The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the objective and the eye-piece is found to be 20cm. The focal lengths of the lenses are
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Solution
Given,
m=9,L=20cm
Let, the focal lengths of the objective and eye-piece are f0 and fe respectively.
We know, m=f0fe=9
⇒f0=9fe....(i)
and L=f0+fe=20....(ii)
Solving equations (i) and (ii), we get
f0=18cm and fe=2cm
Hence, (A) is the correct answer.
Key Concept: Magnifying power of a telescope is the ration of focal length of the objective to that of the eye piece, when the final image formed at infinity.