M.C.Q (1 Marks)

Question 511 Mark

What is the phase difference between electric and magnetic fields in an electromagnetic wave?

Answer

$\pi$

Explanation:
The phase difference between electric and magnetic fields in an electromagnetic wave is$\pi$.

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Question 521 Mark

Which of the following has/have zero average value in a plane electromagnetic wave?

Answer

Both magnetic and electric field

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Question 531 Mark

A plane electromagnetic wave with a single frequency moves in vacuum in the positive x direction. Its amplitude is uniform over the yz plane its wavelength:

Answer

Same

Explanation:
The same amount of energy passes through equal areas parallel to the yz plane as the wave travels in the +x direction, so the amplitude and the intensity, which is proportional to the square of the amplitude, do not change.

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Question 541 Mark

Choose the correct answer from the alternatives given.
Displacement current goes through the gap between the plates of a capacitor when the charge of the capacitor.

Answer

increases or decreases

Explanation:
Displacement current inside a capacitor is given by:-
$\text{i}_\text{d}=\epsilon_0\frac{\phi\text{E}}{\text{dt}}$
where $\phi_\text{E}$ is the electric flux inside the capacitor
The displacement current is developed inside a capacitor when there is a change in the electric flux linked with the capacitor.
The change in electric flux can occur in both the cases either the charge increases or decreases on the capacitor. This will lead to a change in flux linked with the coil.

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Question 551 Mark

In the propagation of light waves, the angle between the plane of vibration & plane of polarisation is:

Answer

90°

Explanation:
The plane of polarisations is that plane in which there is no vibration. While a plane including the direction of light propagation and the direction of electric field is called the plane of vibration. The angle between them is 90°.

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Question 561 Mark

Who first proposed the light as an electromagnetic wave?

Answer

James Clerk Maxwell

Explanation:
In 1864, Maxwell predicted the existence of electromagnetic waves, the existence of which had not been confirmed before that time, and out of his prediction came the concept of light being a wave, or more specifically, a type of electromagnetic wave. It is a wide intuition that Albert Einstein proposed the dual nature theory but is not correct.

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Question 571 Mark

A magnetic field can be produced by:

Answer

Both of them.

Explanation:
According to Ampere-Maxwell's Law, a magnetic field is produced due to the conduction current in a conductor and the displacement current. The conduction current is actually the motion of the charge. The displacement current is due to the changing electric field. The displacement current is given by,
$\text{i}_\text{d}=\epsilon_0\frac{\text{d}\phi_\text{E}}{\text{dt}}$ $\big(\because\phi_\text{E}$ is the electric flux$\big)$
Thus, the magnetic field is produced by the moving charge as well as the electric field.

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Question 581 Mark

The wavelength of the wave is.

Answer

7.5m

Explanation
Using $\text{v}=\text{v}\lambda$ where v is the speed of EM wave.
As $\text{V}=\text{c}=3\times\frac{10\text{m}}{\text{s}}$
$\Rightarrow3\times10^8=40\times10^6\lambda$
$\Rightarrow\lambda=7.5\text{m}$

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Question 591 Mark

A linearly polarized electromagnetic wave given as $\text{E}=\text{E}_0\hat{\text{i}}\cos(\text{kz}-\omega\text{t})$ is incident normally on a perfectly reflecting infinite wall at z = a. Assuming that the material of the wall is optically inactive, the reflected wave will be given as:

Answer

$\text{E}_\text{r}=\text{E}_0\hat{\text{i}}\cos(\text{kz}+\omega\text{t})$

Solution:
Key concept: When a wave is reflected from a denser medium or perfectly reflecting wall made with optically inactive material, then the type of wave doesn't change but only its phase changes by 180º or $\hat{\text{I}}€$ radian.

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Question 601 Mark

In case of the electromagnetic waves the angle between the electric and magnetic field vectors is.

Answer

$\frac{\pi}{2}$

Explanation:
Electromagnetic waves are formed when an electric field couples with a magnetic field. The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave, as shown in figure.

