Question

The magnifying power of a compound- microscope increases when ,

• A. the focal length of objective lens is increased and that of eye piece is decreased
• B. the focal length of eye piece is increased and that of objective lens is decreased
• C. focal lengths of both objective and eyepiece are increased
• D. focal lengths of both objective and eyepiece are decreased

Answer 1

The correct option is D focal lengths of both objective and eyepiece are decreased

Magnifying power of a compound- microscope is given by $m=-\frac{l}{f_0}\frac{D}{f_e}$
where, $l$ is length of tube
$D$ is least distance of clear vision
$f_0$ is focal length of objective
$f_e$ is focal length of eyepiece
So, clearly, it can be seen that focal length of objective and eyepiece needs to be decreased so that magnifying power increases.