Key Properties of EM Waves Travelling in Free Space
Physics · Electromagnetic Waves · NEET
In free space an EM wave is transverse (E and B both point across the direction of travel), it moves at one fixed speed c = 1/sqrt(mu0 e0) = 3 x 10^8 m/s, its fields obey E = cB (so E0/B0 = c), E and B stay in step and carry equal energy, and the wave is made by an accelerating charge. Memory hook: "TESA" — Transverse, Equal energy, Speed c, from Accelerating charge.
In free space E (red, vertical) and B (blue) oscillate in phase and perpendicular to each other; the wave travels along x = E x B at speed c, with E0 = cB0 and equal energy in both fields.
Your doubts, answered
Are EM waves transverse or longitudinal?
Transverse. In free space the electric field E, the magnetic field B, and the direction of travel are all mutually perpendicular. E oscillates along one axis, B along a second axis, and the wave moves along the third axis (the E x B direction). Sound is longitudinal, but every EM wave is transverse. NEET tests this directly, so remember: E is perpendicular to B, and both are perpendicular to the propagation direction.
Do E and B reach their peak at the same time?
Yes. E and B oscillate in phase, meaning they hit zero together and hit their maximum together. This is why we can write Ex = E0 sin(kx - wt) and By = B0 sin(kx - wt) with the SAME sine term. A common wrong idea is that B lags E by 90 degrees like in an LC circuit, but in a free-space EM wave they are perfectly in step.
Since E0 = c B0, isn't the electric field far stronger than the magnetic field?
The numbers look very different (E0 is about 3 x 10^8 times B0), but that is only because of the units. When you compute the energy each field stores, the electric energy density uE = (1/2) e0 E^2 exactly equals the magnetic energy density uB = B^2 / (2 mu0). Using E = cB and c = 1/sqrt(mu0 e0) makes them equal. So both fields carry equal energy — a favourite NEET trap.
What kind of charge produces an EM wave?
Only an ACCELERATING charge radiates an EM wave. A stationary charge gives a static electric field (no wave). A charge moving at constant velocity gives a steady magnetic field plus electric field but still no radiation. You need acceleration — for example an oscillating charge in an antenna — to send out an electromagnetic wave. NEET 2016 and NEET 2024 both tested exactly this point.
Is the speed of an EM wave the same for all of them in free space?
Yes. In vacuum every EM wave — radio, light, X-ray, gamma — travels at the same speed c = 1/sqrt(mu0 e0) = 3 x 10^8 m/s. Speed does not depend on frequency or wavelength in free space. Only when the wave enters a material medium does it slow to v = 1/sqrt(mu e) = c/n.
⚠️ The NEET trap ✗ A charge moving at constant (uniform) velocity radiates an EM wave. ✓ Only an ACCELERATING charge radiates. Uniform velocity gives steady fields but no wave. This exact line was the correct 'odd one out' in NEET 2024. 🧠 'EM waves come from charges moving with uniform speed' — sounds right, but it is the ONE false property NTA plants.
Real NEET questions
NEET 2024
The property which is NOT of an electromagnetic wave travelling in free space is that
A · The energy density in the electric field is equal to the energy density in the magnetic field
B · They travel with a speed equal to 1/sqrt(mu0 e0)
C · They originate from charges moving with uniform speed ✓
D · They are transverse in nature
Solution: Check each property of a free-space EM wave. (A) uE = (1/2)e0 E^2 equals uB = B^2/(2 mu0) using E = cB, so equal energy is TRUE. (B) Speed = 1/sqrt(mu0 e0) = 3 x 10^8 m/s is TRUE. (D) E, B and the travel direction are mutually perpendicular, so transverse is TRUE. (C) is FALSE: EM waves come only from ACCELERATING charges, not charges moving with uniform (constant) speed. The question asks for the property that is NOT true, so the answer is (C).
NEET 2020
The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is (c = speed of electromagnetic waves)
A · 1 : c
B · 1 : c^2
C · c : 1
D · 1 : 1 ✓
Solution: Average electric energy density uE = (1/2) e0 E0^2 / 2 and average magnetic energy density uB = B0^2 / (2 mu0) / 2. Use E0 = c B0 and c^2 = 1/(mu0 e0). Then uE = (1/2)e0 (cB0)^2 = (1/2)e0 c^2 B0^2 = (1/2)e0 B0^2/(mu0 e0) = B0^2/(2 mu0) = uB. Since intensity is proportional to energy density, the electric and magnetic contributions are equal. Ratio = 1 : 1, option (D).
NEET 2016
Out of the following options which one can be used to produce a propagating electromagnetic wave?
A · A charge moving at constant velocity
B · A stationary charge
C · A chargeless particle
D · An accelerating charge ✓
Solution: A stationary charge makes only a static electric field. A charge at constant velocity makes steady electric and magnetic fields but no radiation. A chargeless particle makes no field at all. Only an ACCELERATING charge produces time-varying (coupled) E and B fields that detach and propagate as an EM wave. Answer: (D) an accelerating charge.
Solved Electromagnetic Waves NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the main properties of EM waves in free space?
They are transverse; travel at a single speed c = 1/sqrt(mu0 e0) = 3 x 10^8 m/s; have E perpendicular to B with both perpendicular to travel (E x B gives the direction); satisfy E0/B0 = c; carry equal electric and magnetic energy; are produced by accelerating charges; carry energy and momentum; and need no medium.
Why do E and B carry equal energy?
Because uE = (1/2)e0 E^2 and uB = B^2/(2 mu0), and substituting E = cB with c = 1/sqrt(mu0 e0) makes the two expressions identical. So half the wave's energy is in the electric field and half in the magnetic field at every instant.
Do EM waves need a medium to travel?
No. EM waves travel through vacuum. Changing E and B fields regenerate each other, so no material is needed — this is why sunlight reaches us through empty space. (See the next concept: Do EM Waves Need a Medium?)
What is the relation between E0 and B0 in an EM wave?
E0 = c B0, so E0/B0 = c = 3 x 10^8 m/s. The electric amplitude is about 3 x 10^8 times the magnetic amplitude in SI units, but both fields still carry equal energy.
Is the direction of an EM wave along E, along B, or along E x B?
Along E x B. Take the cross product of the electric field direction with the magnetic field direction and it points in the direction the wave travels. For example, E along +y and B along +z gives travel along +x.
