Where does this come from, and can I check it?

Light in glass runs at about two thirds of its vacuum speed, setting a floor on any message crossing an ocean. The figure is taken from Britannica, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.

Is this really a technology fact?

Yes. It sits on the Technology shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.

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The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.

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No internet link can beat the speed of light

Light in glass runs at about two thirds of its vacuum speed, setting a floor on any message crossing an ocean In a vacuum light covers 300 kilometres per millisecond; inside silica fibre the refractive index slows it to roughly 200 kilometres per millisecond. A message from London to New York, some 5,600 kilometres apart, therefore needs at least 28 milliseconds each way before any electronics are counted. Trading firms pay for unusually straight cable routes because every extra kilometre adds delay. It sits in Mondo's technology collection, one of 200 facts that each trace back to the reference below.

Where this comes from

Britannica

Facts that can change over time are reviewed and updated. If something here looks wrong, Mondo would rather know than be confidently incorrect.

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Tags: latency, fibre-optics, physics

No internet link can beat the speed of light: the short answers

Where does this come from, and can I check it?: Light in glass runs at about two thirds of its vacuum speed, setting a floor on any message crossing an ocean. The figure is taken from Britannica, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
Is this really a technology fact?: Yes. It sits on the Technology shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
What else can I read about it?: The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.