Question
The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are
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10 cm, 10 cm
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15 cm, 5 cm
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18 cm, 2cm
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11 cm, 9 cm
Solution
C.
18 cm, 2cm
Given M = fo/fe =9 and fo+fe = 20
fo = 9fe
So, 9fe+fe = 20
fe = 2cm
fo = 9 x2
fo =18
