Electromagnetic Waves Notes for NEET, JEE Main 2027: Key Points, PDF

Electromagnetic Waves is Chapter 8 of NCERT Class 12 Physics Part I and Unit 15 of both the NEET UG and JEE Main physics syllabi. These notes cover displacement current, the Ampere-Maxwell law, how accelerated charges produce electromagnetic waves, their transverse nature, the relation c = 1/√(μ₀ε₀) and the full electromagnetic spectrum with uses, plus a free notes PDF.

Electromagnetic Waves Notes for NEET and JEE Main

Section-wise key points from NCERT Chapter 8, a spectrum table, high-yield facts and common mistakes for 2027 revision.

Electromagnetic Waves at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus released by NTA on 31 October 2025, which stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.

What is displacement current?

Displacement current is the current term Maxwell added to Ampere’s circuital law: it is produced by a changing electric field, not by moving charges, and it is a source of magnetic field just like conduction current.

• Maxwell found an inconsistency when applying Ampere’s circuital law to a charging capacitor: a flat surface through the wire gives a magnetic field at point P, but a pot-shaped surface passing between the plates (where no charge flows) gives zero.
• The missing term is the displacement current, i_d = ε₀ (dΦ_E/dt), where Φ_E is the electric flux through the surface.
• Total current i = i_c + i_d. Outside the capacitor plates there is only conduction current (i_d = 0); between the plates there is only displacement current, and i_d equals the current in the wires.
• Ampere-Maxwell law: ∮B·dl = μ₀ i_c + μ₀ε₀ (dΦ_E/dt).
• For steady electric fields (for example a steady current in a wire) the displacement current is zero, because E does not change with time.
• Faraday’s law says a changing magnetic field produces an electric field; the displacement current means a changing electric field produces a magnetic field. This symmetry is what makes electromagnetic waves possible.
• The laws are still not perfectly symmetrical: there are no known magnetic monopoles to act as sources of magnetic field the way charges do for electric field.

How are electromagnetic waves produced?

Accelerated charges radiate electromagnetic waves; neither stationary charges nor charges in uniform motion (steady currents) can.

• A stationary charge gives only an electrostatic field; a steady current gives a magnetic field that does not vary with time.
• An oscillating charge (an example of an accelerated charge) sets up an oscillating electric field, which produces an oscillating magnetic field, and so on: the fields regenerate each other as the wave travels.
• The frequency of the wave equals the frequency of oscillation of the charge, and the energy of the wave comes from the source.
• Visible light could not be tested with an ac circuit: yellow light has a frequency of about 6 × 10¹⁴ Hz, while electronic circuits reach only about 10¹¹ Hz. So the first test was in the radio region.
• Hertz (1887) produced and detected electromagnetic waves in the laboratory, confirming Maxwell’s prediction.
• Jagdish Chandra Bose, in Calcutta, produced much shorter waves (25 mm to 5 mm). Marconi transmitted electromagnetic waves over many kilometres, the start of wireless communication.

What is the nature of electromagnetic waves?

Electromagnetic waves are transverse: the electric field, the magnetic field and the direction of propagation are mutually perpendicular, and both fields oscillate in phase.

• For a wave along z: E_x = E₀ sin(kz − ωt) and B_y = B₀ sin(kz − ωt), with k = 2π/λ.
• ω = ck, which gives νλ = c.
• Speed in vacuum: c = 1/√(μ₀ε₀) = 3 × 10⁸ m/s, equal to the measured speed of light, so light is an electromagnetic wave.
• Field amplitudes: B₀ = E₀/c (equivalently E₀/B₀ = c).
• The direction of propagation is along E × B. Example from NCERT: E along +y and wave along +x means B is along +z.
• In a material medium the speed is v = 1/√(με), so it depends on the electric and magnetic properties of the medium.
• No material medium is needed: the waves are self-sustaining oscillations of E and B in free space.
• All electromagnetic waves travel at the same speed in vacuum, independent of wavelength. This value is so well known that it is used to define the standard of length.
• Electromagnetic waves carry energy, for example sunlight reaching the earth and radio and TV signals.

Electromagnetic Spectrum: Production, Detection and Uses

The electromagnetic spectrum is the classification of electromagnetic waves by frequency or wavelength; in order of increasing wavelength it runs gamma rays, X-rays, ultraviolet, visible, infrared, microwaves and radio waves (about 10⁻¹² m to 10⁶ m).

