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

Asked in NEET 2018 · Resolving power and limits of ray optics

Answer: (1) large focal length and large diameter

Step-by-step solution

Given: a refracting telescope wanted to have both large angular magnification and high angular resolution.

Idea: magnification and resolution depend on two different properties of the objective, so check each in turn.

Angular magnification M=(fₒ)/(fₑ), so a large objective focal length fₒ raises it.

Resolving power R=a/(1.22λ), where a is the objective's diameter.

A large diameter therefore raises the resolution, and the two requirements do not conflict.

The objective should have a large focal length and a large diameter.

Why the other options are wrong

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