Question

It 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

The correct option is D microscope will decrease but that of telescope will increase

Magnifying power of a microscope $m_m=\left(\frac{L}{f_0}\right)\left(\frac{D}{f_e}\right)$

where $f_0$ is the focal length of objective lens.

$⟹$ $m_m\propto \frac{1}{f_0}$

Thus if $f_0$ increases, then $m_m$ will decrease.

Magnifying power of telescope $m_t=\frac{f_0}{f_e}$

$⟹$ $m_t\propto f_0$

Thus if $f_0$ is increased, then $m_t$ will increase.