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Four double convex lenses with following...

Four double convex lenses with following specifications are available
Lens, A, B, C, D,
focal length, 100 cm, 100 cm, 10 cm, 5 cm,
aperture, 10 cm, 5 cm,2 cm, 2 cm.
Which of the given four lenses should be selected as objected and eyepiece to construct an astronomical telescope and why ? What will be the magnifying power and length of the tube of the telescope ?

Text Solution

Verified by Experts

The objective of a telescope must have large aperture to gather more light. It should also have large focal length `(m = (f_(0))/(f_(e)))`. Therefore, lens A is selected as objective lens.
The eye piece should have small aperture and small focal length. Therefore, lens `D` is selected as eye lens. In normal adjustment, Magnifying power, `m = (f_(0))/(f_(e)) = (100)/(5) = 20`
`L = f_(0) + f_(e) = 100 + 5 = 105 cm`.
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