# Speed of Light in Vacuum, Glass and Water

As a child, looking at the sun rays that come through one of the glass tile among the tiled roof of our ancestral home I have wondered how they reach to the earth! More than finding a scientific answer, I was curious to imagine and mix the presence of mythological gods with those light rays. Sun is a God in epics of many countries and He was one of heroes in my childhood stories and dramas we played. Sun being the provider of light and thus the source of energy in our planet can not less than a God!

Light travels with the highest known speed in the universe. Some years back, I heard that a scientist Mr. Sudarshan had put forward the theory of Tachyons that travels faster than light. Our professor told, if the theory of Tachyons is proved to be correct, it may shake some of Einstein’s principles. I did not concentrate much on that direction as it has never proved or did not shake any of Einstein’s principles.

The sunlight takes around 8 minutes and 18 seconds to reach to the earth. So if we see sunlight at 6’O clock in the morning that light should have started its journey from sun before 6’O clock, exactly at 5:51:42 AM. This was told to us by teachers to understand the speed of light. But in actual sunlight is there always in the universe. Sun continuously emits light. But the light particles originated at sun takes around 8 minutes and 18 seconds to reach to the earth.

Michelson and Morley experimented with light to find out the speed of light. He installed many mirrors and sent light from the first mirror. The light travelled to the last mirror by reflecting at many mirrors. Thus the length the light needed to travel was sufficiently enough to ensure accuracy of the measurement. He found that the light travels at a speed of 298,160 kilometers per second with a variation of around 48 kilometers per second.

The speed of light in vacuum is a universal physical constant and is calculated exactly as 299792.458 kilometers per second, According to special relativity, this is the maximum speed at which all matter and information in the universe can travel. It is the speed at which all mass-less particles and changes of the associated fields including electromagnetic radiation such as light and gravitational waves travel in vacuum. There are many new theories proposed in the scientific world to claim that there are particles that can travel at speeds higher than that of light. For example, E.C George Sudarshan’s theory of Tachyons

If we know the speed of light in vacuum and the refractive index of a material it easy to calculate the speed of light in that material. A silica glass having a refractive index of 1.46 will allow the light to travel at as speed of 205,337.3 kilometers per second. Speed of light in water (refractive index of 1.33) will be 225,407.86 kilometers per second.

#### Author: Praveen Gupta

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