Electromagnetic Waves
Bihar Board - Class 12 Physics
Chapter Overview
Electromagnetic waves are a form of energy that can travel through space. Unlike mechanical waves, they do not require a medium to propagate. This chapter covers the nature, properties, spectrum, and applications of electromagnetic waves.
Key Concepts: Electromagnetic spectrum, Properties of EM waves, Maxwell's equations, Production and detection of EM waves, Applications.
Introduction to Electromagnetic Waves
Definition: Electromagnetic waves are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum.
Electromagnetic waves are transverse waves consisting of varying electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation.
James Clerk Maxwell first predicted the existence of electromagnetic waves in 1865 through his famous equations. Heinrich Hertz later experimentally confirmed their existence in 1887.
Characteristics of Electromagnetic Waves
- They are transverse in nature
- They can travel through vacuum
- They travel at the speed of light (3 × 10⁸ m/s in vacuum)
- They obey the wave equation: c = fλ
- They carry energy and momentum
- They can be reflected, refracted, diffracted, and polarized
Where: c = speed of light (3 × 10⁸ m/s), f = frequency (Hz), λ = wavelength (m)
Electromagnetic Spectrum
The electromagnetic spectrum is the range of all types of electromagnetic radiation, arranged according to frequency or wavelength.
Visible Light Spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. It ranges from violet (shorter wavelength) to red (longer wavelength).
| Color | Wavelength Range (nm) | Frequency Range (THz) |
|---|---|---|
| Violet | 380 - 450 | 668 - 789 |
| Blue | 450 - 495 | 606 - 668 |
| Green | 495 - 570 | 526 - 606 |
| Yellow | 570 - 590 | 508 - 526 |
| Orange | 590 - 620 | 484 - 508 |
| Red | 620 - 750 | 400 - 484 |
Properties of Electromagnetic Waves
Transverse Nature
Electromagnetic waves are transverse waves, meaning the oscillations of the electric and magnetic fields are perpendicular to the direction of wave propagation.
Speed in Vacuum
All electromagnetic waves travel at the same speed in vacuum, which is approximately 3 × 10⁸ m/s. This speed is denoted by 'c'.
Where μ₀ is the permeability of free space and ε₀ is the permittivity of free space.
Energy Carried by EM Waves
Electromagnetic waves carry energy. The energy density (u) of an electromagnetic wave is given by:
For electromagnetic waves, the electric and magnetic field energies are equal, so:
Momentum and Radiation Pressure
Electromagnetic waves also carry momentum. When EM waves strike a surface, they exert pressure called radiation pressure.
p = 2U/c (for perfect reflection)
Where p is momentum, U is energy, and c is the speed of light.
Production of Electromagnetic Waves
Accelerated Charges
Electromagnetic waves are produced by accelerated charges. When charged particles accelerate, they produce changing electric and magnetic fields that propagate as electromagnetic waves.
Sources of Different EM Waves
| Type of EM Wave | Common Sources |
|---|---|
| Radio Waves | Oscillating currents in antennas |
| Microwaves | Klystron tubes, magnetrons |
| Infrared | Hot bodies, molecules |
| Visible Light | Sun, lamps, LEDs |
| Ultraviolet | Sun, mercury vapor lamps |
| X-rays | X-ray tubes, inner shell electron transitions |
| Gamma Rays | Radioactive nuclei, nuclear reactions |
Applications of Electromagnetic Waves
Radio Waves
- Broadcasting (radio and television)
- Communication (mobile phones, Wi-Fi)
- Radar systems
- GPS navigation
Microwaves
- Microwave ovens
- Satellite communication
- Radar technology
- Wireless networks
Infrared Waves
- Remote controls
- Thermal imaging
- Night vision devices
- Infrared spectroscopy
Visible Light
- Vision and photography
- Fiber optic communication
- Lasers in various applications
Ultraviolet Radiation
- Sterilization
- Forensic analysis
- Vitamin D production in skin
- Black lights
X-rays
- Medical imaging (radiography)
- Security screening
- Material analysis
- Cancer treatment
Gamma Rays
- Cancer treatment (radiotherapy)
- Sterilization of medical equipment
- Nuclear medicine
- Astrophysical observations
Chapter Exercises
1. What are electromagnetic waves? Name the scientist who first predicted their existence.
2. Arrange the following in decreasing order of wavelength: X-rays, radio waves, blue light, infrared radiation.
3. Why are electromagnetic waves called transverse waves?
4. What is the speed of electromagnetic waves in vacuum? Does it depend on the wavelength?
5. Name the electromagnetic radiation used for the following:
(a) Treatment of cancer
(b) Aircraft navigation
(c) Sterilization of surgical instruments
(d) Remote controls of electronic devices
6. How are electromagnetic waves produced? What is the source of energy of these waves?
7. Give two uses each of:
(a) Microwaves
(b) Infrared radiation
(c) Ultraviolet radiation
(d) Gamma rays
8. Why is the ozone layer important for life on Earth? Which radiation does it absorb?
9. A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?
10. The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B₀ = 510 nT. What is the amplitude of the electric field part of the wave?
Displacement Current
Definition: Displacement current is the current due to changing electric field and is given by Iₓ = ε₀(dΦᴇ/dt)
Maxwell introduced the concept of displacement current to modify Ampere's circuital law and make it consistent with time-varying electric fields.
Maxwell's Equations
| Equation | Name | Significance |
|---|---|---|
| ∮E·dA = Q/ε₀ | Gauss's Law for Electricity | Electric flux through closed surface equals charge enclosed |
| ∮B·dA = 0 | Gauss's Law for Magnetism | No magnetic monopoles exist |
| ∮E·dl = -dΦʙ/dt | Faraday's Law | Changing magnetic field produces electric field |
| ∮B·dl = μ₀(I + ε₀dΦᴇ/dt) | Ampere-Maxwell Law | Electric current and changing electric field produce magnetic field |
Previous Years Board Questions
2023 Questions
- Define displacement current. Write Maxwell's equations. (5 marks)
- Name the electromagnetic waves used in remote controls. (1 mark)
2022 Questions
- What are electromagnetic waves? Write any four properties. (3 marks)
- Arrange in increasing frequency: Microwaves, X-rays, Radio waves (1 mark)
Numerical Problems
1. Calculate the wavelength of electromagnetic wave of frequency 15 MHz. [Ans: 20 m]
2. The magnetic field in a plane EM wave is 6 × 10⁻⁸ T. Calculate the electric field amplitude. [Ans: 18 V/m]
Memory Tips
Electromagnetic Spectrum Order (Increasing Frequency):
Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma
Remember: "Rabbits Mate In Very Unusual X-ray Gardens"
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