7500\times10^{-10}\text{m}$
$\text{f}=\frac{3\times10^8}{7500\times10^{-10}}$
$=4\times10^{14}\text{Hz}$
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Question 1051 Mark
An electromagnetic wave going through vacuum is described by $\text{E}=\text{E}_0\sin(\text{kx}-\omega\text{t});\text{B}=\text{B}_0\sin(\text{kx}-\omega\text{t}).$ Which of the following equation is true?
Answer
  1. $\text{E}_0\text{k}=\text{B}_0\omega$
Explanation:
$\frac{\text{E}_0}{\text{B}_0}=\text{c},$ also $\frac{2\pi}{\lambda}$ and $\omega=2\pi\text{v}.$
$\Rightarrow\frac{\text{E}_0}{\text{B}_0}=\frac{\omega}{\text{k}}$
$\text{E}_0\text{k}=\text{B}_0\omega$
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Question 1061 Mark
X−ray falling on a material.
Answer
  1. Transfer energy to it
Explanation:
The emitted X-rays transfer energy to the material on which it is falling.
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Question 1071 Mark
If $\overrightarrow{\text{E}}$ and $\overrightarrow{\text{B}}$ are the electric and magnetic field vectors of electromagnetic waves then the direction of propagation of electromagnetic wave is along the direction of:
Answer
  1. $\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$
Explanation:
The direction of propagation of electromagnetic wave is perpendicular to the variation of electric field $\overrightarrow{\text{E}}$ as well as to the magnetic field $\overrightarrow{\text{B}}$
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Question 1081 Mark
Which of the following conclusion can be drawn from the result $\oint\overline{\text{B}}\cdot\text{d}\overline{\text{A}}=0$
Answer
  1. Magnetic monopole cannot exist
Explanation:
Flux of certain closed surface is zero and so it tells that net magnetic charge is equal to zero. This is possible when there are two equal and opposite poles.
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Question 1091 Mark
Beyond which frequency, the ionosphere bands any incident electromagnetic radiation but do not reflect it back towards the earth?
Answer
  1. 40MHz
Explanation:
The ionosphere can reflect electromagnetic waves of frequency less than 40MHz but not of frequency more than 40MHz.
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Question 1101 Mark
The ratio of amplitude of a magnetic field to the amplitude of electric field for an electromagnetic wave propagating in vacuum is equal to:
Answer
  1. Reciprocal of speed of light in vacuum
Explanation:
Both electric field and magnetic field are vector quantities.
They both are components of electromagnetic waves.
We know that,
$\text{c}=\frac{\text{E}_0}{\text{B}_0}$
$\frac{\text{B}_0}{\text{E}_0}=\frac{1}{\text{c}}$
 
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Question 1111 Mark
Waves in decreasing order of their wavelength are:
Answer
  1. radio waves, infrared rays, visible rays, X-rays.
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Question 1121 Mark
The displacement current flows in the dielectric of a capacitor when the potential difference across its plates.
Answer
  1. is increasing with time
Explanation:
According to Maxwell's hypothesis, a displacement current will flow through a capacitor when the potential difference across its plates is varying. Thus a varying electric field will exist between the plates and this displacement current is same in magnitude to the current flowing in outer circuit. When a D.C voltage applied across its plates, constant voltage appears across its plates and so there will be no displacement current flowing through the capacitor. Thus the displacement current will flow when the potential is increasing with time.
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Question 1131 Mark
Which is the frequency range of gamma rays from the following?
Answer
The frequency range of Gamma rays is $3 \times 10^{18}$ to $5 \times 10^{22 }Hz.$ These rays have a wavelength of $6 \times 10^{-13}$ to $10^{-10}m.$ Gamma rays are produced in nuclear reactions and are also emitted by radioactive nuclei.
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Question 1141 Mark
When light propagates in vacuum there is an electric field and a magnetic field. These fields.
Answer
  1. have zero average value
  2. are perpendicular to the direction of propagation of light.
  3. are mutually perpendicular
Explanation:
When light propagates in vacuum there is an electric field and a magnetic field which has zero average value. They are perpendicular to each other and also perpendicular to the direction of the propagation of light.
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Question 1151 Mark
On what basis is the classification of electromagnetic waves done?
Answer
  1. Electromagnetic spectrum
Explanation:
The classification of electromagnetic waves done according to the frequency called the electromagnetic spectrum. The basic difference various type of electromagnetic waves lies in their wavelength or frequency since all of them travel through vacuum at the same speed and also, the waves differ in their mode of interaction with matter.
