1.1.31 · Cell Structure
Eyepiece Graticules and Stage Micrometers — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
1.1.3 Eyepiece Graticules and Stage Micrometers — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Calibrate an eyepiece graticule with a stage micrometer and use it to measure the length of specimens.Eyepiece graticule — an engraved ruler seen through the eyepiece, of…
- Why calibrate
- Calibration equation
- Magnification factor
- Recalibration
- Wrong edge
- Single calibration
Section 1 of 8The two scales
1Eyepiece graticule
- Eyepiece graticule an engraved ruler seen through the eyepiece, often divided into 100 graticule divisions (eyepiece units).
- Its divisions have no fixed real length.
2Stage micrometer
- Stage micrometer a slide carrying a tiny ruler with an accurate known scale.
- Typical divisions are 0.1 mm (100 µm) and 0.01 mm (10 µm).
3Why calibrate
- Why calibrate the real value of a graticule division changes with the objective lens, so the graticule must be calibrated for each magnification.
Section 2 of 8Calibration procedure
1
Place the graticule in the eyepiece and the stage micrometer on the stage.
2
Focus on the micrometer scale using the chosen objective lens.
3
Rotate the eyepiece and move the stage so the two scales lie side by side with the zero marks aligned.
4
Find a point further along where a graticule line exactly coincides with a micrometer line.
5
Count the graticule divisions and the micrometer divisions between the zero and that point.
6
Convert the micrometer divisions to µm.
Section 3 of 8Calculating the calibration value
Calibration equation
- Calibration equation length of one graticule division (µm) = micrometer length (µm) ÷ number of graticule divisions.
- For instance, if 100 µm equals 40 graticule divisions, one division is 2.5 µm.
Magnification factor
- Magnification factor this value is the µm per graticule division at that objective lens.
Section 4 of 8Measuring a specimen
1
Remove the stage micrometer and place the specimen slide on the stage without changing the objective lens.
2
Count the graticule divisions that the object covers.
3
Multiply the number of divisions by the calibration value to get the length in µm.
4
Measure several cells and calculate a mean.
- Recalibration
- if the objective lens is changed, calibrate again.
Section 5 of 8Sources of error
Wrong edge
- Wrong edge counting from the wrong edge gives over- or under-counted divisions.
Single calibration
- Single calibration using one calibration for all objective lenses gives wrong sizes.
Unit slips
- Unit slips forgetting to convert stage micrometer divisions (10 µm) into µm.
Section 6 of 8More from the specification
1Eyepiece graticule calibration with sample
- Eyepiece graticule calibration with sample aligning stage micrometer divisions against the eyepiece scale ensures exact dimensional measurement of the biological sample.
2Magnification Equation and Specimen Calculations
- The magnification formula M = I / A relates image size (I), actual specimen size (A),
- and magnification (M), requiring all units to be converted consistently.
- (e.g., millimetres to micrometres, µm)
3Eyepiece Graticule Calibration
- An eyepiece graticule scale consists of arbitrary divisions that must be calibrated against a stage micrometer of known physical distance for each microscope objective lens magnification.
4Resolution Limits of Light Microscopy
- Resolution is the minimum distance between two points at which they can still be distinguished as separate;
- in light microscopes, resolution is limited by the wavelength of visible light to ~200 nm.
5Conventions for Biological Drawings
- Accurate biological diagrams of specimens viewed under a microscope require clear unbroken lines without shading, accurate proportioning,
- and ruled label lines pointing directly to cell structures.
6Magnification Equation and Specimen Calculations
- The magnification formula M = I / A relates image size (I), actual specimen size (A),
- and magnification (M), requiring all units to be converted consistently.
- (e.g., millimetres to micrometres, µm)
7Eyepiece Graticule Calibration
- An eyepiece graticule scale consists of arbitrary divisions that must be calibrated against a stage micrometer of known physical distance for each microscope objective lens magnification.
8Resolution Limits of Light Microscopy
- Resolution is the minimum distance between two points at which they can still be distinguished as separate;
- in light microscopes, resolution is limited by the wavelength of visible light to ~200 nm.
9Conventions for Biological Drawings
- Accurate biological diagrams of specimens viewed under a microscope require clear unbroken lines without shading, accurate proportioning,
- and ruled label lines pointing directly to cell structures.
10Resolution
- The ability to distinguish between two objects that are close together.
11Starch
- An insoluble polysaccharide of glucose used by plants for energy storage.
12Biological drawing
- A labelled, clear line drawing recording what is observed.
13Light microscope
- Microscope using light, magnifying up to about x1500.
14Starch grain
- Store of starch in the stroma.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Eyepiece graticule | An engraved scale in the eyepiece, in arbitrary divisions, used to measure specimens. |
| Stage micrometer | A slide with a ruler of known scale used for calibration. |
| Graticule division | One division of the eyepiece graticule scale. |
| Calibration | Finding the real length that one graticule division represents. |
| Magnification factor | The µm per graticule division at a given objective lens. |
| Objective lens | The lens nearest the specimen that sets the magnification. |
Section 8 of 8Exam technique
1
Calibrating once for all objective lenses — calibrate separately for each objective lens.
2
Using the stage micrometer to measure cells — it is only used to calibrate the graticule.
3
Not lining up the zero marks — alignment is needed before counting divisions.
4
Dividing the wrong way round — one graticule division = micrometer length ÷ graticule divisions.
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