When an astronomical telescope is in normal adjustment, the focal length of the objective lens is 50cm and…

Analyze the question: An astronomical telescope in normal adjustment means the final image is formed at infinity. We need the distance between the objective and the eyepiece.

Concept: In normal adjustment, the objective lens forms its image at its focal point. That image must also sit at the focal point of the eyepiece, so the eyepiece produces parallel rays (image at infinity). The two focal points coincide, so:

Distance between lenses = f₀ + fₑ

Solution:

• Focal length of objective, f₀ = 50 cm
• Focal length of eyepiece, fₑ = 5.0 cm

Distance = 50 + 5.0 = 55.0 cm

(The magnification would be f₀/fₑ = 50/5 = 10, which is a useful check, but it is not what the question asks for.)

Answer: D. 55.0 cm.

Tip: Remember "Normal adjustment = add the focal lengths" for the lens separation (L = f₀ + fₑ), and "divide them" for the magnification (M = f₀/fₑ). Options like 10.0 cm (the magnification value) and 45 cm (the difference) are common traps.