Production and propagation of electromagnetic waves
Electromagnetic waves are produced by accelerating charges and propagate through space, carrying energy and momentum, with their direction determined by the orientation of the electric and magnetic fields.
Why this shows up in the exam
NEET often asks about how electromagnetic waves are generated and how their energy and direction are determined.
How NEET tests this
Learn the idea
Accelerating charges emit electromagnetic waves; the wave consists of mutually perpendicular electric (E) and magnetic (B) fields that travel together, and the energy flows in the direction of the Poynting vector S = (E×B)/μ₀, not along either field alone.
🧠 Memory hook: Think of the EM wave as a perfect dance: the electric partner and magnetic partner always stay at right angles, and together they step forward in the direction given by their crossed hands (right‑hand rule).
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Radiation occurs only when a charge experiences acceleration (including SHM).
- In an EM wave E, B and the direction of propagation are mutually perpendicular (right‑hand rule).
- Energy flux is given by the Poynting vector S = (E×B)/μ₀.
- In vacuum EM waves travel at speed c and satisfy c = λν.
- The magnetic field magnitude is B = E/c in a plane wave.
How to approach it
- 1Identify whether the source involves an accelerating charge or time‑varying dipole moment.
- 2Confirm that the E and B fields are at right angles; use the right‑hand rule to get the propagation direction (E×B).
- 3If the question asks about energy flow, apply the Poynting vector, not the direction of a single field.
- 4For frequency‑related problems (e.g., microwave oven), recall that the field makes polar molecules oscillate, converting field energy to heat.
Worked example — watch it click
Statement I: Electromagnetic waves are radiated when a charged particle undergoes SHM. Statement II: Electromagnetic waves propagate energy in the direction of the electric field of the wave.
- A)Both Statement I and Statement II are true.
- ✅Statement I is true but Statement II is false.
- C)Statement I is false but Statement II is true.
- D)Both Statement I and Statement II are false.
The concept behind this problem
The example tests whether you know that radiation needs acceleration (true for SHM) and that the energy of an EM wave travels along the Poynting vector, which is perpendicular to both E and B, not along the electric field.
Step by step
- 1Statement I: EM waves radiated when charged particle undergoes SHM - TRUE.
- 2Accelerating charges radiate EM waves;
- 3SHM involves acceleration.
- 4Statement II: EM waves propagate energy in direction of E field - FALSE.
- 5Energy propagates in direction of Poynting vector S = (E × B)/μ₀, which is perpendicular to both E and B.
- 6Statement I true, II false.
Watch out
Students often answer that energy moves in the direction of the electric field, forgetting the Poynting vector rule.
Common slip-ups that cost marks
- •Confusing the direction of energy transport with the direction of the electric field.
- •Assuming uniform linear motion or simple vibration without acceleration can radiate – only accelerated motion radiates.
- •Mixing up wavelength with the size of the antenna or the source frequency.
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Practise it
These are real questions from past NEET papers that test this exact idea.
Out of the following options which one can be used to produce a propagating electromagnetic wave?
Push further
More challenging15 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.
Identify the correct pairing of an electromagnetic wave and its typical source.
More from Electromagnetic Waves
Mathematical relations in electromagnetic waves
Key mathematical relationships in electromagnetic waves include the connection between electric and magnetic field amplitudes, wave speed, wavelength, frequency, and the wave equation parameters.
Nature and properties of electromagnetic waves
Electromagnetic waves are transverse waves consisting of mutually perpendicular oscillating electric and magnetic fields, both perpendicular to the direction of propagation, and exhibit properties such as speed, polarization, and ability to travel through vacuum.
Energy, momentum, and radiation pressure of electromagnetic waves
Electromagnetic waves transport energy and momentum, exerting radiation pressure and force on surfaces, with quantifiable energy density and intensity.
Electromagnetic spectrum
The electromagnetic spectrum encompasses all types of electromagnetic waves, classified by wavelength or frequency, with each region having characteristic properties and applications.
How Electromagnetic Waves Are Produced
Electromagnetic radiation is generated by accelerated charges or time-varying currents. In a source-free region, coupled time-varying electric and magnetic fields propagate without requiring a material medium.
Maxwell Equations as a Unified Picture
The integral Maxwell equations are Gauss's electric law, Gauss's magnetic law, Faraday's induction law, and the Ampere-Maxwell law. Together in a charge-free, current-free region they imply electromagnetic wave equations.