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Optical Instruments — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsOptical Instruments2 Marks
Question
A Galilean telescope is $27\ cm$ long when focused to form an image at infinity. If the objective has a focal length of $30\ cm,$ 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$)f_0 = 30\ cm, L = 27\ cm$
Since $L = f_0 - |f_e|$
$[$Since, concave eyepiece lens is used in Galilean Telescope$]$
$\Rightarrow f_e = f_0 - L = 30 - 27 = 3\ cm$

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