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Class 9 Microscope and Microscopy (Advanced): cheat sheet
CBSE Science. How microscopes see the very small: total magnification, real size, field of view, resolution, and choosing between light, phase-contrast, fluorescence, TEM and SEM.
Key ideas
- The objective forms a real, inverted, magnified image; the eyepiece magnifies it again into a virtual image.
- More magnification is not always better: without enough resolution the bigger image is just blurred.
- The eye resolves about 0.1 mm, a light microscope about 0.2 µm and an electron microscope about 0.2 nm.
- Phase-contrast shows living cells without stains; fluorescence makes dyed structures glow against a dark background.
- TEM passes electrons through ultra-thin sections for internal details in 2D; SEM scans a metal-coated surface for 3D-like images.
- Electron microscopes need a vacuum and chemical preparation, so they cannot show living cells.
- Temporary mounts dry out; permanent slides are fixed, stained, dehydrated and sealed in Canada balsam or DPX.
Remember
| Total magnification | M = objective × eyepiece |
| Real size | real size = image size ÷ M; image size = real size × M |
| Field of view | inversely proportional to M; cell size ≈ field width ÷ number of cells across |
| Units | 1 cm = 10 mm; 1 mm = 1,000 µm; 1 µm = 1,000 nm |
| Resolving power | 1 ÷ resolution (the smaller the resolution, the greater the power) |
Watch out
- Adding the eyepiece and objective powers instead of multiplying them.
- Forgetting to change mm to µm before dividing by the magnification.
- Thinking the field of view gets bigger at higher magnification.
Worked example
A TEM photograph at 5,000X shows a structure 42 mm long. Find its real length (a) in µm and (b) in nm. (c) Could a light microscope, which resolves about 0.2 µm, show its two ends as separate points? (Yes or No) (Give the answer to 2 decimal places where needed.)
Working: (a) 42 mm = 42,000 µm, ÷ 5,000; (b) × 1,000; (c) compare with 0.2 µm
Answer: (a) 8.4 µm; (b) 8,400 nm; (c) Yes
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