A Galilean telescope (objective convex, eyepiece concave) has objective focal length 20 cm and eyepiece focal length −5 cm. In normal adjustment, the tube length and angular magnification are
A{'text': 'tube 25 cm, m = 4', 'label': 'A'}
B{'text': 'tube 15 cm, m = 4', 'label': 'B'}
C{'text': 'tube 25 cm, m = −4', 'label': 'C'}
D{'text': 'tube 15 cm, m = −4', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'tube 15 cm, m = 4', 'label': 'B'}
1. Galilean (terrestrial) telescope in normal adjustment: tube length L = f_o − |f_e| (eyepiece is diverging, sits between objective and its focal plane).
2. L = 20 − 5 = 15 cm.
3. m = f_o / |f_e| = 20 / 5 = 4 (image is erect — the sign is positive in this convention since the eyepiece is concave).
_Source: NCERT Class 12 Physics Part II, Ch 9 §9.7.2 (Galilean variant)_
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