Question

The velocity of light in Vaccum is 3product 10power10 cm/s .The velocity of light in a medium refrctive index is 1.5

Answer 1

heard these statements before? They are often quoted as results of Einstein's theory of relativity. Unfortunately, these statements are somewhat misleading. Let's add a few words to them to clarify. "Nothing can travel faster than the speed of light in a vacuum." "Light in a vacuum always travels at the same speed."  Those additional three words in a vacuum are very important. A vacuum is a region with no matter in it. So a vacuum would not contain any dust particles (unlike a vacuum cleaner, which is generally full of dust particles).



Light traveling through anything other than a perfect vacuum will scatter off off whatever particles exist, as illustrated below.

 

In vacuum the speed of light is

c = 2.99792458 x 108 m/s 
 

This vacuum speed of light, c, is what the statements from relativity describe. Whenever light is in a vacuum, its speed has that exact value, no matter who measures it. Even if the vacuum is inside a box in a rocket traveling away from earth, both an astronaut in the rocket and a hypothetical observer on earth will measure the speed of light moving through that box to be exactly c. No one will measure a faster speed. Indeed, c is the ultimate speed limit of the universe.