A plane mirror generally forms a virtual image. Under what conditions it can form a real image?
A virtual image is one that can be seen, and a real image can be projected. Although we usually use plane mirrors for the purpose of producting virtual images, that is being able to see an object in the mirror. It possible for a plane mirror to produce a real image in conjuction with another mirror. In order for a real image to be produced the mirror must bend the incident light rays in such a way that they converge to a single point. According to the law of reflection the angle of incidence of an incident ray equals the angle of reflection of a reflected ray, so parallel light rays are reflected back at the same angle producing a virtual image of a real object.
In order for a plane mirror to produce a real image ir must be reflecting the virtual image produced by another mirror or a lens. In this context we call the virtual image a virtual object. If the light rays coming from the virtual object converge at a point behind the plane mirror the reflected rays will converge in front of the mirror, forming a real image.
There are three types of mirrors. A plane mirror, a concave mirror, and a convex mirror.
Plane mirrors always produce virtual images, because they never focus light into a single converging point. Plane mirrors use perfect regular reflection, which creates an upright, virtual image. The mirror also keeps everything in correct proportional sizes. The only "weird" thing that a plane mirror does is reverse the image from left to right.
A convex mirror also always produces a virtual, upright image. But unlike a plane mirror, the image is always reduced in size.
A concave mirror is the only type of mirror that can produce a real and a virtual image. If the image is real, it will also be an inverted image. The size depends on the distance of the original object from the focal point of the mirror. If the image is virtual, it will be an upright and enlarged image.