Question

For an astronomical telescope, the eye-piece has a focal length of 10 cm. When tube length is 1 m, the telescope is focussed for normal Vision of distant objects. The focal length of objections

• A. 10 cm
• B. 45 cm
• C. 90 cm
• D. 100 cm

Answer 1

Answer: (C)

90 cm

For tube length L
$L_{\infty }=f_o+f_e$
where, $f_o$ is focal, length of objective and 4 is focal length of eye-piece
Given, L=1m=100 cm, $f_e=10cm$
$\therefore$ Focal length of objectivo $f_o=100-10=90m$