Introduction
Electromagnetic Waves is an important chapter of Class 12 Physics and is usually short but very high-scoring in board exams. Questions from this chapter are frequently asked in CBSE Board Exams, NEET, JEE Main, and other competitive exams.
This chapter explains how energy can travel through space in the form of waves without the need for any material medium. Light, radio waves, X-rays, and gamma rays are all examples of electromagnetic waves that we experience in daily life.
In these Class 12 Physics Electromagnetic Waves Notes, every topic is explained step by step, in easy language, so that students can easily understand concepts, remember facts, and write accurate answers in exams.
Class 12 Physics Electromagnetic Waves Notes PDF | EM Waves NCERT Notes
What are Electromagnetic Waves?
Electromagnetic waves are waves formed by oscillating electric and magnetic fields. These waves do not require any medium to propagate and can travel through vacuum.
An electromagnetic wave consists of:
- An electric field (E)
- A magnetic field (B)
Both fields:
- Oscillate perpendicular to each other
- Are perpendicular to the direction of propagation
Key Points:
- Electromagnetic waves can travel in vacuum
- They carry energy and momentum
- Speed in vacuum = 3 × 10⁸ m/s
Production of Electromagnetic Waves
Electromagnetic waves are produced by accelerated charges. When an electric charge oscillates or accelerates, it produces changing electric and magnetic fields. These changing fields sustain each other and move through space as electromagnetic waves.
Example:
- Oscillating charges in an antenna produce radio waves
- Rapid deceleration of electrons produces X-rays
Maxwell’s Contribution
James Clerk Maxwell gave the theoretical explanation of electromagnetic waves. He showed that:
- A changing electric field produces a magnetic field
- A changing magnetic field produces an electric field
This continuous process allows electromagnetic waves to propagate through space.
Important Result:
Maxwell calculated the speed of electromagnetic waves and found it equal to the speed of light. This proved that light itself is an electromagnetic wave.
Nature of Electromagnetic Waves
Electromagnetic waves have the following important properties:
- Transverse in nature
The electric and magnetic fields vibrate perpendicular to the direction of propagation. - No medium required
They can travel through vacuum. - Travel at the speed of light
In vacuum, speed = 3 × 10⁸ m/s. - Carry energy and momentum
They can exert radiation pressure. - Obey laws of reflection and refraction
Electromagnetic Spectrum
The electromagnetic spectrum is the complete range of electromagnetic waves, arranged according to increasing frequency or decreasing wavelength.
Order of Electromagnetic Spectrum:
- Radio Waves
- Microwaves
- Infrared Rays
- Visible Light
- Ultraviolet Rays
- X-Rays
- Gamma Rays
Radio Waves
Radio waves have the longest wavelength and lowest frequency.
Properties:
- Wavelength: More than 0.1 m
- Produced by oscillating charges in antennas
Uses:
- Radio broadcasting
- Television communication
- Mobile communication
Microwaves
Microwaves have shorter wavelength than radio waves.
Properties:
- Produced by electronic devices like klystrons
- High penetration power
Uses:
- Microwave ovens
- Radar systems
- Satellite communication
Infrared Rays (IR)
Infrared rays lie just beyond red light in the spectrum.
Properties:
- Emitted by hot objects
- Cause heating effect
Uses:
- Night vision devices
- Remote controls
- Physiotherapy
Visible Light
Visible light is the only part of the spectrum visible to human eyes.
Wavelength Range:
- 400 nm (violet) to 700 nm (red)
Colors Order:
VIBGYOR
(Violet, Indigo, Blue, Green, Yellow, Orange, Red)
Ultraviolet Rays (UV)
Ultraviolet rays lie beyond violet light.
Properties:
- High frequency
- Harmful in large amounts
Uses:
- Sterilization of water
- Detection of forged documents
- Medical applications
X-Rays
X-rays have very high energy and strong penetrating power.
Properties:
- Produced when fast electrons are suddenly stopped
- Can penetrate soft tissues
Uses:
- Medical imaging
- Airport security scanning
- Industrial testing
Gamma Rays
Gamma rays have the highest frequency and shortest wavelength.
Properties:
- Emitted by radioactive nuclei
- Extremely penetrating
Uses:
- Cancer treatment
- Sterilization of medical equipment
- Nuclear research
Important Relations in Electromagnetic Waves
Relation between Speed, Frequency, and Wavelength:
c = \nu \lambda
Where:
- c = speed of light
- ν = frequency
- λ = wavelength
Displacement Current
Displacement current is not a real current but arises due to a changing electric field.
Importance:
- Helps maintain continuity of current
- Explains electromagnetic wave propagation
Energy in Electromagnetic Waves
Energy in electromagnetic waves is equally shared between:
- Electric field
- Magnetic field
The flow of energy per unit area is given by Poynting vector.
