"Light reflects off mirror" in Chinese
When light, heat, or other rays reflect off a surface or when a
  surface reflects them, they are sent back FROM the surface and do not
  pass through it.


“Light reflects off mirror.” means 光线从镜子反射出来/过来/回来/开来. "Off" denotes the sense of 'from' in this case. 

Compared to this sentence:


  The sun reflected off the snow-covered mountains. // 阳光从被雪覆盖的山峦反射回来。


As a sidenote, from English perspective, maybe it's better to put: 


  Light reflects off mirrors. Or, Light reflects off the/that mirror.


But I am not a native English speaker. It's all opinion-based. Hope some English native speakers can correct me if I'm wrong. Thanks!