Question

Two plane mirrors are placed parallel to each other at a distance $L$ apart. $A$ point object $O$ is placed between them at a distance $\frac{L}{3}_{}$ from one mirror. Both mirrors form multiple images. What cannot be the distance between any two images ?

• A. $\frac{3 L}{2}$
• B. $\frac{2 L}{3}$
• C. $2L$
• D. none of these

Answer 1

Answer: (A)

The reflection between parallel mirrors take place in such a way that the second image of one mirror is formed due to the first image on the other mirror.
When the separation between the object and the two mirrors is taken as $L/3$ and $2L/3$
the separation between images will be given by double the separation between the object and the opposite mirror .
The possible separations are : $2L/3,4L/3,2L,8L/3.....$
As we see, the separation between the images should always be a multiple of $2L/3$ and this is the only option which is not a multiple.