Applications of Electromagnetic Waves in Daily Life
- Communication systems
- Medical diagnosis
- Cooking food
- Remote sensing
- Wireless technology
Advantages of Electromagnetic Waves
- Can travel long distances
- No medium required
- Fast transmission of information
Exam-Oriented Important Points
- EM waves are transverse
- Speed in vacuum is constant
- Do not get deflected by electric or magnetic fields
- Energy is proportional to frequency
Frequently Asked Questions (Repeated Questions)
Q1. Do electromagnetic waves require a medium?
No, electromagnetic waves can travel through vacuum.
Q2. Which EM wave has maximum frequency?
Gamma rays.
Q3. Which EM waves are used in microwave ovens?
Microwaves.
Q4. What is the speed of EM waves in vacuum?
3 × 10⁸ m/s.
Q5. Who predicted electromagnetic waves?
James Clerk Maxwell.
Conclusion–Class 12 Physics Electromagnetic Waves Notes PDF
Electromagnetic Waves is a small but very important chapter of Class 12 Physics. Understanding this chapter helps students score easy marks because most questions are theory-based and direct. From radio waves to gamma rays, electromagnetic waves play a vital role in science, technology, and daily life.
These Class 12 Physics Electromagnetic Waves Notes are designed to help students understand every concept clearly and prepare confidently for exams.
Repeated Questions – Class 12 Physics
Electromagnetic Waves
Very Short Answer Questions (1 Mark)
- What are electromagnetic waves?
- Do electromagnetic waves require a medium for propagation?
- Name the scientist who predicted electromagnetic waves theoretically.
- What is the speed of electromagnetic waves in vacuum?
- Which electromagnetic wave has the longest wavelength?
- Which electromagnetic wave has the highest frequency?
- Are electromagnetic waves transverse or longitudinal?
- Which part of the electromagnetic spectrum is visible to human eye?
- Which electromagnetic waves are used in TV remote controls?
- Which EM waves are produced during nuclear reactions?
Short Answer Questions (2 Marks)
- Explain how electromagnetic waves are produced.
- Write any two properties of electromagnetic waves.
- Why are electromagnetic waves called transverse waves?
- What is meant by electromagnetic spectrum?
- Write two uses of infrared rays.
- Give two uses of ultraviolet rays.
- Why are microwaves preferred for satellite communication?
- Write two differences between radio waves and gamma rays.
- What is displacement current? Why is it important?
- Why do electromagnetic waves not get deflected by electric or magnetic fields?
Short Answer Questions (3 Marks)
- Explain Maxwell’s contribution to electromagnetic waves.
- Describe the nature of electromagnetic waves.
- Write three important uses of X-rays.
- Explain the production of X-rays in brief.
- State three important properties of electromagnetic waves.
- Explain why gamma rays have the highest penetrating power.
- Write three applications of electromagnetic waves in daily life.
- Explain why infrared rays are called heat waves.
Long Answer Questions (5 Marks)
- Explain electromagnetic waves with the help of Maxwell’s equations.
- Describe the electromagnetic spectrum in detail. Write the properties and uses of each region.
- What are electromagnetic waves? Explain their production and characteristics.
- Explain displacement current and its role in electromagnetic wave propagation.
- Draw a neat diagram of electromagnetic waves and explain its features.
Assertion–Reason Type Questions (Repeated)
- Assertion (A): Electromagnetic waves can travel through vacuum.
Reason (R): They do not require a material medium for propagation. - Assertion (A): Gamma rays have maximum penetrating power.
Reason (R): They have very high frequency and short wavelength. - Assertion (A): Electromagnetic waves carry momentum.
Reason (R): They consist of oscillating electric and magnetic fields.
Case Study / Competency Based Questions (New Pattern)
- A student observes that microwaves are used in satellite communication.
- (a) Name the region of electromagnetic spectrum involved.
- (b) Give one reason why these waves are preferred.
- (c) Mention one more application of these waves.
- During medical diagnosis, X-rays are used to detect bone fractures.
- (a) Why can X-rays pass through soft tissues?
- (b) Mention one harmful effect of X-rays.
Numerical / Formula-Based Repeated Questions
- Write the relation between speed, wavelength, and frequency of EM waves.
- Calculate the frequency of an electromagnetic wave having wavelength 600 nm.
- If the frequency of an EM wave increases, what happens to its wavelength?
Most Important Exam Tips ⭐
- Always remember the order of electromagnetic spectrum
- Learn uses of each EM wave properly
- Displacement current is a very important concept
- Speed of EM waves in vacuum is constant