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Question 1161 Mark
Wavelength of, monochromatic light is $5000A^\circ.$ It's wave number is$:$
Answer
Wave number $=\frac{1}{\text{wavelength}}$
$=\frac{1}{5000\times10^{-10}}$
$=2\times10^6\text{m}^{-1}$
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Question 1181 Mark
The amplitude of the sinusoidally oscillating electric field of a plane wave is $60\ v/m.$ Then the amplitude of the magnetic field is$:$
Answer
Given: Electric field $= 60\ v/m$
$C = 3\times 10^8\ m/s^2$
To find: Magnetic field
Solution$:$
We know, $\text{C}=\frac{\text{E}_0}{\text{B}_0}$
Therefore, $\text{B}_0=\frac{\text{E}_0}{\text{C}}$
$\frac{60}{3\times10^8}=2\times10^7\text{T}$
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Question 1201 Mark
Which one of the following is the primary effect of UV radiation caused due to depletion of ozone layer?
Answer
  1. Skin cancer
Explanation:
The ozone layer prevents most harmful UV wavelengths of ultraviolet light (UV light) from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn and cataracts, which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals.
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Question 1211 Mark
Displacement current is continuous:
Answer
  1. when electric field is changing in the circuit
Explanation:
Displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field.
When electric field is changing with time continuously, the displacement current is constant.
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Question 1221 Mark
 If a variable frequency ac source is connected to a capacitor then with decrease in frequency the displacement current will:
Answer
  1. Decrease
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Question 1241 Mark
In Maxwell's velocity distribution curve area under the graph:
Answer
  1. Remains same at all temperature.
Explanation:
Area under the Maxwell's velocity distribution curve gives the number of particles. Since number of particles remains the same at all the temperatures, so the area under the curve also remains the same at all temperature.
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Question 1251 Mark
In an electromagnetic wave, the electric magnetising fields are $\frac{100\text{V}}{\text{m}}$ and $\frac{0.265\text{A}}{\text{m}}$ The maximum energy flow is?
Answer
Maximum rate of energy flow, $S = E_0 ​\times H_0$
Given, $\text{E}_0=\frac{100\text{V}}{\text{m}},\text{H}_0=\frac{0.265\text{A}}{\text{m}}$
$\therefore\text{S}=100\times0.265=\frac{26.5\text{W}}{\text{m}^2}$​
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Question 1261 Mark
The figure here gives the electric field of an EM wave at a certain poiunt and a certain instrant. the wave is transporting energy in the negative z direction. what is tha direction of the magnetic field of the wave at that point and instant.
Answer
  1. Towards +X direction
Explanation:
The direction of EM wave is given by the direction of $\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$
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Question 1271 Mark
The speeds of microwaves, infrared waves, and ultraviolet waves are $V_m, V_i,$ and $V_u$ respectively. Identify the correct combination showing the different waves in vacuum.
Answer
The correct combination is $➔ Vm = Vi = Vu.$ This is because, in vacuum, all the electromagnetic waves in question will travel at the same speed. The speed with which they travel in vacuum is the speed of light. $(c = 3 \times 108\ m/s).$
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Question 1281 Mark
Which of the following electromagnetic waves is used in medicine to destroy cancer cells?
Answer
  1. Gamma rays
Explanation:
Gamma rays has property to kill cancer cell because the energy released by gamma ray is perfect to kill and leave out the healthy ones.
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Question 1291 Mark
Choose the correct answer from alternatives given.
If $\overrightarrow{\text{E}}$ and $\overrightarrow{\text{B}}$ represent electric and magnetic field vectors of an electromagnetic wave, the direction of propagation of the wave is along.
Answer
  1. $\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$
Explanation:
Electromagnet waves have electric field as well as magnetic field which are perpendicular to each other and the electromagnetic waves propagate in a direction
which is perpendicular to both the fields.
