Solved: PHYSICS Question for NEET

NEET PHYSICSMedium

An astronomical refracting telescope will have large angular magnification and high angular resolution when it has an objective lens of:

small focal length and large diameter.

large focal length and small diameter.

large focal length and large diameter.

small focal length and small diameter.

Angular Magnification: The magnifying power ( $m$ ) of an astronomical refracting telescope in normal adjustment is given by the ratio of the focal length of the objective lens ( $f_o$ ) to the focal length of the eyepiece ( $f_e$ ), i.e., $m = \frac{f_o}{f_e}$ . For a large angular magnification, the focal length of the objective lens ( $f_o$ ) must be large.
Angular Resolution: The angular resolution (or resolving power) of a telescope depends on the diameter ( $D$ ) of its objective lens. The limit of resolution is $\Delta\theta = \frac{1.22\lambda}{D}$ . A smaller limit of resolution means a higher resolving power. Therefore, to achieve high angular resolution, the diameter of the objective lens ( $D$ ) must be large.
Conclusion: Combining both requirements, the objective lens should have a large focal length and a large diameter.

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