# Class 12th Physics – 8 Electromagnetic Waves MCQs

#### The displacement current between the plates of the capacitor is due to ____

Correct! Wrong!

stationary charge and constant electric field cannot produce magnetic field, which cannot induce current between the plates of the capacitor. The change in electric field produces a varying magnetic field, which can induce the current between the plates of the capacitor, called displacement current.

#### Which of the following electromagnetic waves are used in night vision cameras?

Correct! Wrong!

Infrared Waves are used in night vision cameras, remote switches and earth satellites.

#### Infrared waves are detected by ____.

Correct! Wrong!

Infrared waves can be detected by Bolometer, Thermopile and photographic film.

#### Which of the following electromagnetic wave has high energy?

Correct! Wrong!

The frequency of Gamma Rays is high compared with all the electromagnetic waves.
We know that the energy of electromagnetic waves can be represented by, E=hν
Where h is the Planck's constant and ν is the frequency.
As h is constant, E∝ν.

#### Which of the following Maxwell's equation represents Gauss' law of magnetism?

Correct! Wrong!

According to Gauss's law of magnetism, the magnetic flux through a closed surface is zero. That is, ∮B. dA = 0

#### The magnetic field of an electromagnetic wave is given by, By=8×10−6sin (1.5×103x+2×1011t)T. then the wavelength of the wave is ____.

Correct! Wrong!

Given,
The magnetic field of electromagnetic wave is, By=8×10−6 sin (1.5×103x+2×1011t)T
We know that the general form of the magnetic field of electromagnetic wave is,
By=B0sin(kx−ωt)
On comparing the two equations, we get k=1.5×103
But, we know that k=2π/λ
Then the wavelength, λ=2π/k
=2×3.14/1.5×103
=4.187×10−3m
=0.4187cm

#### Which of the following electromagnetic wave has highest wavelength?

Correct! Wrong!

Radio waves have highest wavelength, out of all the electromagnetic waves; its value is greater than the, out of all the electromagnetic waves, its value is greater than 0.1m. Whereas Gamma rays has the lowest wavelength of all the electromagnetic waves, its value is in the range of 10−14m to 10−10m

#### The depletion of Ozone layer in the atmosphere is due to ____.

Correct! Wrong!

The ozone layer in the atmosphere absorbs the harmful UV rays emitted by the Sun. But the Ozone layer gets depleted due to chlorofluorocarbon gas such as Freon.

#### The electromagnetic waves are produced by ____.

Correct! Wrong!

An accelerating charge produces a variable electric field and the variable electric field produces a magnetic field changing with time. These electric field and magnetic field oscillate perpendicular to each other and are perpendicular to the direction of propagation of the wave.

#### An oscillating charge about a mean position produces electric oscillating electric field which produces an oscillating magnetic field, called electromagnetic wave. The electric field and magnetic field oscillates perpendicular to each other and perpendicular to the direction of propagation of the wave. If the oscillating frequency of the charge is 108Hz, the frequency of the electromagnetic wave is ____

Correct! Wrong!

We know that accelerated charges can produce electromagnetic wave. This can be observed by considering a charge oscillating about a mean position. The oscillating charge produces an oscillating electric field, which produces an oscillating magnetic field. This is combined known as electromagnetic wave. The frequency of the electric wave, magnetic wave and the electromagnetic wave is equal to the frequency of the oscillating charge.

#### Which of the following property is same for all types of electromagnetic waves?

Correct! Wrong!

The different types of electromagnetic waves have different values of frequency, wavelength and energy. But all the electromagnetic waves travel with the same velocity in air or vacuum. That is, 3×108ms−1

#### In an electric circuit, a capacitor is connected to a battery through a resistance. If a compass needle is placed in the gap of the parallel plate capacitor, then the compass needle ____.

Correct! Wrong!

When the battery is connected in the circuit, the plates of the capacitor are charging. As the charge is increasing on the plates of the capacitor, it produces a change in electric field between the plates of the capacitor which produces a magnetic field. Then the compass deflects. After the time constant of the RC-circuit, the plates of the capacitor charged to its maximum value. Then the electric field become constant, which cannot produce magnetic field. Then the compass needle comes to its original value.

#### Statement P: Gamma rays are produced by the radioactive decay of nuclei Statement Q: Micro Waves are used in spectroscopy

Correct! Wrong!

Gamma rays are produced in nuclear reaction and also emitted by the radioactive decay of nuclei. Hence the statement P is true.

#### Which of the following equation is not a Maxwell's equation?

Correct! Wrong!

In the given equations, the option C is not a Maxwell's equation.
The correct equation is,
∮B.dl = μ0ic0ϵ0 (dΦE/dt)

#### Which of the following experiment confirms that the ether medium does not exist?

Correct! Wrong!

