The physical quantity which is same for ultraviolet rays and red light travelling in free space is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Speed
All electromagnetic waves, whatever their frequency or wavelength, travel in free space with the same speed c, fixed by the permeability and permittivity of free space.
\( \displaystyle c = \frac{1}{\sqrt{\mu_0\varepsilon_0}} \)
\( \displaystyle c = 3\times10^{8}\,\mathrm{m\,s^{-1}} \)
Ultraviolet rays and red light differ only in frequency and wavelength (\( c = \nu\lambda \)).
NCERT (2026-27):
Points to Ponder – “… The basic difference between various types of electromagnetic waves lies in their wavelengths or frequencies since all of them travel through vacuum with the same speed. Consequently, the waves differ considerably in their mode of interaction with matter.”