Thus the propagation vector of EM waves $\overrightarrow{\text{k}}=\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$
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Question 1301 Mark
Assume you are sitting in sun for $2.5$ hours. The area of your body exposed normally to sun rays $1.3m^2.$ The intensity of sun rays is $\frac{1.1 \ \text{Kilowatt}}{\text{m}^2}$ If your body completely absorbs the sun rays then the momentum transferred to your body will be $($in $\ Kg - \frac{\text{m}}{\text{s}}):$
Answer
Power $=$ intensity $\times$ area
Intensity and area are given,
Power $= 1.3 \times 1.1 \ KW$
Energy $=$ power $\times$ time
Time is $2.5\ hr$
Energy $= 1300 \times 1.1 \times 2.5 \times 3600J$
$= 12870000J$
$\text{Momentum}=\frac{\text{energy}}{\text{c}}$
$=\frac{12870000}{3\times10^8}$
$=0.043$
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Question 1311 Mark
In the propagation of electromagnetic waves the angle between the direction of propagation and the plane of vibration is ________
Answer
  1. $\frac{\pi}{2}$
Explanation:
Axis of propagation always lie perpendicular to the plane of vibrations, therefor angle between them is $\frac{\pi}{2}$
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Question 1321 Mark
 Which of the following statement is false for the properties of em waves?
Answer
  1. Both electric and magnetic field vectors are parallel to each and perpendicular to the direction of propagation of wave.
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Question 1331 Mark
Which of the following statement is false for the properties of electromagnetic waves?
Answer
  1. Both electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of wave.
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Question 1351 Mark
A compass needle is placed in the gap of a parallel plate capacitor. The capacitor is connected to a battery through a resistance. The compass needle:
Answer
  1. Deflects and gradually comes to the original position in a time that is large compared to the time constant.
Explanation:
The compass needle deflects due to the presence of the magnetic field. Inside the capacitor, a magnetic field is produced when there is a changing electric field inside it. As the capacitor is connected across the battery, the charge on its plates at a certain time tis given by,
$\text{Q}=\text{CV}\Big(1-\text{e}^{-\tau/\text{RC}}\Big),$
Q = Charge developed on the plates of the capacitor.
R = Resistance of the resistor connected in series with the capacitor.
C = Capacitance of the capacitor.
V = Potential difference of the battery.
The time constant of the capacitor is given, $\tau=\text{RC}$
The capacitor keeps on charging up to the time $\tau$. The development of charge on the plates will be gradual after $​​\tau=\text{RC}$ The change in electric field will be up to the time the charge is developing on the plates of the capacitor. Thus, the compass needle deflects and gradually comes to the original position in a time that is large compared to the time constant.
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Question 1361 Mark
If E and B represent electric and magnetic field vectors of the electromagnetic wave, the direction of propagation of electromagnetic wave is along,
Answer
  1. E × B.
Solution:
Key concept: A changing electric field produces a changing magnetic field and vice versa which gives rise to a transverse wave known as electromagnetic wave. The time varying electric and magnetic field are mutually perpendicular to each other and also perpendicular to the direction of propagation of this wave. The electric vector is responsible for the optical effects of an EM wave and is called the light vector.

The direction of propagation of electromagnetic wave is perpendicular to both electric field vector $(\vec{\text{E}})$ and $\vec{\text{B}}$ magnetic field vector B, i.e., in the direction of $\vec{\text{E}}\times\vec{\text{B}}$.
Here, elecromagnetic wave is along the z-direction which is given by the cross product of E and B.
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Question 1371 Mark
The period of the wave will be.
Answer
  1. $0.025\mu\text{s}$
Explanation:
Time period, $\text{T}=\frac{1}{\text{v}}$
$\text{T}=\frac{1}{40\times10^6}$
$\Rightarrow\text{T}=0.25\mu\text{s}$
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Question 1381 Mark
For which frequency of light, the human eye is most sensitive?
Answer
Human eye is sensitive to light of wavelength $➔ \lambda = 5550$ angstrom.
So its frequency is $\text{v}=\frac{\text{c}}{\lambda}$
$\text{v} =\frac{5550}{3 \times 10^8} \times 10^{-10}$
$v = 5.405 \times 10^{14} Hz$
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Question 1391 Mark
Which of the following cannot be polarized?
Answer
  1. Ultrasonic waves
Explanation:
All the longitudinal waves like sound etc cannot be polarized because the motion of the particles is already in one dimension that is the direction of propagation of wave.
Thus all the transverse waves like electromagnetic waves can be polarized.
Thus, (B) Ultrasonic waves being sound waves having frequency greater than 20kHz but being longitudinal in nature cannot be polarized
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Question 1401 Mark
If the directions of electric and magnetic field vectors of a plane electromagnetic wave are along positive y-direction and positive z-direction respectively, then the direction of propagation of the wave is along:
Answer
  1. positive x-direction
Explanation:
e = E × B, direction of propagation is always perpendicular to plane of E and B. It will be positive in x-direction.
