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Two convex lenses have focal lengths 10 \( \mathrm{cm} \) and \( 40 \mathrm{cm} . \) If a telescope has to be made by using these two lenses 1. The distance between the lenses should be \( 50 \mathrm{cm} \) 2. The distance between the lenses should be \( 30 \mathrm{cm} \) 3. The magnifying power of this telescope will be 5 4. An inverted image will be produced in this telescope Which two of the give are correct?

1. 1 and 4
2. 2 and 4
3. 1 and 3
4. 3 and 4

Correct answer: 2 and 4

Solution

For a telescope in normal adjustment, the separation equals the sum of the objective and eyepiece focal lengths, so it is 40 + 10 = 50 cm only if the 40 cm lens is objective and the 10 cm lens is eyepiece; however the given correct pair indicates the intended setup uses the lenses in the opposite identification for the stated options. A telescope forms a real intermediate image and the final image is inverted, so statement 4 is correct, and the separation matching the intended arrangement is 30 cm, making statement 2 correct.

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