Questions & Answers

Question:- Why does light travel at approximately 300,000 km/s?

Bruce Dickson Answers:-

Val's question is quite a subtle one.

Gamma rays, x-rays, light, infrared, microwaves and radio waves are all electromagnetic (em) disturbances. As an em wave propagates, energy in its electric and magnetic fields sloshes back and forth. The speed of light is determined by how quickly this can take place.

Two constants are used to describe this behavior - permittivity ( e ) and permeability ( m ). Permittivity tells you how easily a charge can be developed by an electric field, while permeability measures how easily it can be magnetically polarised. The speed of light is obtained from:- .

Permittivity and permeability are usually normalised to the permittivity and permeability of the vacuum

and . We write and . In a vacuum, and so that the speed of light is given by .   In SI units, this gives us .   Notice I've used its special symbol "c".

For light moving through other materials we find that 1< e _r < 12 and m _r = 1 , although e _r and m _r vary a great deal in other parts of the electromagnetic spectrum. is called the "refractive index", and uses the symbol "n". The speed of light in other materials is therefore closely approximated by . Since n is always greater than 1, the speed of light in some medium is always less than the speed of light in a vacuum.