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# In a compound microscope the focal length of objective lens is 1.2 cm and focal length of eye piece is 3.0 cm .When object is kept at 1.25 cm in front of objective , final image is formed at infinity . Magnifying power of the compound microscope should be :

$(a)\;200\qquad(b)\;100\qquad(c)\;400\qquad(d)\;150$

Given $$f_e = 3 cm$$ and
$$f_o = 1.2 cm$$
$$u_o = 1.25 cm$$
$$v_e = \infty$$
From , $$\displaystyle \frac{1}{u_o} + \frac{1}{v_o} = \frac{1}{f_o}$$,
we get $$\displaystyle \frac{1}{v_o} = \frac{1}{1.2} - \frac{1}{1.25}$$
$$v_o = 30$$ cm
Magnification of microscope $$\displaystyle m=\frac{v_o}{u_o} (1+\frac{D}{f_e})$$
$$\displaystyle m = \frac{30}{1.25} (1+\frac{25}{3}) \simeq 200$$            .