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)

microscope will decrease but that of telescope will increase.

Answer 2

Magnifying power of a microscope.
$m= \bigg( \frac{L}{f_0} \bigg) \bigg(\frac{D}{f_e} \bigg)$
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.