speed of light

noun phrase

: a fundamental physical constant that is the speed at which electromagnetic radiation propagates in a vacuum and that has a value fixed by international convention of 299,792,458 meters per second
symbol c

Examples of speed of light in a Sentence

Recent Examples on the Web
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For nearby objects, the speed of light is great enough that the difference in time between source-and-observer can be neglected in most cases. Ethan Siegel, Big Think, 2 Oct. 2025 While the Joliot-Curies were bombarding their samples with alpha particles at relatively low speeds, the Michigan State facility can accelerate stable isotopes up to half the speed of light and smash them on a target to produce new radioisotopes. Andrea Richard, The Conversation, 2 Oct. 2025 The study shows how scientists can essentially turn fiber optic cables into seismometers that return detailed data about earthquakes at the speed of light. Evan Bush, NBC news, 25 Sep. 2025 Additionally, this quasar is also producing jets of particles moving at nearly the speed of light, a rare feature among quasars. Rupendra Brahambhatt, Interesting Engineering, 21 Sep. 2025 See All Example Sentences for speed of light

Word History

First Known Use

1823, in the meaning defined above

Time Traveler
The first known use of speed of light was in 1823

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Cite this Entry

“Speed of light.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/speed%20of%20light. Accessed 16 Oct. 2025.

Kids Definition

speed of light

: a fundamental physical constant that is the speed at which electromagnetic radiation travels in a vacuum and that has a value of 299,792,458 meters per second

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