The prepositional phrase in this sentence is "difficult to understand." It is an adjectival phrase because it gives information about a noun ("physics"), the subject of the sentence.
A prepositional phrase is either used to give information about a verb or a noun in the sentence. It must contain a preposition (like in, to, of, about, at, before, by, during, for, from, over, with, etc.), the object of the preposition, and possibly, words which modify the object.
- It is an adjectival phrase if it modifies a noun (I can't reach the toy on the highest shelf.)
- It is an adverbial phrase if it modifies a verb (He approached with hesitation.)
- It is a noun phrase if it revolves around a subject, like a noun or pronoun (We come from various different backgrounds.)
