The focal length of the objective lens of a telescope is 50 cm. If the

The focal length of the objective lens of a telescope is 50 cm. If the magnification of the telescope is 25, then the focal length of the eye-piece is

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12.5 cm

5 cm

2 cm

10 cm

Answer is Right!

Answer is Wrong!

This question was previously asked in

UPSC NDA-1 – 2019

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The focal length of the eye-piece is 2 cm.

For a refracting telescope used in normal adjustment (final image at infinity), the angular magnification (M) is given by the ratio of the focal length of the objective lens (f_o) to the focal length of the eye-piece lens (f_e): M = f_o / f_e.
Given f_o = 50 cm and M = 25.
We can rearrange the formula to solve for f_e: f_e = f_o / M.
Substituting the given values: f_e = 50 cm / 25 = 2 cm.

A telescope achieves magnification by using an objective lens with a long focal length and an eye-piece lens with a short focal length. The objective forms a real, inverted image of a distant object at its focal plane, and the eye-piece acts as a magnifying glass to view this intermediate image.

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