Which of the following statements is not true with respect to electromagnetic waves?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → They travel with speed given by \( v = \sqrt{\mu_0\varepsilon_0} \) in vacuum
Option (1) They can be polarised — True. Electromagnetic waves are transverse, so they can be polarised.
Option (2) They carry energy and momentum — True. The waves transport energy and also exert radiation pressure, which shows that they carry momentum.
Option (3) They travel with speed given by \( v = \sqrt{\mu_0\varepsilon_0} \) in vacuum — Not true. The speed of electromagnetic waves in vacuum is \( c = \displaystyle\frac{1}{\sqrt{\mu_0\varepsilon_0}} \approx 3\times10^{8}\,\mathrm{m\,s^{-1}} \); the expression \( \sqrt{\mu_0\varepsilon_0} \) is its reciprocal and has the dimension of \( \mathrm{s\,m^{-1}} \), not of speed.
Option (4) They do not require material medium to propagate — True. The changing electric and magnetic fields sustain each other, so the waves travel through vacuum.
NCERT (2026-27):
Nature of Electromagnetic Waves – “… Using Eqs. [8.7(a) and (b)] for Ex and By and Maxwell’s equations, one finds that ω = ck, where,
\( \displaystyle c = 1/\sqrt{\mu_0 \varepsilon_0} \qquad [8.9(a)] \)”
Nature of Electromagnetic Waves – “… They differ from all the other waves we have studied so far, in respect that no material medium is involved in the vibrations of the electric and magnetic fields.”