Latest practice questions with clear explanations — built like a modern knowledge base.
Oil left hardens, attracts dust, damages lens coating and can seep into objective. Clean with lens paper and xylene-free cleaner after each session, never use tissue.
Ref: NCERT Biology Laboratory Manual, Class XI, Mounting precautions and identification of artifacts.
Parfocal: minimal refocus needed between objectives. Parcentric: field stays centered. Saves time and prevents collision. Calibrated at factory.
Ref: NCBI Bookshelf, Molecular Biology of the Cell: Oil immersion principle and numerical aperture.
Köhler gives uniform illumination, reduces glare, maximizes NA. Steps: focus specimen, close field diaphragm, focus condenser, center, reopen to fill FOV, set aperture to 70% objective NA.
Ref: Nikon MicroscopyU, Iris diaphragm control: Contrast versus resolution and diffraction effects.
DOF ∝ λ / NA². Oil immersion 1.4 NA has DOF ~0.2µm vs 4x 0.1 NA DOF ~50µm. So bacteria at different depths need refocus.
Ref: NCERT Biology Laboratory Manual, Class XI, Precautions for high power and collision avoidance.
FOV = FN / objective mag. FN 18 mm /40 = 0.45 mm. Eyepiece mag does not affect FOV diameter formula, but affects total mag (600x). High WF eyepieces increase FN.
Ref: NCBI Bookshelf, Molecular Biology of Cell: Resolving power, numerical aperture, and Abbe limit.
Air bubbles are thick dark outline with bright center due to total internal reflection. Not to be confused with cells. Avoid by lowering coverslip at angle.
Ref: NCERT Physics, Class XII, Chapter 9: Magnification calculation and field of view relation.
Without oil, light refracts from glass to air (n=1.0) and is lost by total internal reflection. Oil bridges gap, allowing higher NA and resolution.
Ref: Nikon MicroscopyU, Parfocality and field loss during objective switching, troubleshooting.
Iris controls condenser NA. Over-closing lowers effective NA, increases contrast but introduces diffraction artifacts and reduces resolving power. Optimal ~70-80% open.
Ref: NCBI Bookshelf, Molecular Biology of the Cell: Troubleshooting high power blur and coverslip thickness.
High power objectives have very short working distance. Always watch from side when lowering stage/objective, use fine focus only at 40x/100x.
Ref: NCERT Biology Laboratory Manual, Class XI, Recommended low power scanning procedure.
Abbe limit: d = 0.61λ/NA. With λ~550nm, NA 1.4, d~0.2µm. Cannot resolve viruses, only bacteria.
Ref: NCERT Physics, Class XII, Chapter 9: Ray Optics, Compound microscope image formation.
Vignetting = dark edges. Raise condenser, center it, ensure objective is fully clicked, open field diaphragm.
Ref: NCERT Biology Laboratory Manual, Class XI, Illumination adjustment during magnification change.
Parfocality should maintain focus, but slight refocus with fine knob needed. Also check oil, dirty lens, wrong coverslip (#1.5 required).
Ref: Nikon MicroscopyU, Microscope Condenser: Height adjustment and illumination quality.
