We use "the" when there's only one of something but why do we use "Speed of light" instead of "Speed of the light"?

  • Oh but we don't use "the" when there's only one of something. Or do you always address your mother as "the mother"? – RegDwigнt Jul 16 '15 at 14:28
  • And we don't capitalize it. – Drew Jul 16 '15 at 14:30
  • @ RegDwigнt♦- source of this claim is Cambridge English Grammar in Use. – Rwy5 Jul 16 '15 at 14:39
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    When you're speaking of 'light' in general (all light), it is 'the speed of light'. In some very particular context where you might be talking about the light of a lamp, you might say appropriately 'the speed of the light (from the lamp)'. So you can say both, but if you're speaking scientifically about the general phenomenon of light, then it's 'the speed of light'. – Mitch Jul 16 '15 at 14:51

The "speed of light" is a term of art in physics meaning the speed of electromagnetic radiation in a vacuum. The definite article isn't used because that singular speed applies to all such radiation. It is possible to talk about a specific instance of light, however:

The speed of the light injected into a Bose-Einstein Condensate (BEC) can be slowed to a tiny fraction of the speed of light in a vacuum.

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  • @AndyT Read the question and my answer again. The OP is under the impression that there is only one speed of light, so he's puzzled why the definite article isn't applied to the word "light." – deadrat Jul 16 '15 at 15:58
  • @AndyT Yes, and as my answer indicates, we would use "the light" if there was only one light. For instance the only light injected into the apparatus containing a Bose-Einstein Condensate. Did you miss "It is possible to talk about a specific instance of light"? What's so hard about this? – deadrat Jul 16 '15 at 18:09

In fact we use "the" with "speed" because there is only one speed of light. [Note to physicists: I'm aware that there's more to it than this but we're in a language forum!]

Therefore we talk about "The speed of light."

With regard to "light" versus "the light", it's a lot more complicated. Here's my simplistic answer:

We say "I like to drink water." Now you might argue that there is only one water and it is spread around the universe in pockets. However "I like to drink the water" refers to a particular body of water. Example: "I like to drink the water at my local spa."

Similarly "Light moves." refers to the ubiquitous electromagnetic phenomenon, whereas "The light moves." would indicate that we are referring to a particular source of illumination.

I suggest you search online for the many excellent EFL resources that discuss the use of the definite article.

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The "Speed of Light" is a universal constant that describes the speed of light in a vacuum.

The "Speed of the Light" would refer to a specific light's speed.

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It really has to do with what we are speaking of. The use of the word 'the' as a definite article, on a grammatical sense, tells us that we are speaking of something specifically.

In this case, the lack of a 'the' speaks of the 'light' in a general sense. We use 'the' when we speak of one thing because the definite article is necessary to convey that we are speaking of this specific one thing.

So, for the speed of light (in general), we say 'the speed of light'. For the speed of a certain light in a certain medium, we would say 'the speed of the light'.

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