An astronomical telescope has magnifying power 6 for distant objects. The separation between the objective and the eyepiece is 42 cm and the final image is formed at infinity. The focal lengths fₒ of the objective and fₑ of the eyepiece are (in cm):
Options

Question

An astronomical telescope has magnifying power 6 for distant objects. The separation between the objective and the eyepiece is 42 cm and the final image is formed at infinity. The focal lengths fₒ of the objective and fₑ of the eyepiece are (in cm):

Options

G1 Physics · Accepted Answer

For normal adjustment, |M| = fₒ/fₑ = 6 and tube length L = fₒ + fₑ = 42 cm. Thus fₒ = 6fₑ, so 7fₑ = 42 and fₑ = 6 cm. Hence fₒ = 36 cm. Therefore option A is correct.