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Question 611 Mark

In an electromagnetic wave, the direction of the magnetic induction B is

Answer

perpendicular to the electric field $\overrightarrow{\text{S}}$

Explanation:
The electric field is always perpendicular to the magnetic field, and both fields are directed at right-angles to the direction of propagation of the wave. In fact, the wave propagates in the direction $\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$ Electromagnetic waves are clearly a type of transverse wave.

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Question 621 Mark

Electromagnetic waves are produced by:

Answer

An accelerating charge.

Explanation:
A static charge produces an electrostatic field. A moving charge produces a magnetic field. Electromagnetic waves are produced by an accelerating charge.

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Question 631 Mark

According to Maxwell’s Hypothesis, a changing electric field gives rise to?

Answer

Magnetic Field

Explanation:
A changing electric field gives rise to a magnetic field.

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Question 641 Mark

Pick out the correct increasing order of energy of electromagnetic waves from the following:

Answer

The energy of electromagnetic waves is directly proportional to the frequency of the electromagnetic waves.
So the order of frequency is given as:
$v_{micro }< v_{infrared} < v_{visible }< v_{ultraviolet} < v_{gamma}$
Since $E = hv$
$➔ \text{E} \propto\text{v}$
The order of energy is as follows:
$E_{micro} < E_{infrared} < E_{visible} < E_{ultraviolet} < E_{gamma}$

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Question 651 Mark

An electromognetic wave travels in vacuum along z direction: $\text{E}=\big(\text{E}_1\hat{\text{i}}+\text{E}_2\hat{\text{j}}\big)\cos(\text{kz}-\omega\text{t})$. Choose the correct options from the following:

Answer

The associated magnetic field is given as $\text{B}=\frac{1}{\text{c}}\big(\text{E}_1\hat{\text{i}}+\text{E}_2\hat{\text{j}}\big)\cos(\text{kz}-\omega\text{t})$.

The given electromagnetic wave is plane polarised.

Solution:
We are given that the electric field vector of an electromagnetic wave travels in a vacuum along z-direction as,
$\vec{\text{E}}=\big(\text{E}_2\hat{\text{i}}+\text{E}_2\hat{\text{j}}\big)\cos(\text{kz}-\omega\text{t})$
The magnitude of the electric and the magnetic fields in an electronagnetic wave are related as
$\text{B}_0=\frac{\text{E}_0}{\text{c}}$
$\vec{\text{B}}=\frac{\vec{\text{E}}}{\text{c}}=\frac{\text{E}_1\text{i}+\text{E}_2\text{i}}{\text{c}}\cos(\text{kz}-\omega\text{t})$
Also, $\vec{\text{E}}$ and $\vec{\text{B}}$ are perpendicular to each other and the propagation of electromagnetic wave is perpendicular to $\vec{\text{E}}$ as well as $\vec{\text{B}}$, so the given electromagnetic wave is plane polarized.

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Question 661 Mark

Unpolarized light falls first on polarizer (P) and then on analyzer (A). If the intensity of the transmitted light from the analyser is $\frac{1}{8}\text{th}$ of the incident unpolarized light. What will be the angle between optic axes of P and A?

Answer

60°

Explanation:
Given,
$\text{I}=\frac{\text{I}_\text{0}}{2}....(\text{i})$
$\text{I}=\text{I}\cos^2\theta$ $\Big(\because\text{I}=\frac{\text{I}_0}{8}\Big)$
$\therefore\frac{\text{I}_0}{8}=\frac{\text{I}_0}{2}\cos^2\theta$
From the equation (i), we have
$\frac{1}{4}=\cos^2\theta$
$\Rightarrow\cos\theta=\frac{1}{2}$
$\Rightarrow\cos\theta=\cos60^\circ$
$\Rightarrow\theta=60^\circ$

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Question 671 Mark

Radiations of intensity $\frac{0.5\text{W}}{\text{m}^2}$ are striking a metal plate. The pressure on the plate is.

