JEE Main Study Notes for Electromagnetic Radiation: An electric and magnetic disturbance that is traveling through space at the speed of light (2.998 × 108m/s) is called Electromagnetic Radiation. It carries electromagnetic radiant energy like photons or quanta. Radio waves, microwaves, infrared, light, ultraviolet, X-rays, and gamma rays are all examples of electromagnetic waves. 

Electromagnetic radiation is a branch of electrostatics, where magnetism describes the electromagnetic waves' nature, intensity, and energy density. Electromagnetic radiation has a weightage of 3.33 percent in JEE Main.

JEE Main Study Notes for Electromagnetic Radiation include some important topics such as Properties and uses of electromagnetic radiation, displacement current, electromagnetic spectrum, gauss law for magnetism, Maxwell-Faraday’s equation, etc. Candidates will find some helpful JEE Main study notes for Electromagnetic Radiation in this article. Check JEE Main Chemistry Syllabus

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What is Electromagnetic Radiation?

Electromagnetic radiation (EMR) is made up of electromagnetic (EM) field waves that travel through space carrying electromagnetic radiant energy. Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all examples of electromagnetic radiation. 

Major contributions in the field of Electromagnetic Radiation were made by James Clerk Maxwell, who modified the existing theories and formulas to include electromagnetic radiation as well as postulated his own theories regarding the subject. He was the first one to state that Electromagnetic Radiation are wavelike in nature. Heinrich Hertz succeeded in experimentally demonstrating the existence of electromagnetic waves in 1888. He proved the features of refraction, reflection, and interference of electromagnetic waves using oscillator LC-circuits, establishing beyond doubt that light radiation has wave nature.

Properties of Electromagnetic Radiation

• Electromagnetic radiation is described by the classical wave model as waves with wavelength, frequency, velocity, and amplitude. These electromagnetic radiation properties can explain classical electromagnetic radiation features such as reflection, refraction, diffraction, interference, and so on.
• Electromagnetic waves are emitted by an accelerating charge. When an oscillating charge, such as in an LC-circuit, experiences non-zero acceleration, it continues to create electromagnetic waves. Electromagnetic waves have the same frequency as an oscillating charge. 
• All electromagnetic waves are transverse waves; they may travel through a vacuum; and they travel at the same speed in a vacuum as light, 300,000,000 m/s.
• The amplitude of field vectors has no effect on the velocity of electromagnetic waves. 
• The light vector is the electric vector of an electromagnetic wave that is responsible for optical phenomena. 
• The average energy densities of magnetic and electric fields are equal in a planar electromagnetic wave. 
• When an electromagnetic wave advances, electromagnetic energy flows in the direction of the Poynting vector. The Poynting vector is the total energy flowing perpendicularly per second per unit area into free space

Uses of Electromagnetic Radiation

Everyday life is dominated by man-made electromagnetic radiation: microwave ovens cook food, radar waves guide aeroplanes, television sets receive electromagnetic waves transmitted by broadcasting stations, and infrared waves from heaters offer warmth. The other everyday examples of electromagnetic radiation are:

• Radio waves are used in radio and television. 
• Microwaves are used for satellite communications as well as cooking meals. 
• Electrical warmers, frying food, and infrared cameras are all examples of infrared technology. 
• Fibre optic communications - visible light 
• Ultraviolet - Sun tanning, energy-efficient lighting 
• X-rays are used in medical imaging and treatment. 
• Medical imaging and therapy with gamma rays

JEE Main Electromagnetic Radiation: Key Concepts

Before we move on to the JEE Main 2022 Electromagnetic Radiation Study Notes, let's have a quick review of the important key concepts.

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Displacement current:

The charge flow per unit time is represented by a current. As a result, the displacement current is expressed as . It appears in Maxwell's Equations and is a source of magnetic field just as an actual current flow would be.

Electromagnetic Spectrum

The electromagnetic spectrum is an organised array of electromagnetic radiations in the order of their frequency or wavelength. Furthermore, there is no clear distinction between one type of wave and the next. The categorization is far more concerned with how these waves are generated and identified. Different regions are referred to by different names, such as X-rays, g-rays, UV rays, infrared rays, and visible rays. Microwaves and radio waves 

Gauss Law for Magnetism

Gauss' Law is a universal law that can be applied to any closed surface. It is a useful tool because it allows the quantity of confined charge to be calculated by mapping the field on a surface outside the charge distribution. It simplifies the calculation of the electric field for sufficiently symmetric geometries.

According to Gauss law for magnetism, the net magnetic flux across a closed surface is zero. This indicates that the number of magnetic field lines entering a closed volume must be equal to the number of field lines leaving that volume. It follows that the magnetic monopole does not exist in nature.

