Q.

Magnifying power of an astronomical telescope is 3 for normal vision and when the final image is formed at least distance of distinct vision, the magnifying power is 9. Then tube length of the telescope for normal vision is [Least distance for distinct vision = 25 cm]

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a

200 cm

b

150 cm

c

175 cm

d

250 cm

answer is A.

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Detailed Solution

For normal vision, f0fe=3When the image is formed at least distance of distinct vision, f0fe1+feD=9⇒31+fe25=9⇒fe=50 cm∴f0=3fe=150 cm∴For normal vision, L=f0+fe=150+50 cm=200 cm
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