Q 12-09-157JEE MainJEE Main 2021 (25 Feb, Shift 1)Easy
The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is ______ cm.
Numerical value type. Enter your answer.
Answer: 15
One image is real and inverted, the other virtual and erect, with equal size: $m_1 = -m_2$. For a lens $m = \dfrac{f}{f + u}$ with $u$ negative:
$$\frac{f}{f - 20} = -\frac{f}{f - 10} \Rightarrow f - 20 = -(f - 10) \Rightarrow f = 15\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics