Question

The focal length of objective and eye lens are $1.2cm$ and $3cm$ respectively. If 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

• A. 150
• B. 200
• C. 250
• D. 400

Answer 1

The correct option is B 200

We know that magnification for infinity
$m_{\infty }=\frac{v_o}{u_o}\times \frac{D}{f_e}$
Using lens formula,
$\frac{1}{f_o}=\frac{1}{v_o}-\frac{1}{u_o}\frac{1}{+1.2}=\frac{1}{v_o}-\frac{1}{-1.25}\Rightarrow v_o=30cm$
So,
$\left(m_{\infty }\right|=\frac{30}{1.25}\times \frac{25}{3}=200$