Multiple Plane Mirrors: How Many Images Will Be Formed?

[sachin123]

Homework Statement

How many images of a symmetrically placed object will be formed if 2 plane mirrors are fitted on ADJACENT walls of a room and another one on the top(roof)?
(mirrors are big)

2. The attempt at a solution
According to me,9.
because,3 direct images of the object are formed by the 3 mirrors.And the image formed by each mirror serves as an object for the other 2 mirrors thus forming 2 x 3 = 6 more images,thus a total of 9 images.

Am I correct?

 

[chrisbaird]

Not exactly. Try drawing a diagram. Each mirror doubles the system. So three mirrors would create 2^3 objects total. The question asks for images, so we must subtract the real objects which is not an image: 2^3 - 1 = 7 images. You have over-counted the cross images by neglecting the fact that they line up and become the same image.