Question

A telescope, when in normal adjustment, has a magnifying power of $6$ and the objective and eye-piece are $14cm$ apart. The focal lengths of the eye-piece and the objective, respectively, are

• A. $3cm$ and $14cm$
• B. $3cm$ and $12cm$
• C. $2cm$ and $14cm$
• D. $2cm$ and $12cm$

Answer 1

Answer: (D)

$2cm$ and $12cm$

Given,

magnification=6,

length=14.

We know,

In a telescope,

Length of the telescope=$f_o+f_e$ -(1)

Magnification=$\frac{f_o}{f_e}=6$

$f_o=6f_e$

Putting in equation 1.

$6f_e+f_e=14$

$f_e=2cm$

$f_o=12cm$

Hence option $\text{D}$ is correct.