Question

(e) There are four convex lenses of focal lengths 3, 4, 25
and 100 cm. To obtain maximum magnifying power
which of the lenses should be used as an objective
and an eyepiece of an astronomical telescope?
2​

Answer 1

Answer:

The convex lens with focal length of 100cm should be used for magnifying

The convex lens of focal length 3cm for the eyepiece of telescope

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Answer 2

The lens with a focal length of 100cm should be used as the objective and the lens with a focal length of 3cm should be used as the eyepiece.

• We know that the magnifying power of an astronomical telescope for normal adjustment is given by $m=\frac{f_o}{f_e}$, where $f_o$ is the focal length of the lens used for the objective and $f_e$ is the focal length of the eyepiece lens.
• To maximize the magnifying power we need to maximize the focal length of the objective and minimize the focal length of the eyepiece.
• Here, the maximum and minimum focal lengths given to us are 100cm and 3cm respectively.
• Hence, the lens with a focal length of 100cm should be used as the objective and the lens with a focal length of 3cm should be used as the eyepiece.