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Optical Instruments — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsOptical Instruments2 Marks
Question
A Galilean telescope is 27cm long when focused to form an image at infinity. If the objective has a focal length of 30cm, what is the focal length of the eyepiece?
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Answer

For the given Galilean telescope, (When the image is formed at infinity)$f_0 = 30cm, L = 27cm$
Since$ L = f_0 - |f_e|$
[Since, concave eyepiece lens is used in Galilean Telescope]
$\Rightarrow f_e = f_0 - L = 30 - 27 = 3cm$

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