Learn French: Le faisceau de ce vieux phare est puissant. - The beam of this old lighthouse is powerful.

It means both! Originally meaning "lighthouse," the word phare was later adopted for car "headlights" because they also project a beam of light. You will usually rely on context to tell them apart.

Why is vieux used here instead of vieil?

You only use vieil before a masculine singular noun starting with a vowel or a mute h (like un vieil ami). Because phare starts with a consonant, it takes the standard masculine singular form vieux. It comes before the noun because it describes age, which is one of the common adjective categories that precede nouns in French.

Is faisceau only used for beams of light?

Not exclusively. While it frequently means a beam of light, the word generally refers to a "bundle" or "group" of things converging. At this level, you might encounter the useful expression un faisceau d'indices, which translates to a "body of evidence" or a "bundle of clues" in an investigation.

If I wanted to practice this lesson's grammar by adding "even if it is old" to this sentence, would I use the subjunctive?

No. The most important rule for même si (even if / even though) is that it is always followed by the indicative mood, never the subjunctive. You would say, "Le faisceau de ce phare est puissant, même s'il est vieux." Notice that si and il must contract into s'il.