Q44. A Galilean telescope and a Keplerian telescope both have objective focal length 120 cm and eyepiece focal-length magnitude 10 cm. Both are in normal adjustment. Their tube-length difference is:
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Q44. A Galilean telescope and a Keplerian telescope both have objective focal length 120 cm and eyepiece focal-length magnitude 10 cm. Both are in normal adjustment. Their tube-length difference is:

G1 Physics · Accepted Answer

GIVEN: Both telescopes use fₒ = 120 cm and eyepiece focal-length magnitude 10 cm; final image at ∞. Step 1: Keplerian telescope has convex eyepiece (+10 cm): Lₖ = 120 + 10 = 130 cm. Step 2: Galilean telescope has concave eyepiece (−10 cm): Lɢ = 120 − 10 = 110 cm. Step 3: Difference = 130 − 110 = 20 cm. Step 4: General shortcut for same |fₑ|: Lₖ − Lɢ = 2|fₑ| = 2 × 10 = 20 cm. FINAL ANSWER: 20 cm. Option C.