Answer

$\frac{0.166\times10^{-8}\text{N}}{\text{m}^2}$

Explanation:
Intensity or power per unit area of the radiations,
P = pv
$\Rightarrow\text{P}=\frac{\text{P}}{\text{v}}=\frac{0.5}{3\times10^8}=\frac{0.166\times10^{-8}\text{N}}{\text{m}^2}$

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Question 681 Mark

A parallel plate capacitor consists of two circular plates each of radius 12cm and separated by 5.0mm. The capacitor is being charged by an external source. The charging current is constant and is equal to 0.15A.The displacement current is:

Answer

0.15A

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.
Here, the current in the outer circuit is 0.15 A. Thus 0.15A will be the displacement current.

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Question 691 Mark

Infrared radiation are detected by:

Answer

Photometre

Explanation:
Infrared radiation is detected by photometer.

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Question 701 Mark

According to the electromagnetic wave theory, light consists of electric and magnetic fields which are_____________.

Answer

perpendicular to each other

Explanation:
Light consists of electric and magnetic field that are perpendicular 90° to each other.
APPOACH by example
Electric field inside plates. The magnetic field this given rise to via the displacement current is along the perimeter of the circle parallel to capauatates plates.
So B and E are perpendicular in this case.

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Question 711 Mark

A charged particle oscillates about its mean equilibrium position with a frequency of $10^9 \ Hz.$ The electromagnetic waves produced$:$

Answer

Here we are given the frequency by which the charged particles oscillates about its mean equilibrium position, it is equal to $10^9 \ Hz.$ The frequency of electromagnetic waves produced by a charged particle is equal to the frequency by which it oscillates about its mean equilibrium position.
So, frequency of electromagnetic waves produced by the charged particle is $v = 10^9 \ Hz.$
Wavelength $\lambda=\frac{\text{c}}{\text{v}}=\frac{3\times10^8}{10^9}=0.3\text{m}$
The frequency of $10^9 \ Hz$ falls in the region of radiowaves.

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Question 721 Mark

An electromagnetic wave is propagating along Y-axis. Then.

Answer

oscillating electric field is along Z-axis and oscillating magnetic field is along X-axis

Explanation:
electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vaccum. The oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. so if propogation is along Y-direction ,Electric field will be along X or Z, if it is along Z -direction than Magnetic field has to be in X -direction.

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Question 731 Mark

The electric field intensity produced by the radiations coming from 100W bulb at a 3 m distance is E. The electric field intensity produced by the radiations coming from 50W bulb at the same distance is:

Answer

$\sqrt{2}\text{E}.$

Solution:
We know the electric field intensity on a surface due to incident rediation is,
$\text{I}_\text{av}\propto\text{E}_0^2$
$\frac{\text{P}_\text{av}}{\text{A}}\propto\text{E}_0^2$
Here $\text{P}_\text{av}\propto\text{E}_0^2$ [$\because$ A is same in both cases]
We know that, $\text{E}_0\propto\sqrt{\text{P}_\text{av}}$
$\therefore\ \frac{(\text{E}_0)_1}{(\text{E}_0)_1}=\sqrt{\frac{(\text{P}_\text{av})_1}{(\text{P}_\text{av})_2}}\ .....(\text{i})$
$\Rightarrow\ \frac{\text{E}}{(\text{E}_0)_2}=\sqrt{\frac{1000}{5}}$
$(\text{E}_0)_2=\frac{\text{E}}{\sqrt{2}}$
Nowa according to question, P' = 50W, P = 100W
$\therefore$ Putting these value in Eq. (i), we get
$\frac{\text{E}'}{\text{E}}=\frac{50}{100}\Rightarrow\ \frac{\text{E}'}{\text{E}}=\frac{1}{2}\Rightarrow\ \text{E}'=\frac{\text{E}}{2}$

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Question 741 Mark

Which among the following is an application of microwaves?

Answer

Ovens

Explanation:
Microwave ovens are an application of microwaves. In microwave ovens, the frequency of microwave produced is matched with the natural frequency of water molecules so that resonance occurs and water molecules in the material vibrate at the higher amplitude and transfer energy to nearby food molecules, results in heating the food.

