When is the speed of light a constant 299,792,458 meters per second

The speed of light is constant in vacuum and is equal to 299,792,258 meters per second.

Explanation:

The whole topic of constant speed of light is a bit misleading. The question that arises is:

Is the speed of light is constant everywhere? and if it is not constant everywhere, where does it change?

If we are talking about the interstellar space, the speed of light is constant everywhere in vacuum because astronomers do not need to consider speed of light in any media.

However, the speed of light changes when it goes from one medium to another. Like when light enters in a denser medium, its frequency remains constant but the wavelength decreases which is also the reason behind bending of light.

We know that the speed of light 'v' is given as:

v = fλ              where f = frequency and λ = wavelength

This equation implies that constant frequency and decrease in wavelength means decreased speed.

Similarly, the speed of light will increase when light travels from denser medium to a lighter medium as wavelength increases and frequency remains constant. As there is no medium lighter than vacuum, the speed of light is maximum there and is always constant.