Electromagnetic Waves - Notes, Topics, Types, Formulas, Books, FAQs

Electromagnetic Waves Class 12 Topics(NCERT Syllabus)

1. Introduction

When we listen to the radio, watch television, use Wi-Fi or talk on mobile phones, all of these work because of invisible waves traveling through space. These are called electromagnetic waves. They are produced by changing electric and magnetic fields and travel with the speed of light. Electromagnetic waves include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, each used in different technologies.

2. Displacement Current

When a capacitor is charging, no conduction current flows between its plates, yet a magnetic field exists. To explain this, Maxwell introduced displacement current, which is caused by a changing electric field.

$
I_d=\varepsilon_0 \frac{d \Phi_E}{d t}
$

3. Electromagnetic Waves

Electromagnetic (EM) waves are formed when a time-varying electric field generates a magnetic field and vice versa. These waves consist of oscillating electric and magnetic fields, which are perpendicular to each other and also perpendicular to the direction of wave propagation.

EM waves do not require any medium to travel.

They propagate at the speed of light,
$c=3 \times 10^8 \mathrm{~m} / \mathrm{s}$

They transfer both energy and momentum.

• Nature of electromagnetic waves
• Propagation Of Electromagnetic Waves

4. Electromagnetic Spectrum

The electromagnetic spectrum is the range of all possible electromagnetic waves, arranged according to their frequency or wavelength. It extends from very short wavelength $\gamma$-rays to very long wavelength radio waves.

1. $\gamma$-rays - Produced in nuclear reactions; used in cancer treatment.
2. X-rays - Used in medical imaging.
3. Ultraviolet (UV) rays - Cause tanning, sterilization.
4. Visible light - The only part visible to the human eye.
5. Infrared (IR) rays - Used in remote controls, night vision.
6. Microwaves - Used in ovens, radar, and communication.
7. Radio waves - Used for radio, TV, and wireless communication.

NCERT Notes Subject-Wise Link:

• NCERT notes Class 11 Maths
• NCERT notes Class 11 Physics
• NCERT notes Class 11 Chemistry
• NCERT notes Class 11 Biology

Electromagnetic Waves Formulas

1. Speed of EM Wave

$
c=\frac{1}{\sqrt{\mu_0 \epsilon_0}} \quad\left(c=3 \times 10^8 \mathrm{~m} / \mathrm{s}\right)
$

2. Wave Relation

$
c=\lambda \nu
$

• $\lambda=$ wavelength
• $\nu=$ frequency

3. Electric & Magnetic Field Relation

$
\frac{E_0}{B_0}=c
$

4. Energy Density

$
u=\epsilon_0 E^2=\frac{B^2}{\mu_0}
$

5. Intensity of EM Wave

$
I=\frac{1}{2} \epsilon_0 c E_0^2=\frac{c}{2 \mu_0} B_0^2
$

Exam-wise Weightage of Electromagnetic waves

Electromagnetic waves Real-life application

• Radio Waves – Used in radio, TV, and mobile communication.
• Microwaves – Used in microwave ovens, radar systems, and satellite communication.
• Infrared Rays – Used in remote controls, night-vision cameras, and thermal imaging.
• Visible Light – Enables vision; used in photography, lasers, and optical instruments.
• Ultraviolet Rays – Used for sterilization, detecting fake currency, and in medical treatments.
• X-Rays – Used in medical imaging (scans), airport security, and material testing.
• Gamma Rays – Used in cancer treatment, nuclear energy, and sterilization of equipment.

How to prepare Electromagnetic waves

• First, you should have good command over the concepts of Electromagnetic waves and you should also know how to apply them well at the time of the entrance exam or while solving questions.
• Please try to understand each concept from this chapter, with the help of theory, questions with solutions, and video lectures on each important concept.
• For each concept, practice enough problems so that you have a thorough understanding of the concept.
• Solve all the questions at home with proper concentration and try to do all the calculations by yourself without seeing the solution first.
• While solving questions of wave equations, you should also consider the direction of corresponding vectors.
• Please have a thorough knowledge of the electromagnetic wave spectrum and the application of various Electromagnetic waves.
• Remember all the formulas of Energy, momentum, and frequency of Electromagnetic waves. This will help you while solving questions.

NCERT Solutions Subject-Wise Link:

• NCERT solutions for class 11 Physics.
• NCERT solutions for class 11 Chemistry.
• NCERT solutions for class 11 Mathematics.
• NCERT solutions for class 11 Biology.

Electromagnetic Waves Books

For Electromagnetic Waves, chapter concepts in NCERT are enough, but you will have to practice lots of questions, including previous year questions, and you can follow other standard books available for competitive exam preparation like Concepts of Physics (H. C. Verma) and Understanding Physics by D. C. Pandey (Arihant Publications).

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