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Asked in GUJCET 2017 · Telescopes
Given: objective focal length f₀, eye-piece focal length fₑ, normal adjustment.
Idea: the objective forms a real image of the distant object at its own focal plane, and the eye-piece then views that image as a magnifier.
For the final image to be at infinity, the intermediate image must sit exactly at the eye-piece's first focal point.
So the two lenses are f₀ plus fₑ apart: L=f₀+fₑ.
The tube can never be shorter than that, which is what L≥ f₀+fₑ records.
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