In languages that distinguish definiteness (e.g. English) usually if you’re talking about a “kind of thing”, you can use either the definite or indefinite form and make sense. Only if you’re talking about a specific thing does the distinction matter: “a mirror” = a mirror I’m now introducing and you don’t know about yet, “the mirror” = the mirror we talked about before and you already know about; but either form can mean “mirrors in general”. There are slight stylistic differences what’s preferred in what contexts depending on the language, but in German too you can say “in den Spiegel schauen”.
In languages that distinguish definiteness (e.g. English) usually if you’re talking about a “kind of thing”, you can use either the definite or indefinite form and make sense. Only if you’re talking about a specific thing does the distinction matter: “a mirror” = a mirror I’m now introducing and you don’t know about yet, “the mirror” = the mirror we talked about before and you already know about; but either form can mean “mirrors in general”. There are slight stylistic differences what’s preferred in what contexts depending on the language, but in German too you can say “in den Spiegel schauen”.