The problem is the question 5.67 of Hecht book – Optics
Suppose two mirrors (A and B) are 25 cm apart and a small candle is placed in between them such that you
cannot see it. You see the images at $Q_{1}$ and $Q_{3}$, and they seem to be $10$
cm apart. Where is the candle located?
But in optics textbook by S. Parkinson
published in 1884. he proves that $\overline{Q_{1}Q_{3}} = 2\overline{AB}$
and then, $\overline{QQ_{3}} – \overline{QQ_{1}} = 2c = 2\overline{AB} $.
