Test: Electromagnetic Waves - 1 - JEE MCQ

Explanation:

• Maxwell's equations describe how electric and magnetic fields interact with each other.


• One of Maxwell's equations states that a time-varying magnetic field acts as a source of electric field.


• This means that when a magnetic field changes over time, it induces an electric field in the surrounding space.


• Similarly, another Maxwell's equation states that a time-varying electric field acts as a source of magnetic field.


• Therefore, time-varying magnetic fields and time-varying electric fields are interconnected and can generate each other.


 

Option 2 : Microwaves

Concept:

• Frequencies range of Radio waves:  3KHz to 300GHz
• Frequencies range of Micro waves:  1GHz to 300GHz
• Frequencies range of Infrared waves:  300GHz to 400THz
• Frequencies range of light waves:  430THz to 750THz

The electromagnetic wave travels in vacuum along z-direction. Then, the electric field vector and magnetic field vector will be in the XY plane.
Frequency of the wave = 30 MHz
Converting into Hz, v = 30× 10⁶ Hz.
speed of light, c = 3 × 10⁸ m/s
Wavelength of the wave can be calculated using the following relation:
λ = c/v
⇒ λ = 3 × 10⁸/30 × 10⁶
⇒λ = 10 m

The frequency of radiation arising from the two close energy levels in hydrogen known as Lamb shift i.e. 1057 MHz is radio waves as it belongs to the short wavelength end of the electromagnetic spectrum.

The frequency of radiation arising from the two close energy levels in hydrogen known as Lamb shift i.e. 1057 MHz is radio waves as it belongs to the short wavelength end of the electromagnetic spectrum. this wavelength correspond to microwave region of electromagnetic waves.

Long distance radio broadcasts use short-wave bands because they can be reflected by the ionosphere of the earth's atmosphere and thus can be send to longer distances.

The electromagnetic waves are produced by vibration of an electric charge. This creates a wave which is both as electric and magnetic components. This way oscillates in perpendicular planes with respect to each other and in a phase.This creates all electromagnetic waves begin with an oscillating charged particle which creates an oscillating electric and magnetic field.

Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF (millimeter wave) bands.
Answer B and D both are correct.

Magnetic field part of a harmonic electromagnetic wave in vacuum
,B₀​=510×10⁻⁹T
Speed of light,
C=3×10⁸m/s
E=cB_o​=153N/C

TV signals being of high frequency are not reflected by the ionosphere. Therefore, to reflect these signals, satellites are needed. That is why, satellites are used for long distance TV transmission.
 Most long-distance shortwave (high frequency) radio communication—between 3 and 30 MHz—is a result of skywave propagation.
This 3-30 MHz is a range of frequencies which are used in sky waves propagation so that the ionosphere is capable of reflecting it.

Long distance radio broadcasts use shortwave bands because only these bands can be refracted by the ionosphere.

Maxwell’s Fourth Equation
It is based on Ampere’s circuital law. To understand Maxwell’s fourth equation it is crucial to understand Ampere’s circuital law,
Consider a wire of current-carrying conductor with the current I, since there is an electric field there has to be a magnetic field vector around it. Ampere’s circuit law states that “The closed line integral of magnetic field vector is always equal to the total amount of scalar electric field enclosed within the path of any shape” which means the current flowing along the wire(which is a scalar quantity) is equal to the magnetic field vector (which is a vector quantity)

The direction of propagation of the electromagnetic wave is always perpendicular to the plane in which
So, the direction of the propagation of the wave, 
Hence, option B is the correct answer.

Infrared waves are emitted by hot bodies. They are produced due to the de-excitation of atoms.
They are called Heat waves as they produce heat falling on matter. This is because water molecules present in most materials readily absorb infrared waves. After absorption, their thermal motion increases, that is, they heat up and heat their surroundings.
Uses: Infra red lamps; play an important role in maintaining warmth through greenhouse effect.

