I am doing the Plane Mirror Lab in physics class. I place a pin a distance of 4.9 cm from a plane mirror. How far behind the mirror can the image be expected to appear?
4.9 cm. Have a look:
Your eye "sees" a virtual image behind the mirror surface formed by the extension (dotted) of the real beams.
Each beam arrives to your eye after obeying the laws of reflection at the surface of the mirror ( such as: angle of incidence=angle of reflection).