retroreflector
noun
ret·ro·re·flec·tor ˌre-trō-ri-ˈflek-tər 
: a device that reflects radiation (such as light) so that the paths of the reflected rays are parallel to those of the incident rays
Example Sentences
Recent Examples on the Web Instead, a retroreflector directs the light back towards the source. 
Rhett Allain, Wired, 12 Sep. 2021 Its glittering ‘eyes’ were the glass retroreflector beads destined to snoop on unsuspecting targets. David Hambling, Popular Mechanics, 18 Feb. 2020 In practice, these retroreflectors acted as a remote microphone to eavesdrop on any conversation. David Hambling, Popular Mechanics, 18 Feb. 2020
Rhett Allain, Wired, 12 Sep. 2021 Its glittering ‘eyes’ were the glass retroreflector beads destined to snoop on unsuspecting targets. David Hambling, Popular Mechanics, 18 Feb. 2020 In practice, these retroreflectors acted as a remote microphone to eavesdrop on any conversation. David Hambling, Popular Mechanics, 18 Feb. 2020 Word History
First Known Use
1946, in the meaning defined above