When you see your eyes in the mirror, you will probably see the other person’s mouth through the gap. At the same time, the other person will be seeing his or her own eyes, but your mouth. You see a “composite” face made up of alternating horizontal bands of your own features and the other person’s features. The other person has a similar experience.
Your image in the mirror is the same distance “behind” the mirror as your actual face is in front of the mirror. To get your facial features to blend with those of the other person’s, therefore, that person must be just as far away from the mirror as you are, so that his or her face is in the same place as the virtual image of your face.
It may seem odd that your reflection appears to be behind the surface of a plane (flat) mirror rather than at the surface of the mirror. Why is this so?
Light bounces off the tip of your nose as you look into a mirror. That light reflects off the mirror and enters your eyes. Your eyes and brain work together to make a picture of the world. This eye-brain system assumes that the light has traveled in straight lines to reach your eyes. In order for the light to travel in a straight line to your eyes, your head would have to be behind the mirror.
The image below (click to enlarge) shows how, in a plane mirror, the image of a face is the same distance behind the mirror as the face is in front of the mirror.