speed of light

[ speeduhv lahyt ]

noun

Physics, Optics. a fundamental universal constant, the speed at which light and all forms of electromagnetic radiation travel in a vacuum, standardized as 186,282.4 miles per second (299,792,458 meters per second): The speed of light, often represented by the letter c, figures prominently in modern physics, as in Einstein’s famous equation E = mc2, which expresses the relation between mass (m) and energy (E).
an extremely fast rate: They gobbled those appetizers up at the speed of light.

Origin of speed of light

First recorded in 1820–25

Also called light speed .

Words Nearby speed of light

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Cultural definitions for speed of light

speed of light

The distance light can travel in a unit of time through a given substance. Light travels through a vacuum at about 186,000 miles, or 300,000 kilometers, per second. (See E = mc2, electromagnetic waves, relativity, and twin paradox.)

Notes for speed of light

A light year, or the distance light can travel in a year, is over five trillion miles.

Notes for speed of light

Light from the sun takes about eight minutes to reach the Earth.

Notes for speed of light

Light from the moon, and other electromagnetic radiation from the moon, takes about a second and a half to reach the Earth. In conversations between astronauts on the moon and their ground crews, there are lapses of about three seconds between exchanges, because of the time it takes for radio waves to make a round trip between the Earth and the moon.

Notes for speed of light

The special theory of relativity states that the speed of light as measured by all observers is the same.

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