Mirrors and images

Characteristics of the image formed when an object is placed before a plane mirror.

• Image size is same as the size of the object. Magnification factor is one.
• Distance of image from mirror is same as the distance of object from mirror.
• Image is upright and not inverted.
• Image appears left and right reversed.
• Image is virtual. It appears like it is formed in the back of the mirror where the light can not even reach.

Single mirror produces one image. When an object is placed between two mirrors, there will be more than one image. When the mirrors are placed together to form a right angle, then the number of images formed is three - one in each mirror and one in the crease. When the mirrors are placed with 60 degrees, then the number of images formed is five.  When the angle is theeta, the number of images will be (360/theeta -1). 

The more accurate formula depends on the angle and the position of the object.  This was from a paper by V.M. Kulkarni in 1960.

• When 180/theeta is integer x, then the answer is 2*x - 1,
• When 180/theeta is integer x + 0.5, then the answer is 2*x when the object is on angular bisector or symmetrically located with respect to two mirrors or 2*x + 1 otherwise.
• When 180/theeta is integer x + (n/q), then the answer is 2*x or 2*x +1 depending on whether the object is located on central angular sector of (q-2*n) about the angle bisector.
• When 180/theeta is ineger x + (n/q), then the answer is 2*x or 2*x +1 depending on whether the object is located on central angular bisector of (2*n-1) about the angle bisector.

References
Light from Physics Classroom
Patterns in Multiple Reflections