For many years, scientists believed that the entire space is filled with an hypothetical matter called, ether. In 1887, Michelson - Morley experiment confirms that the ether medium does not exist.

#### Which of the following electromagnetic wave has high frequency?

Correct! Wrong!

Gamma Rays has the highest frequency out of all the electromagnetic waves, which is in the range of 1023Hz to 1018Hz
Whereas Radio waves has the lowest frequency out of all the electromagnetic waves, it is in the range of 1012Hz to 108Hz

#### The sunlight reaching the earth has a maximum electric field of 800Vm−1.The maximum magnetic field of that sunlight is ____.

Correct! Wrong!

Given,
Maximum value of electric field, E0=800Vm−1
The velocity of sunlight is equal to the velocity of electromagnetic waves =3×108ms−1
We know that, E0/B0=c
B0=E0c
=800/3×108
=2.67×10−6T
=2.67μT

#### Electromagnetic waves are first produced in the laboratory by ____.

Correct! Wrong!

The existence of electromagnetic waves was first explained by Maxwell. But few years later, Hertz produced the waves in the laboratory, broadcasted the radio waves and detected them.

#### If the charge on the capacitor plates remains constant, then the displacement current between the plates of the capacitor____.

Correct! Wrong!

When the charge on the capacitor plate is maximum, the electric field between the plates of the capacitor is steady. But the steady electric field cannot produce magnetic field. Since the magnetic flux is zero, the displacement current is also zero. Generally, the change in the electric field between the plates will produce magnetic field. Then the change in the magnetic flux produces displacement current.

#### Ultraviolet rays are used in ____.

Correct! Wrong!

Ultraviolet radiations can be focused into a very narrow beams for high precision applications such as LASIK eye surgery.

#### Which of the following property established the wave nature of radiation?

Correct! Wrong!

Hertz showed the existence of electromagnetic waves. He also demonstrated the refraction, diffraction and polarization of the electromagnetic waves.

#### In the 19th century, scientists believed that, there must be some medium filled in the space for the propagation of electric and magnetic fields. The medium is called ____.

Correct! Wrong!

Earlier, scientists believed that there must be some material medium in the space, which is responsible for the propagation of electric field and magnetic fields of electromagnetic wave. That material medium is known as Ether medium.

#### When an electromagnetic wave incidents on a surface, the total energy transferred to the surface is 3×10−4J. Then the total momentum given to the surface is ____.

Correct! Wrong!

Given,
The energy transferred to the surface by the electromagnetic wave is, U=3×10−4J
Then the momentum of the surface is, p=U/c
Where c is the velocity of light.
=3×10−4/3×108
=10−12kgms−1

#### Regarding the electromagnetic waves Assertion (A): A charged particle placed in a plane perpendicular to the propagation of an electromagnetic wave is set in motion by the electric field and magnetic field of the wave. Reason (R): Electromagnetic waves carry energy and momentum

Correct! Wrong!

A charged particle placed in a plane perpendicular to the propagation of an electromagnetic wave is set in motion by the electric field and magnetic field of the wave. Thus, Assertion is true. As the electromagnetic waves carry energy and momentum, they exert pressure on the charged particles placed in its path. Hence the charged particle is set motion by the electric field and magnetic fields of the wave. Hence, Reason is true and it is the correct explanation for the Assertion.

#### A magnetic field can be produced by ____.

Correct! Wrong!

A stationary charge can produce only stationary electric field and constant electric field cannot produce magnetic field. Whereas changing electric field can produce magnetic field.

#### Which if the following statement is false about an electromagnetic wave?

Correct! Wrong!

In an electromagnetic wave, electric field and magnetic field oscillate perpendicular to each other and these oscillations are perpendicular to the direction of propagation. These waves travel with a velocity of 3×108ms−1in vacuum or free space. The electromagnetic waves can travel in vacuum also. Material medium is not necessary for the propagation of electromagnetic waves. When an electromagnetic wave incidents on a surface, it exerts pressure on that surface due to its energy and momentum. The pressure is called radiation pressure.

#### Which of the following electromagnetic waves are also known as heat waves?

Correct! Wrong!

Infrared waves are also known as heat waves. Because, when the infrared waves incident on a material, in most of the cases, the water molecule in the material absorbs the infrared waves and their thermal motion increases. As a result the material and its surroundings get heated. Thus the infrared waves are also known as heat waves.

#### The electric field of an electromagnetic wave propagating in air is 250Vm−1. If the intensity of the electromagnetic wave is amplified and its electric field becomes 400Vm−1. Then the velocity of the electromagnetic wave ___.

Correct! Wrong!

The velocity of the electromagnetic wave remains same. There is no change in its velocity even its intensity is increased. It is a fundamental constant.

Class 12th Physics - 8 Electromagnetic Waves MCQs
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