2021 Ray Optics and Optical InstrumentsOptical Instruments – Astronomical Telescope Easy Question 23 of 89 in Ray Optics and Optical Instruments

A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since:

A a large aperture contributes to the quality and visibility of the images.
B a large area of the objective ensures better light gathering power.
C a large aperture provides a better resolution.
D all of the above.

The answer is D.

Solution

Magnifying power $MP = \frac{f_o}{f_e}$ – a large focal length of the objective gives higher magnification. Resolving power $RP = \frac{a}{1.22\lambda}$ – a large aperture gives better resolution and hence better quality of image. A large area of the objective also gathers more light, improving visibility. So all of the above reasons are correct.

Concept objective of a telescope

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89 questions from this chapter have appeared across 25 exam years.