Question

If the tube length of the astronomical telescope is 105 cm long and magnifying power is 20 for normal setting. Calculate the focal length of the objective. Given magnifying power(m)= $m=\frac{f_0}{f_e}$.

• A. 100 cm
• B. 10 cm
• C. 20 cm
• D. 25 cm

Answer 1

The correct option is A

100 cm

$m=\frac{f_0}{f_e};20=\frac{f_0}{f_e}{f}_e=\frac{f_0}{20}L=f_0+f_{e}105=f_0+f_{e}105=f_0+\frac{f_0}{20}105=\left(\frac{20+1}{20}\right)f_0{f}_0=\frac{105\times 20}{21}{f}_0=100cm$