About the reflection of light....

How an image is formed in the plane mirror can best be illustrated by a ray diagram. The light wave emitted by an object when striking the mirror will obey the law of relection. You just draw two rays from the top of the object and two rays from the bottom of the object and you can see how the image is formed.. It is the characteristics of light rays (obeying the law of reflection) which make the image erect.

As the image formed in the plane mirror is virtual.
All virtual image are erect and behind the mirror.

Also, it can be showed by the ray diagram.