# Light Speed

### What is Light Speed?

The speed of light in vacuum or Light Speed, commonly denoted c, is a fundamental universal constant, the speed at which light and all forms of electromagnetic radiation travel in a vacuum.It is important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s or 186000 mi/s).

1 Light Speed =  299792458 metres per second
1 Light Speed = 1080000000 kilometres per hour
1 Light Speed = 186000 miles per second
1 Light Speed = 173 astronomical units per day
1 Light Speed =  0.307 parsecs per year

Symbol
c

Speed of light is now universally represented by symbol ‘c’. This symbol originated from the initial letter of the Latin word “celerity” meaning “swift” or “quick”.
This symbol was used by Weber and Kohlrausch in their papers in 1856.

Other name

The speed of light, Lightspeed

### History

In ancient times, many scientists believed the speed of light was infinite and could travel any distance instantaneously. Empedocles (c. 490–430 BC) was the first to propose a theory of light  and claimed that light has a finite speed.

The first significant attempt to measure speed of light was made by Galileo in 1638. Galileo Galilei proposed an experiment, with an apparent claim to having performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. He was unable to distinguish whether light travel was instantaneous or not, but concluded that if it were not, it must nevertheless be extraordinarily rapid.

Around 1676, Danish astronomer Ole Roemer became the first person to prove that light travels at a finite speed by studying the apparent motion of Jupiter's moon Io.
In the ensuing centuries, a number of other scientists worked to determine the speed of light and, using improved techniques, came up with increasingly accurate calculations.

French physicist Hippolyte Fizeau is credited with making the first non-astronomical measurement, in 1849, using a method that involved sending light through a rotating toothed wheel then reflecting it back with a mirror located a significant distance away.

One of the first precise calculations of light’s velocity was made in the 1920s by American physicist Albert Michelson, who carried out his research in the mountains of Southern California using an eight-sided rotating mirror apparatus. In 1983, an international commission on weights and measures set the speed of light in a vacuum at the calculation we use today: 299,792,458 meters per second (186,282 miles per second)—a speed that could circle the equator 7.5 times in a single second.

### The importance of light speed

The speed of light is a physical constant of our universe. It is neither important nor unimportant -- it just is what it is. However, knowing the numerical value of the speed of light is essential to understanding and predicting the behavior of our universe in many areas of physics such as electromagnetic wave propagation.

The speed of light was an important clue in the unification of electricity, magnetism and optics. It heralded the advent of modern physics, breaking the
shackles of absolute time and mechanistic view of the physical world. Indirectly, it has led to
the development of quantum electrodynamics, astrophysics and cosmology.