Question

What is the speed of light $c$ ?
(A) $c=3\times {10}^{-8}m{s}^{-1}$
(B) $c=3\times {10}^{8}km{s}^{-1}$
(C) $c=3\times {10}^{8}mm{s}^{-1}$
(D) $c=3\times {10}^{8}m{s}^{-1}$

Answer 1

Hint
We know that light travels a length of $299792458m$ in $1s$ in vacuum. From this knowledge we could easily find the speed of light.
$\Rightarrow v=s/t$
Where, $v$ is the speed, $s$ is the distance travelled and $t$ is the time taken.

Complete step by step answer
We know, Distance of travel of light, $s=299792458m$
Time taken to travel the length, $t=1s$
Now, By the known formula,
Speed of light, $c=v=s/t$
Now, Substituting the values of $s$ and $t$ in the above equation, we get $c=299792458/1m{s}^{-1}=299792458m{s}^{-1}$
After approximation, we get,
 $\Rightarrow c=300000000m{s^{-1}}_{}$
Converting this value in scientific notation, we get
 $\Rightarrow c=3\times {10}^{8}m{s}^{-1}$
Hence, the correct option is (D).

Additional Information
According to the theory of relativity, nothing in the existing universe can move greater than the speed of light (the value that we calculated). If by any means something does flow more than this speed, it will move backwards in time. Or in other words, it becomes eligible for time travel. Till today's existent studies, it is found that neutrinos are something that travels at a speed very close to the speed of light and some are assumed to move even faster, that means they are possibly time travelling.

Note
The distance we took for light travelling in a second is for vacuum. The distance or broadly, the speed of light in any other medium depends on the physical properties of the medium.