The ratio of speeds of X-rays of wavelength \( 10^{-9} \) m and visible light of wavelength \( 10^{-7} \) m in vacuum is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1 : 1
X-rays and visible light are both electromagnetic waves, and all electromagnetic waves travel in vacuum with the same speed \( c = 1/\sqrt{\mu_0 \varepsilon_0} = 3\times10^{8}\,\mathrm{m\,s^{-1}} \), whatever their wavelength.
The wavelength changes the frequency (\( c = \nu\lambda \)), not the speed: the X-rays have 100 times the frequency of the visible light, but the ratio of the speeds is \( 1 : 1 \).
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.”