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