1.

The focal lengths of the eye piece and objective of a compound microscope are `5 cm and 1 cm` respectively, and the length of the tube is `20 cm`. Calculate magnifying power of microscope when the final image is formed at inifinity. The least distance of distinct cision is `25 cm`.

Answer» Correct Answer - 70
Here, `f_e = 5 cm, f_0 = 1 cm, L = 20 cm , m = ?`
`v_e = oo, d = 25 cm`
As final image is at infinity, image formed by objective lies at the focus of eye piece.
`:. v_0 = L - f_e = 20 - 5 = 15 cm`
From `(1)/(u_0)=(1)/(v_0)-(1)/(f_0)=(1)/(15)-(1)/(1)= -(14)/(15)`
`u_0 = -(15)/(14) cm`
As final image is at infinity,
`:. m = (v_0)/(u_0) ((d)/(f_e)) = (15)/(15//14) xx (25)/(5) = 70`.


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