What is the speed of light?
Answers
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s).[Note 3] It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second.[Note 4][3] According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any information can travel through coordinate space.
Speed of light
The distance from the Sun to the Earth is shown as 150 million kilometers, an approximate average. Sizes to scale.
Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth.
Exact values
metres per second
299792458
Approximate values (to three significant digits)
kilometres per hour
1080000000
miles per second
186000
miles per hour[1]
671000000
astronomical units per day
173[Note 1]
parsecs per year
0.307[Note 2]
Approximate light signal travel times
Distance
Time
one foot
1.0 ns
one metre
3.3 ns
from geostationary orbit to Earth
119 ms
the length of Earth's equator
134 ms
from Moon to Earth
1.3 s
from Sun to Earth (1 AU)
8.3 min
one light year
1.0 year
one parsec
3.26 years
from nearest star to Sun (1.3 pc)
4.2 years
from the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth
25000 years
across the Milky Way
100000 years
from the Andromeda Galaxy to Earth
2.5 million years
Explanation:
Speed of light in air is 3×10⁸ m/s and speed of light in common glass is 2*10⁸ m/s.
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