Q38. For a compound microscope in normal adjustment, the distance between the second principal focus of the objective and the first principal focus of the eyepiece is 18 cm. Objective and eyepiece focal lengths are 2 cm and 5 cm. The actual lens separation is:
Options

Question

Q38. For a compound microscope in normal adjustment, the distance between the second principal focus of the objective and the first principal focus of the eyepiece is 18 cm. Objective and eyepiece focal lengths are 2 cm and 5 cm. The actual lens separation is:

Options

G1 Physics · Accepted Answer

GIVEN: Optical tube length Δ (distance from objective's second focus to eyepiece's first focus) = 18 cm; fₒ = 2 cm, fₑ = 5 cm. Step 1: In the usual microscope arrangement, the lens-to-lens separation equals objective focal length + focus-to-focus gap + eyepiece focal length. Step 2: L = fₒ + Δ + fₑ. Step 3: L = 2 + 18 + 5 = 25 cm. Step 4: Check: L − fₒ − fₑ = 25 − 2 − 5 = 18 cm, as given. FINAL ANSWER: Actual lens separation = 25 cm. Option D.