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Question 1411 Mark
A plane electromagnetic wave propagating along $x$ direction can have the following pairs of $E$ and $B.$
Answer
The direction of propagation of electromagnetic wave is perpendicular to both electric field vector $(\vec{\text{E}})$ and $(\vec{\text{B}})$magntic field vector $B,$ i.e., in the direction of $\vec{\text{E}}\times\vec{\text{B}}$.
Here in the question electromagnetic wave is propagating along $x-$direction.
So, electro and magnetic field vectors should have either $y-$direction of $2-$direction.
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Question 1421 Mark
Radiation pressure on any surface:
Answer
  1. Is dependent on nature of surface and intensity of light used
Explanation:
Radiation pressure is given by $\text{P}_\text{R}=\frac{(1+\alpha)\text{I}}{\text{C}}$
where α is the coefficient of reflection of the surface.
For completely reflecting surface $\alpha=1$
For completely absorbing surface $\alpha=0$
So, radiation pressure depends on the nature of surface on which the light is falling but independent of wavelength of light falling.
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Question 1431 Mark
An electromagnetic waves can be produced, when charge is:
Answer
  1. both (b) and (c)
Explanation:
An accelerated charge is the source of electromagnetic waves (EMWs). When the charge is in a circular motion, the direction of its velocity continuously changes and thus it is in accelerated motion and produces EMWs.
A charge falling in an electric field is accelerated by the electric force and thus produces EMWs.
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Question 1441 Mark
The displacement current was first populated by.
Answer
  1. Maxwell
Explanation:
In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field.
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Question 1451 Mark
Speed of electromagnetic waves is the same:
Answer
  1. For all intensities.
Explanation:
For any given medium, the speed (c) of an electromagnetic wave is given by,
$\text{C}=\text{v}\lambda$
Where,
V = Frequency of the electromagnetic wave.
$\lambda=$ wavelength of the electromagnetic wave.
As the frequency and wavelength are changed, the speed of the electromagnetic wave changes. So, the speed of an electromagnetic wave is not same for all wavelengths and all frequencies in any medium. The velocity of an electromagnetic wave changes with change in medium. Also, the speed of an electromagnetic wave is same for all the intensities in any medium.
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Question 1461 Mark
Consider an electromagnetic wave propagating in vacuum. Choose the correct statement:
Answer
  1. For an electromagnetic wave propagating in +x direction the electric field is $\overrightarrow{\text{E}}=\frac{1}{\sqrt{2}}\text{E}_\text{yz}(\text{x,t})(\hat{\text{y}}-\hat{\text{z}})$ and the magnetic field is $\overrightarrow{\text{B}}=\frac{1}{\sqrt{2}}\text{E}_\text{yz}(\text{x,t})(\hat{\text{y}}+\hat{\text{z}})$
Explanation:
Electromagnetic waves travel in the direction perpendicular to electric as well as magnetic field. Cross product of electric and magnetic field should give the direction of electromagnetic wave.
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Question 1481 Mark
An electromagnetic radiation has an energy of 13.2 keV. Then the radiation belongs to the region of.
Answer
  1. X-ray
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Question 1491 Mark
Which of the following have zero average value in a plane electromagnetic wave?
Answer
  1. Electric field.
  2. Magnetic field.
Explanation:
In a plane electromagnetic wave, the electric and the magnetic fields oscillate sinusoidally. For an electromagnetic wave propagating in the z-direction, the electric and magnetic fields are given by,
$\text{E}_\text{x}=\text{E}_0\sin(\text{kz}-\omega\text{t})$
$\text{B}_\text{y}=\text{B}_0\sin(\text{kz}-\omega\text{t})$
These are sinusoidal functions. Therefore, for a fixed value of z. the average value of the electric and magnetic fields are zero.
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Question 1501 Mark
The value of electric field in an electromagnetic wave originating from a point source of light at a distance of  10 meter is $\text{E}=\frac{500\text{Volt}}{\text{m}}$ The electric field at a distance of 5 meter will be.
Answer
  1. $\frac{2000\text{Volt}}{\text{meter}}$
Explanation:
As we know, $\text{E}\propto\text{R}^{-2}$
Therefore, $\text{E}(\text{R}=5)=\frac{500}{10^{-2}}5^{-2}=\frac{2000\text{V}}{\text{m}}$
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M.C.Q (1 Marks) - Page 3 - Physics STD 12 Science Questions - Vidyadip