What is the speed of light in vacuum and what is special about it?

What is dispersion of light?

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in terms of the new unit if light takes 8 min and 20 s to cover this distance ?

Light enters from air to diamond having refractive index 2.4. what is the speed of light in diamond ? Given speed of light in vacuum =3xx10^8 m s^-1 .