Equal to the distance of the object in front.
Explanation
A plane mirror forms an image based on the laws of reflection. The image produced by a plane mirror is always virtual, erect, laterally inverted, and of the same size as the object. A defining characteristic of this image formation is the geometric relationship between the position of the object and the position of the image relative to the mirror surface.
Detailed Analysis
When an object is placed in front of a plane mirror, light rays from the object strike the mirror and reflect according to the laws of reflection (angle of incidence equals angle of reflection). When these reflected rays are extended backward, they appear to meet at a point behind the mirror, forming a virtual image.
Geometrically, the perpendicular distance from the object to the mirror is exactly equal to the perpendicular distance from the image to the mirror. Therefore, if an object is placed at a distance d in front of the mirror, the image is formed at a distance d behind the mirror.
Key Takeaway: In a plane mirror, the image is formed as far behind the mirror as the object is in front of it (Object Distance = Image Distance).