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Question 751 Mark

Generation, propagation and detection of electromagnetic waves is the basis of:

Answer

Radio and television

Explanation:
The communication and broadcasting following the base on generation, propagation, and detection of electromagnetic waves.
The electromagnetic spectrum describes a different range of electromagnetic waves. These EM waves are a special type of wave that can travel without a medium.
Electromagnetic waves are named like this due to the fact that they have both an electric and a magnetic component. In a vacuum, EM waves always travel at the same speed i.e. the speed of light. So, other EM waves besides light are infrared, ultraviolet, radio waves, and microwaves.
Therefore radio and television both are based on EM wave properties. Other options like lasers, reactors, and computers are not guided by EM waves.

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Question 761 Mark

Which of the following is used to investigate the structure of solids?

Answer

X-Rays

Explanation:
X-Rays are used to investigate the structure of solids.

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Question 771 Mark

If a source is transmitting electro$-$magnetic waves of frequency $8.196\times 10^6 \ Hz,$ then the wavelength of the electro$-$magnetic waves transmitted from the source will be$:$

Answer

Given, frequency of $EM$ waves
$v = 8.196 \times 10^6Hz$
velocity of $EM$ waves $(v) = 3 \times 10^8m/s$
Wavelength of $EM$ waves $\lambda=\frac{\text{v}}{\text{v}}$
$=\frac{3 \times 10^8}{8.196 \times 10^6}$
$= 36.60m$
$= 3660\ cm.$

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Question 781 Mark

When light propagates in vacuum there is an electric field and a magnetic field. These fields:

Answer

Have zero average value.
Are perpendicular to the direction of propagation of light.
Are mutually perpendicular.

Explanation:
Light is an electromagnetic wave that propagates through its electric and magnetic field vectors, which are mutually perpendicular to each other, as well as to the direction of propagation of light. The average value of both the fields is zero.

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Question 791 Mark

An electromagnetic wave propagating along north has its electric field vector upwards. Its magnetic field vector point towards.

Answer

East

Explanation:
Electric field and magnetic field vectors for an electromagnetic wave are cross - field vectors.
So, the direction of an electromagnetic wave is given by the product of electric field vector and magnetic field vector.
According to the question, electric field vector is directed upwards and EM wave is directed towards North. So, according to the right - hand thumb rule, the magnetic field vector points towards the East.

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Question 801 Mark

The part of the spectrum of the electromagnetic radiation used to cook food is then:

Answer

microwaves

Explanation:
Microwaves are used to cook food. Microwave oven is a domestic application of these waves.

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Question 811 Mark

The propagation constant of a photon of wavelength $6284 \ A^\circ .$

Answer

The propagation constant can be written as
$\text{K}=\frac{2pi}{\lambda}$
$=\frac{60284}{6284\times10^{-8}}$
$=10^5\text{cm}^{-1}$

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Question 821 Mark

The oscillating electric and magnetic vectors of an electromagnetic wave are oriented along:

Answer

Mutually perpendicular directions and are in phase

Explanation:
$\overrightarrow{E}$ and $\overrightarrow{B}$ are mutually perpendicular to each other and are in phase i.e., they become zero and minimum at the same place and at the same time.

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Question 831 Mark

When light propagates in vaccum there is an electric field and a magnetic field. Which of the following is not true about these field?

Answer

They are constant in time

Explanation:
They vary with time following a wave function (sinuosoidal) and average value of these function is zero and also we can see in figure they are mutually perpendicular and also perpendicular to direction of propagation.

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Question 841 Mark

In electromagnetic spectrum, the frequencies $\gamma-$rays$, X-$rays and ultraviolet rays are denoted by $n_1, n_2$ and $n_3$ respectively then$:$

Answer

From electromagnetic spectrum, frequencies of $\gamma-$rays is greater than frequency of $X-$rays. Frequency of Xrays is greater than frequency of ultraviolet rays.

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Question 851 Mark

A metal block is exposed to beams of X-ray of different wavelength. X-rays of which wavelength penetrate most?

