Question

A giant telescope in an observatory has objective of focal length 19 m and an eyepiece of focal length 1.0 cm. In normal adjustment, the telescope is used to view the moon. What is the diameter of image of the moon formed by the objective?
The diameter of the moon is $3.5\times {10}^6$ m and the radius of the lunar orbit round the earth is $3.8\times {10}^8$ m

• A. 10 cm
• B. 12.5 cm
• C. 15 cm
• D. 17.5 cm

Answer 1

The correct option is D 17.5 cm

$f_0=19m$
$f_e=1cm$

$d=3.5\times {10}^{6}m$

$r=3.8\times {10}^8$
$d^1=\frac{f_0}{f_e}\times \frac{d^1}{l}$

$=\frac{1900}{1}\times \frac{3.5\times {10}^6}{3.8\times {10}^8}m$

$=17.5cm$