Q 12-09-042JEE MainJEE Main 2026 (6 Apr, Shift 2)Easy
A concave mirror of focal length $10$ cm forms an image which is double the size of object when the object is placed at two different positions. The distance between the two positions of the object is ______ cm.
Numerical value type. Enter your answer.
Answer: 10
Concept. For a mirror $m=-\dfrac vu=\dfrac{f}{f-u}$. A double-size image is either real ($m=-2$) or virtual ($m=+2$): two object positions.
Step 1. Real image. $\dfrac{-10}{-10-u}=-2\Rightarrow-10=20+2u\Rightarrow u=-15$ cm.
Step 2. Virtual image. $\dfrac{-10}{-10-u}=2\Rightarrow-10=-20-2u\Rightarrow u=-5$ cm.
Step 3. Separation. $15-5=10$ cm (in general $2|f|/m$).
Answer. $10$.
Solution by Sreeraj P, M.Sc Physics
