If the focal length of objective and eye lens are `1.2 cm` and `3 cm` respectively and the object is put `1.25cm` away from the objective lens and the final image is formed at infinity. The magnifying power of the microscope is

Options

A. 150
B. 200
C. 250
D. 400

Answer

B

Explanation

The microscope here, is a compound microscope which is a combination of objective and eyepiece.
For the objective lens,
`1/(f_(0)) = 1/v_(0) - 1/u_(0)`
Objective lens is convex, so `f_(0) = 1.2` cm
`1/(+1.2) = 1/v_(0) - 1/(-1.25)`
`1/(+1.2) = 1/v_(0) + 1/(1.25)`
`1/v_(0) = 1/1.2 - 1/1.25`
`v_(0) = 30 cm`
When final image is at infinity, Net magnification, `m = m_(0) xx m_( e)`
`m = 30/(-1.25) xx 25/3`
`|m| = 200`