We use the passive voiceThe form of the transitive verb that shows whether the subject acts (active voice) or is acted upon (passive voice). when it is obvious who performs the actionA general term expressing that somebody or something does something. In this case the agentThe doer of an action; the person or the thing that does something. E.g.: True Romance was written by Tarantino. ("Tarantino") is usually not mentioned in the passive sentence.

The murderer has been arrested. (i.e. by the police)

Pictures

Her figure was reflected in the nearby pool.

Passive voice: generalised agent
Uses of passive voice Passive voice: impersonality