One application of a two mirror system is the periscope.
Example
Draw a light cone to show how the eye sees an image of object O reflected by both mirrors.
As O is not directly in front of the mirror we will need to extend the plane of the mirror.
The image of O in mirror M1 is located. We will call this I1.
We can now reflect this image I1 in mirror M2. We will call this I2. I2 is an image of an image, not another image of O.
As always, we start drawing light rays in reverse, starting with the ray(s) entering the eye.
The eye sees an image at I2. because the ray has entered the eye in this direction.
This ray has come from the direction of I1.
Finally we can trace this ray back to its point of origin – the arrow tip of O.
We now have one complete ray drawn leaving O and reflecting twice before entering the eye.
Repeating the above steps for anther ray from O going into the eye we can obtain a light cone.
Notice the distance of I2 from the eye is in fact equal to the distance travelled by the light on its journey from O to the eye.
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