Mirrors reflect light. Objects are reflected in mirrors because light reflects off them, into mirrors.
When light hits a mirror at a particular angle, it reflects back at the same angle, but opposite. The angle that the light hits the mirror is called the angle of incidence, and the angle that it reflects back is called the angle of reflection.
This means that when we look at a mirror straight on, we see our own reflection, because our own image hits the mirror at 90°, and reflects back to us at 90°. When we look at a mirror at an angle, however, we may not see our own reflection. For example, if we look at a mirror at a 45° angle, we will see what is at 45° on the opposite side of the mirror.
In films and TV, we often see actors looking in mirrors. We can see the actor and their reflection, but not the reflection of the camera. This is not fancy editing, removing the camera from the image after filming, but is to do with the angle of the mirror. (In straight on shots, when we see the back of the actor’s head and the mirror image, the camera would be in shot and would need to be edited out afterwards.)
Often, in these shots, the actor wouldn’t actually be able to see their own reflection. The mirror is angled such that the actor can see the camera, and the camera can see the actor in the mirror. Neither the camera nor the actor can see themselves.
Before electric lighting, mirrors were sometimes used to light rooms. A network of mirrors could be set up in a house, such that light reflected back and forth from outside, to inside, illuminating a room with natural sunlight.
Mirrors have also been used to illuminate entire towns. Viganella, a town in Italy, is located in a steep valley and does not receive any natural sunlight for seven weeks in winter. In 2006, a giant mirror was installed to reflect sunlight into the town centre. The mirror is computer controlled to follow the movement of the sun and cost €100,000 to build.