Light - Speed

Light has long been known to be part of a group of waves, called electromagnetic waves, and is one of the characteristics that gather this group at its speed of propagation.

The speed of light in a vacuum, but which actually applies to many other electromagnetic phenomena such as x-rays, gamma rays, radio and tv waves, is characterized by the letter ç, and has an approximate value of 300 thousand kilometers per second, that is:

However, in material environments, light behaves differently as it interacts with matter in the medium. In either of these ways the speed of light v it's smaller than ç.

In environments other than vacuum, the speed also decreases as the frequency increases. Thus the speed of red light is greater than the speed of violet light, for example.

Absolute Refractive Index

For the complete understanding of refraction it is convenient to introduce a new quantity that relates the velocity of monochromatic radiation in vacuum and material media, this quantity is the refractive index of monochromatic light in the presented medium, and is expressed by:

Where no is the absolute refractive index in the middle, being a dimensionless quantity.

It is important to note that the absolute refractive index can never be less than 1 since the highest possible speed in a medium is ç, if the medium considered is the vacuum itself.

For all other material means no is always greater than 1.

Some usual refractive indices:

Material no
Dry air (0 ° C, 1atm) ≈ 1 (1,000292)
Carbon dioxide (0 ° C, 1atm)

≈ 1 (1,00045)

Ice (-8 ° C) 1,310
Water (20 ° C) 1,333
Ethanol (20 ° C) 1,362
Carbon tetrachloride 1,466
Glycerin 1,470
Monochlorobenzene 1,527
Glasses from 1.4 to 1.7
Diamond 2,417
Antimony Sulfide 2,7

Relative refractive index between two media

Relative refractive index between two means is the relationship between the absolute refractive indices of each medium, so that:

But as seen:

Then we can write:

That is:

Note that the relative refractive index between two means can have any positive value, including less than or equal to 1.


We say that one medium is more refreshing than another when its refractive index is higher than the other. That is, ethanol is more refreshing than water.

Otherwise, we can say that one medium is more refreshing than another when light travels through it at a slower speed than the other.