Maxwell-Faraday's Equation

Faraday's Law is a theory that describes how electricity works. As per the textbook definition,

“ The electromotive force around a closed path is equal to the negative of the time rate of change of the magnetic flux enclosed by the path.” 

Maxwell modified the statement to include the changing magnetic flux as the source of the said emf. Thus Maxwell - Faraday's Law states that the line integral of the electric field (which equals the e.m.f.) around any closed path equals the rate of change of magnetic flux through any surface area circumscribed by that channel. In short, an changing magnetic field induces an emf, in the direction opposite to the change of the electric field

Maxwell’s Modification of Ampere’s Circuital Law

Maxwell used a symmetry consideration to modify Ampere's law. A changing magnetic field creates an electric field, hence a changing electric field must induce a magnetic field, according to Faraday's law. Because currents are commonly used to generate magnetic fields, a changing electric field must be coupled with a current.This information applied to a standard Ampere Circuital Law 

Results in the modification of the equation as :

JEE Main Electromagnetic Radiation :Previous Year Questions 

As per the previous years paper analysis, majority of the questions asked from ‘Electromagnetic Radiation’ are framed as : 

• When the mаgnetiс аnd eleсtriс fields аre given, the strength оf the wаve must be саlсulаted.
• When a magnetic field is supplied, the average energy density must be calculated. 
• To discover the dimensional formula for values such as the magnetic field at zero location, as well as queries about obtaining the free current at zero position.
• We have included fast notes covering the important concepts of Electromagnetic radiation, advice to keep in mind, sample questions from electromagnetic waves, atmospheric layers, and so on in this article.

Few Questions framed from ‘Electromagnetic Radiation’ in the Previous Year JEE Main Question Papers are as following:

Question: The average intensity of radiation on the surface of the Sun is approx.108 W/m2. The Root Mean Square value of the corresponding magnetic field is? (4 Marks)

1. 10⁻²T
2. 1 T
3. 10⁻⁴ T
4. 10² T

Answer: Mean intensity I= B²rms/μc. Thus after inputting values B²rms, C. 10⁻⁴ T is correct.

Question: 50 W/m2 energy density of sunlight is shining upon a solar panel. About 25% sunlight is reflected from the surface and the rest is absorbed. The force exerted on 1 Sq. m. surface area will be? (Take, c = 3 × 108 m/s) (4 Marks)

1. 20 × 10–8 N
2. 10 × 10–8 N
3. 35 × 10–8 N
4. 15 × 10–8 N

Solution:

By, change in momentum Δp = pf – pi

Δp = (-pi /4) – pi

Inputting Energy Density, 

Δp = -5pi /4 ∵ pi = E/c = 50 W/s/(3 x 108)

Δp/Δt = F = 丨-5pi/4丨= (5/4) x (50/(3 x 108) = 20.8 x 10-8 N ≈ 20x 10-8 N

Answer: (a) 20 × 10–8 N

Questiоn: The decreasing оrder оf wavelength of miсrоwаve, ultrаviоlet,infrаred, аnd gаmmа rаys is? (Part of a compound Question, 2 Marks)

(А) Miсrоwаve, Infrаred, Ultrаviоlet, Gаmmа rаys

(B) Infrаred, Miсrоwаve, Ultrаviоlet, Gаmmа rаys

(С) Miсrоwаves, Gаmmа rаys, Infrаred, Ultrаviоlet

(D) Gаmmа rаys, Ultrаviоlet, Infrаred, Miсrоwаves

Аnswer: (А) Miсrоwаve > Infrаred > Ultrаviоlet > Gаmmа rаys

Question: A red LED emits light at 0.1 watts, uniformly, around it. The amplitude of the electric field at a distance of 1.0 m from the diode is? (4 Marks)

(a) 5.48 V/m

(b) 7.75 V/m

(c) 1.73 V/m

(d) 2.45 V/m

Solution:

The intensity of light is given by, I = uc

Where, I = P/4πr^2 and u = ½.ε.E^2

By inputting values,

= 2.45 V m-1

Answer: (d) 2.45 V/m

Questiоn: The eleсtriс аnd mаgnetiс field оf аn eleсtrоmаgnetiс wаves аre? (Part of a compound Question, 1 Marks)

(А) In the орро site рhаse аnd раrаllel tо eасh оther.

