Question

Calculate the length of the tube of a simple microscope, if objective and eyepiece focal lengths are $0.5\text{cm}$ and $4\text{cm}$. Given that, the magnifying power of a relaxed eye is $30$.

• A. $3.5\text{cm}$
• B. $7\text{cm}$
• C. $14\text{cm}$
• D. $9\text{cm}$

Answer 1

The correct option is B $7\text{cm}$

Given: $f_0=0.5\text{cm},f_e=4\text{cm},m=30,D=25\text{cm}$

Using, $m=\frac{L-f_0-f_e}{f_0{f}_e}\times D$

Substituting the values we get,

$30=\frac{L-0.5-4}{0.5\times 4}\times 25$

i.e. $\frac{60}{25}=L-4.5$

That gives required tube length as, $6.9\text{cm}\approx 7\text{cm}$