Image Formation for Plane Mirrors
To view an object in any type of mirror, a person must sight along a line at the image of the object. All persons capable of seeing the image must sight along a line of sight directed towards the precise image location. As a person sights in a mirror at the image of an object, there will be reflected rays of light coming from the mirror to that person's eye. The origin of this light ray is the object. A multitude of light rays from the object are incident on the mirror in a variety of directions. Yet as you sight at the image, only a small portion of these rays will reflect off the mirror and travel to your eye. To see an object in a mirror, you must sight at the image; and when you do reflected rays of light will travel from the mirror to your eye along your line of sight.
Not all people who are viewing the object in the mirror will sight along the same geometrical line of sight. The precise direction of the sight line depends on the location of the object, the location of the person, and the type of mirror. Yet all of the lines of sight, regardless of their direction, will pass through the image location. In fact, the image location is defined as the location where it seems to every observer as though light is coming from. Since all people see reflected rays of light as they sight at an image in the mirror, then the image location must be the intersection point of these reflected rays.
In the animation above, an object is positioned in front of a plane mirror. The plane mirror will produce an image of the object on the opposite side of the mirror. The distance from the object to the mirror equals the distance from the image to the mirror. Any person viewing this image must sight at this image location. The animation depicts the path of several rays of light from the object to the mirror. This light subsequently reflects such that observers could sight along a line of sight and view the image. Different people might sight from different locations; yet each person would sight at the same image location. As seen in the animation, the image location is the intersection point of all the reflected rays.
For more information on the ray nature of light, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
The Role of Light to Sight
The Line of Sight
The Law of Reflection
Why is an Image Formed?
Image Characteristics for Plane Mirrors
Ray Diagrams for Plane Mirrors
What Portion of a Mirror is Required to View an Image?