Language of the cosmos: Speed of light

The speed of light in vacuum, commonly denoted

c

, is a universal physical constant important in many areas of physics. Its value is exactly 299792458792458 metres per second, as the length of the metre is defined from this constant and the international standard for time. According to special relativity,

c

is the maximum speed at which all matter and information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in vacuum. Such particles and waves travel at

c

regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity,

c

interrelates space and time, and also appears in the famous equation of mass–energy equivalence

E

mc 2

The speed at which light propagates through transparent materials, such as glass or air, is less than

c

. The ratio between

c

and the speed

v

at which light travels in a material is called the refractive index

n

of the material (

n

c

v

). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at

c

/ 1.5 ≈ 200000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 299700700 km/s or 90 km/s slower than

c

Actually when you see an object it the space literally you are seeing the past of object. The time of the object depends on how far the object is and how much time the light from the object is.

Eg: Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth. Then the sun you are seeing is literally 8mins and 17 sec in the past