Question

If the focal length of objective lens is increased then magnifying power of

• A. microscope will increase but that of telescope decrease
• B. microscope and telescope both will increase.
• C. microscope and telescope both will decrease
• D. microscope will decrease but that of telescope will increase.

Answer 1

Answer: (D)

Magnifying power of a microscope.
$m=\left(\frac{L}{f_{0}}\right)\left(\frac{D}{f_{e}}\right)$
where $f_{o}$ and $f_{c}$ are the focal lengths of the objective and eyepiece respectively and $L$ is the distance between their focal points and $D$ is the least distance of distinct vision.
If $f_{o}$ increases, then $m$ will decrease.
Magnifying power of a telescope, $m=\frac{f_{0}}{f_{e}}$
where $f_{o}$ and $f_{e}$ are the focal lengths of the objective.
and eyepiece respectively.
If $f_{0}$ increases, then $m$ will increase.