Answer

$2\mathring{\text{A}}$

Explanation:
Penetrating power is greater for lower wavelength.

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Question 861 Mark

If the wavelength of electromagnetic radiation is doubled, what will happen to the energy of photons?

Answer

Halved

Explanation:
Energy of a photon,
$\text{E} = \text{hv} = \frac {\text{hc}}{\lambda}.$
$\text{E}\propto\frac{1}{\lambda}.$
When the wavelength of electromagnetic radiation is doubled, the energy of the photons is halved.

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Question 871 Mark

A plane electromagnetic wave is incident on a material surface. The wave delivers momentum p and energy E.

Answer

$\text{p}\neq0,\text{ E}\neq0$

Explanation:
When an electromagnetic wave strikes a material surface, it transports the momentum, as well as the energy, to the surface. The striking electromagnetic wave exerts pressure on the surface. The total energy transferred to the surface by the electromagnetic wave is given by $\text{E}=\text{pc}$ Therefore, $\text{p}\neq0,\text{ E}\neq0$

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Question 881 Mark

Displacement current is.

Answer

continuous when electric field is changing in the circuit

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Question 891 Mark

The frequency of electromagnetic wave in free space is 2 MHz. When it passes through a region of relative permittivity $\epsilon_\text{r}=4.0,$ then its wave length __________ & frequency ______________

Answer

Becomes half, remains constant

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Question 901 Mark

In electromagnetic wave, according to Maxwell, changing electric field gives.

Answer

By Maxwell
$\text{I}_\text{d}=\frac{\epsilon\text{dE}}{\text{dt}}$
$dE$ is electric field
$I_d$ is displacement current per unit area.
Hence changing electric field gives displacement current.

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Question 911 Mark

The ratio of contributions made by the magnetic field and electric field components to the intensity of an EM wave is:

Answer

Explanation:
The ratio of contributions made by the magnetic field and electric field components to the intensity of an EM wave is 1:1.

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Question 921 Mark

Which of the following has zero average value in a plane electromagnetic wave?

Answer

Explanation:
The average of $\sin\theta$ and $\cos\theta$ for whole cycle is is zero.
Step 1: Analyzing the average value of Kinetic energy.
Kinetic Energy is always a positive quantity, therefore its average will also be a positive quantity.
Step 2: Finding the average of electric and the magnetic field.
The equations for the electric field and the magnetic field are given as

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Question 931 Mark

An electromagnetic wave travelling along z-axis is given as: $\text{E}=\text{E}_0\cos(\text{kz}-\omega\text{t})$. Choose the correct options from the following;

Answer

The associated magnetic field is given as $\text{B}=\frac{1}{\text{c}}\text{k}\times\text{E}=\frac{1}{\omega}(\hat{\text{k}}\times\text{E})$.
The electromagnetic field can be written in terms of the associated magnetic field as $\text{E}=\text{c}(\text{B}\times\hat{\text{k}})$.
$\hat{\text{k}}.\text{E}=0,\hat{\text{k}}.\text{B}=0.$

Solution:

The direction of propagation of an eletromagnetic wave is always along the direction of vector product $\vec{\text{E}}\times\vec{\text{B}}$. Refer to Figure.

$\vec{\text{B}}=\text{B}\hat{\text{j}}=\text{B}(\hat{\text{k}}\times\hat{\text{i}})=\frac{\text{E}}{\text{C}}(\hat{\text{k}}\times\hat{\text{i}})$
$=\frac{1}{\text{c}}[\text{k}\times\text{E}\hat{\text{i}}]=\frac{1}{\text{c}}[\hat{\text{k}}\times\vec{\text{E}}]\bigg(\text{as}\frac{\text{E}}{\text{B}}=\text{c}\bigg)$

