The four characteristics of a plane mirror image that you have to know. A good way of explaining the (quite) confusing concept of real and virtual images. 

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An excerpt:

In the case of plane mirrors, the image is said to be a virtual image. Virtual images are images which are formed in locations where light does not actually reach. Light does not actually pass through the location on the other side of the mirror; it only appears to an observer as though the light is coming from this location. Whenever a mirror (whether a plane mirror or otherwise) creates an image which is virtual, it will be located behind the mirror where light does not really come from. Later in this unit, we will study instances in which real images are formed by curved mirrors. Such images are formed on the same side of the mirror as the object and light passes through the actual image location.

Besides the fact that plane mirror images are virtual, there are several other characteristics which are worth noting. The second characteristic has to do with the orientation of the image. If you view an image of yourself in a plane mirror (perhaps a bathroom mirror), you will quickly notice that there is an apparent left-right reversal of the image. That is, if you raise your left hand, you will notice that the image raises what would seem to be it’s right hand. If you raise your right hand, the image raises what would seem to be its left hand. This is often termed left-right reversal. This characteristic becomes even more obvious if you wear a shirt with lettering. For example, a shirt displaying the word “NIKE” will read “EKIN” when viewed in the mirror; a shirt displaying the word “ILLINOIS” will read “SIONILLI;” and a shirt displaying the word “BOB” will read “BOB.”

A third characteristic of plane mirror images pertains to the relationship between the object’s distance to the mirror and the image’s distance to the mirror. For plane mirrors, the object distance (often represented by the symbol d_o) is equal to the image distance (often represented by the symbol d_i). That is the image is the same distance behind the mirror as the object is in front of the mirror. If you stand a distance of 2 meters from a plane mirror, you must focus at a location 2 meters behind the mirror in order to view your image.

A fourth and final characteristic of plane mirror images is that the dimensions of the image are the same as the dimensions of the object. If a 1.6-meter tall person stands in front of a mirror, he/she will see an image which is 1.6-meters tall. If a penny with a diameter of 18-mm is placed in front of a plane mirror, the image of the penny has a diameter of 18 mm. The ratio of the image dimensions to the object dimensions is termed the magnification. Plane mirrors produce images which have a magnification of 1.