Q 12-09-189JEE MainJEE Main 2020 (4 Sep, Shift 2)Medium
The distance between an object and a screen is $100\ \text{cm}$. A lens can produce real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is $40\ \text{cm}$. If the power of the lens is close to $\left(\dfrac{N}{100}\right)$ D where $N$ is an integer, the value of $N$ is ______.
Numerical value type. Enter your answer.
Answer: 476
Displacement method: $f = \dfrac{D^{2} - d^{2}}{4D} = \dfrac{100^{2} - 40^{2}}{400} = 21$ cm.
$$P = \frac{100}{21} \approx 4.76\ \text{D} = \frac{476}{100}\ \text{D}$$
Solution by Sreeraj P, M.Sc Physics