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Question
A giant refracting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece lens of focal length 1.0 cm is used, find the angular magnification of the telescope. If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.42 × 106 m and the radius of the lunar orbit is 3.8 × 108 m.
Solution
Angular magnification = `- f_0/f_0 (1 +(f_e)/D) = - 1500/1 (1 +1/25) =-1560`
Negative sign indicates that the image is inverted.
Diameter of the image of the moon formed by the objective lens = d (say)
`tan alpha≈alpha = (\text (diameter of moon))/(\text { radius of the orbit}) =d/f_0`
`or ,(3.42 xx 10^6)/(3.8 xx 10^8) = d/15 => d =0.135 m`
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