"What is the speed of light?" is a lot more natural in English. "What is the limit ..." requires a twisted story about two researchers trying to get an upper or lower bound on the velocity.
In English the science behind this is wrong there is no limit of the speed of light it is a constant at least in traditional science.
Why is it "da" instead of "de" here?
Because we say "the light" and not just "light"
Why? And when do we say "the [noun]" and when do we not we say "the [noun]" and how do we know? Or do we just have to stumble through Portuguese one noun at a time figuring it out?
Most of the time, the definite article is used, except when the noun is the subject of the sentence.