Question

Speed of light in vacuum is

• A. highest attainable speed
• B. attainable by small mass particles in vacuum
• C. attainable by dust particles in interstellar space
• D. None of the above

Answer 1

Answer: (A)

Light travels with a speed of $c=2.99792458\times 10^{8}m{s}^{-1}$.
For many purposes, it suffices to take $c=3\times 10^{8}m{s}^{-1}$.
The speed of light in vacuum is the highest speed attainable in nature.