Physics · Ch 6 — Optics
Compound Microscope
Compound Microscope
A compound microscope uses two lenses: the objective, near the object, forms a real, inverted, magnified image; this image acts as the object for the second lens, the eyepiece, which -- functioning exactly like a simple microscope -- produces a further-enlarged, virtual final image. The first (objective) image is positioned close to, but just within, the eyepiece's own focal plane, so the final image forms nearly at infinity (normal focusing) or at the near point …
What this figure shows. An object AB near the objective lens (focal length fo) forms a real, inverted, enlarged intermediate image A''B'' inside the tube, close to the focal plane of the second lens, the eyepiece (focal length fe); the eyepiece then acts as a simple microscope, magnifying this intermediate image further into a large, still-inverted, final virtual image seen by the eye at E. The tube length L, the distance between the objective's second focal point and the eyepiece's first focal point, and the object distance u, are marked on the ray diagram exa …
Worked out. A compound microscope has objective focal length fo = 5 cm, eyepiece focal length fe = 50 cm, and tube length L = 30 cm, with near point D = 25 cm. (i) For near-point focusing, m = (L/fo)(1 + D/fe) = (30/5)(1 + 25/50) = 6 times 1.5 = 9. (ii) For normal focusing, m = (L/fo)(D/fe) = 6 times (25/50) = 6 times 0.5 = 3. The noticeably higher near-point-focusing magnification (9, versus 3 for normal focusing) illustrates the general trade-off between higher magnification and the more comfortable, relaxed viewing normal focusing provides -- the same trade-off already seen for the simple microscope, now carried through to the two-l …