• There is no sharp boundary between regions; the ranges overlap. NCERT’s text, for example, gives ultraviolet as 400 nm down to 0.6 nm and X-rays as 10 nm down to 10⁻⁴ nm, slightly different from Table 8.1.
• The basic difference between the types is wavelength or frequency, since all travel at the same speed in vacuum; so they interact with matter differently.
• Exposure to ultraviolet makes the skin produce more melanin (tanning). Depletion of the ozone layer by chlorofluorocarbons (CFCs) such as freon is an international concern.
• Infrared waves vibrate whole atoms and molecules, which raises internal energy and temperature; that is why they are called heat waves.

Most-asked Facts for NEET and JEE Main: Electromagnetic Waves

Revise these one-line facts and formulas from the NCERT chapter just before the exam.

• Displacement current: i_d = ε₀ dΦ_E/dt; it exists between capacitor plates and equals the conduction current in the wires.
• Ampere-Maxwell law: ∮B·dl = μ₀ i_c + μ₀ε₀ dΦ_E/dt.
• Only accelerated charges radiate electromagnetic waves; wave frequency = frequency of the oscillating charge.
• E, B and the direction of propagation are mutually perpendicular; propagation is along E × B.
• E and B oscillate in phase, with B₀ = E₀/c.
• c = 1/√(μ₀ε₀) = 3 × 10⁸ m/s in vacuum; v = 1/√(με) in a medium.
• Hertz (1887) first produced and detected electromagnetic waves; J. C. Bose made waves of 25 mm to 5 mm.
• Microwaves are produced by klystrons, magnetrons and Gunn diodes; microwave ovens match the resonant frequency of water molecules.
• Infrared = heat waves; detected by thermopiles and bolometers.
• Visible range: about 400 nm to 700 nm.
• Ozone layer absorbs most solar ultraviolet; ordinary glass also absorbs ultraviolet.
• X-rays: produced by bombarding a metal target with high-energy electrons. Gamma rays: from radioactive nuclei.
• Increasing frequency: radio < microwave < infrared < visible < ultraviolet < X-rays < gamma rays.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Treating displacement current as a flow of charge between the capacitor plates. It is due to the changing electric field; no charge crosses the gap.
• Writing B₀ = E₀c. The correct relation is B₀ = E₀/c, so the magnetic field amplitude in tesla is much smaller than E₀ in V/m.
• Saying E and B are 90° out of phase. In a plane electromagnetic wave they oscillate in phase.
• Thinking charges moving with constant velocity radiate. Only accelerated charges produce electromagnetic waves.
• Assuming electromagnetic waves need a medium. They travel through vacuum, at the same speed for all wavelengths.
• Getting the direction of B wrong. Use E × B along the direction of propagation.
• Mixing up the order of the spectrum: X-rays have shorter wavelength than ultraviolet, and microwaves shorter than radio waves.
• Saying microwave ovens heat food with infrared. They use microwaves tuned to the resonant frequency of water molecules.

Electromagnetic Waves Notes PDF and Related Pages

Download the AglaSem notes PDF that covers Electromagnetic Waves (AglaSem Class 12 notes for Electromagnetic Waves and Optics: key notes and important questions with answers) free: View PDF

• All chapters: Physics notes for NEET and JEE Main
• Full topic list: NEET syllabus and JEE Main syllabus
• Practice: NEET question papers and JEE Main question papers
• Source text: NCERT Chapter 8 (NCERT, Class 12 Physics Part I)

Physics Notes for NEET and JEE Main: All Chapters

• Units and Measurement
• Motion in a Straight Line
• Motion in a Plane
• Laws of Motion
• Work, Energy and Power
• Systems of Particles and Rotational Motion
• Gravitation
• Mechanical Properties of Solids
• Mechanical Properties of Fluids
• Thermodynamics
• Kinetic Theory of Gases
• Oscillations
• Waves
• Thermal Properties of Matter
• Electric Charges and Fields
• Electrostatic Potential and Capacitance
• Current Electricity
• Moving Charges and Magnetism
• Magnetism and Matter
• Electromagnetic Induction
• Electromagnetic Waves
• Ray Optics and Optical Instruments
• Wave Optics
• Dual Nature of Radiation and Matter
• Atoms
• Nuclei
• Semiconductor Electronics

All chapters with key points and notes PDFs are on the Physics notes for NEET and JEE Main page.

Electromagnetic Waves Notes for NEET and JEE Main FAQs