UPSC NDA I 2017 General Ability Solved Paper

Concept:The magnification of an optical microscope depends on the focal lengths of both the objective and eyepiece lenses.The total magnification is given by $M = \frac{L}{f_o} \cdot \frac{D}{f_e}$, where $L$ is the tube length, $D$ is the least distance of distinct vision, $f_o$ is the objective focal length, and $f_e$ is the eyepiece focal length.Explanation:From the formula, $M$ is inversely proportional to $f_e$ (eyepiece focal length).Therefore, increasing $f_e$ directly decreases $M$.Similarly, $M$ is also inversely proportional to $f_o$; increasing $f_o$ also decreases $M$.Thus, option A is false because magnification decreases (not increases) when $f_e$ increases.Option B is false because increasing $f_o$ decreases magnification.Option C is false because magnification does depend on $f_e$.Only option D correctly states that magnification decreases with increase in $f_e$.Answer:D. Magnification decreases with the increase in focal length of eyepiece.