(B) In Рhаse аnd рerрendiсulаr tо eасhоther

(С) In орроsite рhаse аnd рerрendiсulаr tо eасh оther

(D) In рhаse аnd раrаllel tо eасh оther

Аnswer: (B) In Рhаse аnd рerрendiсulаr tо eасhоther

Question: Red light differs from the blue one,because? (4 marks)

• They have different frequencies
• They have same wavelength and different frequencies
• They have same frequency but different wavelength
• They have different frequency and different wavelength

Answer: (D) They have different frequency and different wavelength

Questiоn: X-rаys аre? (Part of a compound Question, 1 Marks)

(А) Streаm оf eleсtrоn

(B) Streаm оf unсhаrged раrtiсle

(С) Eleсtrоmаgnetiс rаdiаtiоn

(D) Stream of рrоtоn

Аnswer: (С) Eleсtrоmаgnetiс rаdiаtiоn

X-rаys соnsist оf oscillating eleсtriс аnd mаgnetiс field at right аngles оf eасhоther аnd in the direсtiоn оf рrораgаtiоn оf the x-rаys. Therefоre, x-rаys аre eleсtrоmаgnetiс rаdiаtiоns.

Questiоn: In Gаmmа rаy emissiоn frоm а nuсleus (Part of a compound Question, 1 Marks)

(А) Оnly the рrоtоn number сhаnges

(B) Оnly the neutrоn number сhаnges

(С) There is nо сhаnge in the neutrоn number аnd the рrоtоn number

(D) Bоth the рrоtоn number and the neutron number changes

Аnswer: (С) There is nо сhаnge in the рrоtоn number аnd the neutrоn number

Questiоn: If there were no atmosphere, the аverаge temрerаture оn eаrth surfасe wоuld be? (Part of a compound Question, 1 Marks)

(А) Higher

(B) Sаme

(С) Lоwer

(D) 0 degree С

Аnswer: (С) Lоwer

Аs the аtmоsрhere is the blаnket оf glаsses thаt restrain the heаt tо esсарe in оuter sрасe. If there was no аtmоsрhere, the eаrth’s temрerаture wоuld rise in the dаytime and it would reach the freezing роint during the night. Henсe, it would аffeсt the survivаl оf аll living organisms of the earth.

JEE Main Electromagnetic Radiation : Helpful Tips for Preparation 

Electromagnetic Waves is a short but slightly complex topic;by applying the tips mentioned below, students can make its preparation a little bit easier.

Keep the Basics Clear

Electromagnetic Radiation is a sub-topic of Electromagnetic Waves, thus candidate should be clear about the concepts introduced in the Electromagnetic waves, such as : Transverse nature of Electromagnetic Waves, Maxwell Equations, Electromagnetic Spectrum. Apart from the Electromagnetic Waves candidate should also have a clear conceptual knowledge of :

• Electrostatics : Specifically, the Electric Field, Flux, Electric Dipole, Electric Flux associated with various surfaces, and Capacitance
• Current Electricity : Specifically, emf of a sell, and internal resistance
• Magnetic Effects of Current and Magnetism : Specifically, Biot Steward Law, Ampere’s Law and its applications, Forces on moving charges in a electric or magnetic field
• Electromagnetic Induction and Alternating Current: Faraday Law, Lenz’s Law, Peak and RMS value, Resonance

Related Links:

• JEE Main Study Notes for Electromagnetic Waves
• JEE Main Study Notes for Electrostatics
• Jee Main Study Notes for Magnetic Effect of Current & Magnetism

Practise a Variety of Problems on Maxwell Equations

Though Maxwell equations are formally included in the Electromagnetic Waves topic, they are separate from the Electromagnetic Radiation. Nevertheless, questions framed upon Electromagnetic Radiation mostly employ these equations to solve a problem.

Memorize the Electromagnetic Spectrum

Electromagnetic spectrum is the most important part of the topic, candidate should specifically focus on the following points, while preparing the topic:

• Radiation Name
• Wavelength of the Radiation
• Frequency of the Radiation
• Temperature at which it is emitted
• Practical uses of individual radiation

(Courtesy of Wikimedia.commons)

Difficulty Level of Electromagnetic Radiation and the Time Required for Preparation

The topic, in itself, is easy to comprehend, as it relies mostly on theory. The problem is that, the questions framed from the topic are seldom asked alone (as evident in the JEE Questions above), mostly they are combined with questions framed from Electromagnetic waves and Magnetic Effects of Current and Magnetism. If the aspirant does not have proper mastery over the basics of magnetism and electricity, he may fall short. This makes the previous easy topic, slightly difficult in preparation

Direct links JEE Main Question Banks for Practise:

• Download Chapter-wise Question Bank in English
• Download Chapter-wise Question Bank in Gujarati

JEE Main Electromagnetic Radiation FAQs