The light's path or light's path? Can I use articles with apostrophes?

Question

I've been told that "The Dirac's equation" is not grammatically acceptable. The correct should be "Dirac's equation" or "The Dirac equation".

I'm wondering if this has something to do with Dirac being the name of someone or if this is a general rule. My case, specifically, concerns the phrase I used in the title. Which one is the correct version:

a - "(...) the correction of the light's path"

b - "(...) the correction of light's path"

c - "(...) the correction of the light path"

What I mean is the path that a ray of light follows.

Thank you very much.

Answer 1

Yes, you can. The important thing to remember (and the reason why "The Dirac's equation" is not acceptable) is that when you place an article before a possessive construction [Noun 1]'s [Noun 2], it always goes with the first noun rather than the second. The second noun does not need and cannot take an article. So you cannot say "the Dirac's equation" because that would mean "the equation of the Dirac." (And we don't use the before names.)

With "the light's path," on the other hand, "light" is a common noun, and so you should use the definite article with it when appropriate. In this context, it should be definite, so you do need the article.

Therfore, we would say "the light's path," meaning "the path of the light."

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In fact, this answer is not complete: there are some exceptions to the rule that I describe in the first paragraph. Read this question to see some examples: articles with the possessive nouns in the plural