Learn Russian: Учёный меняет направление света посредством этой линзы. - The scientist changes the direction of the light by means of this lens.

The formal preposition посредством means "by means of" or "via" and always requires the genitive case. Here, it changes эта линза to the genitive этой линзы. It is highly characteristic of scientific, technical, or formal writing.

Could I just use the instrumental case without a preposition here, like этой линзой?

Yes, you can. Saying меняет направление этой линзой is grammatically correct and means essentially the same thing. Using посредством is a stylistic choice that makes the sentence sound more academic and explicitly highlights the mechanism being used.

Could I use с помощью or при помощи instead of посредством?

Absolutely. С помощью этой линзы or при помощи этой линзы would also mean "by means of this lens" or "with the help of this lens" and both also require the genitive case. Посредством is just the most formal option among these compound prepositions.

Why does the noun учёный have an adjective ending?

Учёный is a substantivated adjective. This means it started as an adjective but now functions entirely as a noun (literally a "learned" person). Even though it acts as the subject of the sentence here, it still declines exactly like a standard masculine adjective.