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

73. 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 increase

Answer :  (b) microscope and telescope both will increase

For Microscope Magnification formula:

\( M = \left( 1 - \frac{v_0}{f_0} \right) \left( 1 + \frac{D}{f_e} \right) \)

As $f_o$ increases we see that $M_o$ increases, so M will increase.

For a telescope (normal adjustment, final image at infinity):

\( M = \frac{f_0}{f_e} \)

As $f_o$ increases we see M increases.