Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The speed of electromagnetic waves in vacuum is used to define a standard of length.
Reason (R): The constancy of the speed of electromagnetic waves in vacuum is strongly supported by experiments, and its value is known very accurately.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A) The speed of electromagnetic waves in vacuum is used to define a standard of length. — True. The speed of electromagnetic waves in free space is a fundamental constant. So a length can be fixed as the distance these waves cover in vacuum in a given time.
Reason (R) The constancy of the speed of electromagnetic waves in vacuum is strongly supported by experiments, and its value is known very accurately. — True. Measurements on waves of different wavelengths give the same speed, to within a few metres per second out of \( 3\times10^{8}\,\mathrm{m\,s^{-1}} \). The value itself is now very well known.
Why R explains A?
A standard has to give the same result whatever is used to measure it. The speed in vacuum does not depend on the wavelength, so every kind of electromagnetic wave gives the same value.
The value is also known so precisely that a length worked out from it is as exact as the time measured. This constancy and accuracy are the reason the speed serves as a standard of length.
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. The constancy of the velocity of em waves in vacuum is so strongly supported by experiments and the actual value is so well known now that this is used to define a standard of length.”