# An optical instrument uses a lens of power 100 D for objective lens and 50 D for its eyepiece. When the tube length is kept at 25 cm. the final image is formed at infinity.(a) Identify the optical instrument(b) Calculate the magnification produced by the instrument.

The optical instrument is therefore a compound microscope.

When the final image is formed at infinity, the magnification of a compound
microscope equals

L=25cm D=25cm

f0=1 cm

fe=2cm

$m=(\frac{L}{f_o})(\frac{D}{f_e})=(25/1)(25/2)=312.5$

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