A body at temperature T produces a continuous spectrum of wavelengths. For a black body, the wavelength corresponding to maximum intensity of radiation is given according to Planck's law by the relation, m=0.29cmK/T. For m=106m,T=2900K.Temperatures for other wavelengths can be found. These numbers tell us the temperature ranges required for obtaining radiations in different parts of the electromagnetic spectrum. Thus, to obtain visible radiation, say  =5×107m, the source should have a temperature of about 6000K. A lower temperature will also produce this wavelength but not the maximum intensity.

X-rays are absorbed by the atmosphere and therefore the source of X-rays must lie outside the atmosphere to carry out X-ray astronomy and therefore satellites orbiting the earth are necessary but radio waves and visible light can penetrate through the atmosphere and therefore optical and radio telescopes can be built on the ground.

E is the electric field vector, and B is the magnetic field vector of the EM wave. For electromagnetic waves E and B are always perpendicular to each other and perpendicular to the direction of propagation. Electromagnetic waves are transverse waves. The wave number is k = 2π/λ, where λ is the wavelength of the wave.

Explanation:


Region between the plates:
- The region between the plates of the capacitor is where the concept of displacement current is most crucial.
- Without displacement current, the changing electric field between the plates would not be taken into account, leading to significant errors in the calculation of the magnetic field in this region.
- Displacement current ensures that Ampere's law is correctly applied in this region, considering both the current in the circuit and the changing electric field between the plates.
 

The visible light spectrum is the segment of the electromagnetic spectrum that the human eye can view. More simply, this range of wavelengths is called visible light. Typically, the human eye can detect wavelengths from 380 to 700 nanometers.
Option C is the closest so the answer is option C.
 

Radio waves have wavelengths of 1mm to 100km. Hence, 21cm lies at short wavelength end of radio waves.

The speed of any electromagnetic waves in free space is the speed of light c = 3x10⁸ m/s. Electromagnetic waves can have any wavelength λ or frequency f as long as λf = c.

UVA radiation is what makes people tan. UVA rays penetrate to the lower layers of the epidermis, where they trigger cells called melanocytes (pronounced: mel-an-oh-sites) to produce melanin. Melanin is the brown pigment that causes tanning. Melanin is the body's way of protecting skin from burning.

The frequency of an electromagnetic wave produced by the oscillator is same as that of a charged particle oscillating about its mean position i.e., 10⁹Hz.

The radiation of infrared rays of the sun keeps the earth's surface warm even at night due to the greenhouse effect of the atmosphere. Heavy gases likely CO_2​ present in the earth's atmosphere reflect infra-red radiation back towards the earth's surface. Due to which earth's atmosphere becomes richer in infrared radiation. If the atmosphere is absent then earth temperature falls off.

The electromagnetic waves are not mechanical waves. There are vibrations of electric vectors and magnetic vectors in them. These vibrations do not need any particles present in the medium for their propagation. That's why electromagnetic waves do not require any medium for propagation.This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

Gamma-ray photons have the highest energy in the EMR spectrum and their waves have the shortest wavelength. Scientists measure the energy of photons in electron volts (eV). X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-ray photons generally have energies greater than 100 keV.

f​₁=7.5×10⁶ Hz
f₂​=12×10⁶ Hz
λ_1​=c/f₁​=40 m
λ₂​=c/f₂​=25 m
 
So, the range is 40m to 25m

The speed of any electromagnetic waves in free space is the speed of light c = 3x10⁸ m/s. Electromagnetic waves can have any wavelength λ or frequency f as long as λf = c.

when white light enters the prism it has actually entered out atmosphere and so the parts of the EM spectrum like UV don't appear. After the light comes out of the prism all the parts separate but we can only see the visible part i.e. 400-700nm range. So we look at only a part of the EM spectrum. Hence option B is correct.

The use of nuclear weapons would cause the formation of smoke clouds preventing the light from the sun reaching earth surface and it will also deplete the atmosphere and therefore stopping the greenhouse effect and thus doubling the cooling effect.
Hence the correct answer is option C.