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Question:

A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance: 54.0cm, 37.3cm, 46.0cm, 50.0cm

46.0cm

37.3cm

54.0cm

50.0cm

Solution:

On applying the lens formula for the objective lens:
1/fo = 1/vo - 1/uo
1/vo = 1/fo + 1/uo
1/vo = 1/40 + 1/200
vo = 50 cm
Therefore, tube length = vo + fe = 50 + 4 = 54 cm