$\vec{\text{E}}=\text{E}\hat{\text{i}}=\text{cB}(\hat{\text{j}}\times\hat{\text{k}})=\text{c}(\text{B}\hat{\text{j}}\times\text{k})=\text{c}(\vec{\text{B}}\times\hat{\text{k}})$
$\hat{\text{k}}.\vec{\text{E}}=\hat{\text{k}}.(\text{E}\hat{\text{i}})=0,\vec{\text{k}}.\vec{\text{B}}=\vec{\text{k}}.(\text{B}\hat{\text{j}})=0$
$\hat{\text{k}}\times\vec{\text{E}}=\hat{\text{k}}\times(\text{E}\hat{\text{i}})=\text{E}(\hat{\text{k}}\times\hat{\text{i}})=\text{E}\hat{\text{j}}$ and $\hat{\text{k}}\times\vec{\text{B}}=\hat{\text{k}}\times(\text{B}\hat{\text{j}})=\text{B}(\hat{\text{k}}\times\hat{\text{j}})=-\text{B}\hat{\text{i}}$.

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Question 941 Mark

An electromagnetic wave going through vacuum is described by $\text{E}=\text{E}_0\sin(\text{kx}-\omega\text{t}),\text{ B}=\text{B}_0\sin(\text{kx}-\omega\text{t})$ Then:

Answer

The relation between $E_0$ and $B_0$ id given by $\frac{\text{E}_0}{\text{B}_0}=\text{c}\ ....(\text{i})$
Here, $c =$ Speed of the electromagnetic wave,
The relation between $\omega ($the angular frequency$)$ and $k($wave number$),$
$\frac{\omega}{\text{k}}=\text{c}\ ...(ii)$
Therefore, from $(i)$ and $(ii),$ we get
$\frac{\text{E}_0}{\text{B}_0}=\frac{\omega}{\text{k}}=\text{c}$
$\text{E}_0\text{k}=\text{B}_0\omega$

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Question 951 Mark

The electric field intensity at a point in vacuum is equal to:

Answer

Force a unit positive charge would experience there.

Explanation:
The electric field intensity at a point in a vacuum is equal to force experienced by a unit positive charge placed at that point.

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Question 961 Mark

An electromagnetic wave radiates outwards from a dipole antenna, with $E_0$ as the amplitude of its electric field vector. The electric field $B_0$ which transports significant energy from the source falls off as$:$

Answer

An antenna that produces the Electromagnetic wave are radiated outwards. The amplitude of electric field vector $(E_0).$ This electric field vector transports the energy from the source through the medium.
The electric field intensity of the wave from the source at a distance is inversely proportional to the distance between the source and the point. $\text{E}_0=\frac{1}{\text{r}}$

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Question 971 Mark

Huygens' principle of secondary wavelets may be used to:

Answer

Find the new position of a wavefront.
Explain Snell's law.

Explanation:
Huygen's wave theory explains the origin of points for the new wavefront proceeding successively. It also explains the variation in speed of light on moving from one medium to another, i.e. it proves Snell's Law.

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Question 981 Mark

A charged particle oscillates about its mean equilibrium position with a frequency of $10^9\ Hz.$ The frequency of electromagnetic waves produced by the oscillator is$:$

Answer

The frequency of the electromagnetic wave is same as that of oscillating charged particle about its equilibrium position, which is $10^9\ Hz.$

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Question 991 Mark

According to Maxwell's hypothesis, changing of electric filed give rise to.

Answer

magnetic field

Explanation:
According to Maxwell's hypothesis, changing of electric field gives rise to Magnetic field.
We know that F = qE,, where F is force and E is electric field.
We can relate magnetic field and force by F = qvB, where v is velocity and B is the magnetic field.
Therefore we can obtain magnetic field by changing electric field.

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Question 1001 Mark

Two waves having same velocity enter electric and magnetic fields respectively. If $\lambda_1$ and $\lambda_2$ are their wavelengths as they move in the fields, then.

Answer

$\lambda_1$ and $\lambda_2$ are variable

Explanation:
Velocity of a wave is given by:
$\text{v}=\frac{\text{E}}{\text{B}}$
Hence wave velocity change in both the cases.
Frequency of the wave remains the same.
Using $\text{v}=\text{f}\lambda,$ it can be concluded that both $\lambda_1$ and $\lambda_2$ are variable.

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Physics M.C.Q (1 Marks)