Which of the following statement is not true for the electromagnetic waves?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → These waves travel at different speeds in vacuum depending upon their frequency
In vacuum all electromagnetic waves travel with the same speed \( c = 1/\sqrt{\mu_0\varepsilon_0} = 3\times10^{8}\,\mathrm{m\,s^{-1}} \), whatever their frequency. In a material medium the speed is \( c/\mu \), and \( \mu \) depends on the frequency.
Option (1) These waves have momentum — True. An electromagnetic wave carrying energy \( U \) also carries momentum, \( U/c \); this is why light exerts radiation pressure.
Option (2) These waves travel at different speeds in vacuum depending upon their frequency — Not true. The speed in vacuum depends only on \( \mu_0 \) and \( \varepsilon_0 \), so it is the same for every frequency.
Option (3) These waves transport energy — True. The oscillating electric and magnetic fields carry energy along the direction of travel; sunlight warming the earth is an example.
Option (4) These waves travel at different speeds in a medium depending upon their frequency. — True. In a medium the refractive index changes with frequency, which is the cause of dispersion of white light by a prism.
NCERT (2026-27):
Nature of Electromagnetic Waves – “… The velocity of electromagnetic waves in free space or vacuum is an important fundamental constant. It has been shown by experiments on electromagnetic waves of different wavelengths that this velocity is the same (independent of wavelength) to within a few metres per second, out of a value of 3×108 m/s.”