The value of **velocity of light** in vacuum is of great importance in science. The following are some of the important fields where the value of velocity of light is used.

**(1) Frequency – wavelength relation:** From the relation c = vλ, the frequency of electromagnetic radiations can be calculated if the wavelength is known and vice versa.

**(2) Relativistic mass variation with velocity:** Theory of relativity has shown that the mass m of a moving particle varies with its velocity v, according to the relation m = m_{0}/√{1 –(v^{2}/c^{2})}.

Here m_{0} is the rest mass of the particle.

**(3) Mass – Energy relation:** E = mc^{2} represents conversion of mass into energy and energy into mass. The energy released in nuclear fission and fusion is calculated using this relation.

**(4) Measurement of large distance in Astronomy:** Light year is a unit of distance used in astronomy. A light year is the distance travelled by light in one year. It is equal to 9.46 x 10^{15} metre.

**(5) Refractive index:** The refractive index μ of a medium is given by

μ = [velocity of light in vacuum / velocity of light in medium] = c/v