interesting question, considering that the angle of reflection is equal to the angle of incidence, which means that the reflection returns the light to its place of birth. Let's ask the question, where is the light source located? 1 - If the source is located outside the mirrors, which means a beam of light from the source is reflected from mirror A to mirror B, from which it returns to mirror A ... where this returning light is reflected in the sense: the angle of reflection is equal to the angle of incidence ? 2 - if the source is placed between opposite mirrors, so that each reflection of light returns to the source and through it in the unaltered direction to the opposite mirror (which is probably your case) Practical meaning By measuring the time between sending the light and its return, we calculate the distance reflection point (mirror) from us. In the 2nd case, using such a measurement, we can determine the distances of the source from both mirrors and at the same time the distance of the mirrors between each other.