Question
The speed of anything depends upon the medium in which they are traveling. According to Einstein, an electromagnetic wave has the maximum speed and no other thing can cross its speed. This is the ultimate speed, which is equal to $299,792,458 \ ms^{-1}$. Now since light is that part of the electromagnetic spectrum, which is visible to us, light is the fastest among all other options.
The speed of light in a vacuum is $299,792,458 \ ms^{-1}$ which is also denoted by ‘c’. Note that it’s important to mention the medium ‘vacuum’ here as the speed of light depends upon the medium in which it is traveling. Every medium has a unique property called its refractive index. Refractive index ($\mu$) is the ratio of speed of light in vacuum to that in that medium $\left(\mu=\dfrac{c}v \right)$ and hence $\mu$>1 and equals 1 for vacuum.
Note: In case of air, $\mu$= 1.0003 hence the speed of light in air is: $v=\dfrac{299,792,458}{1.0003}=299,702,547 ms^{-1}$
We can see that the speed of light in both air and vacuum is almost the same. Hence we can take it equal to $3\times 10^8 ms^{-1}$ for easier calculations, which won’t affect our results much. This is the reason why we take the speed of light in the air as $3\times 10^8 ms^{-1}$.