Lesson 5 of 6 / Cells and microscopy
Calibrate a microscope measurement
What does one eyepiece division represent at this objective?
I can use a stage micrometer, graticule, scale bars and cautious image interpretation.
I can recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.Syllabus
Syllabus 9477, Core 1(b). Recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.
About 7 min
Make a guess. You are not marked.
You calibrate an eyepiece graticule at x10 objective. You then switch to x40. Does each division still represent the same length?
Show the answer
No. It now represents a quarter of that length.
Each division spans a smaller part of the specimen at higher magnification. Recalibrate at every objective.
The key idea
Calibrate the eyepiece scale at each objective before converting a specimen measurement to a real length.
Explore the idea
Calibrate first, then measure
Align the zero marks. Read a second pair of coincident marks to connect eyepiece divisions to a known length.
The stage micrometer here has 10-micrometre divisions. The eyepiece scale has arbitrary units. The orange guides mark the two positions used for calibration.
The drawn cell is illustrative. Calculate with the calibrated scales, never the number of pixels or centimetres on your screen. Actual slide mounting, focusing and calibration require laboratory practice.
Explanation
An eyepiece graticule has arbitrary divisions. A stage micrometer has a known physical scale. Superimpose the scales at the chosen objective, align one pair of marks, and find another matching pair. Divide the known micrometer distance by the corresponding eyepiece divisions.
If 40 eyepiece divisions span 100 micrometres, one division represents 2.5 micrometres. A structure spanning 12 divisions is then 30 micrometres long. Changing objective changes the relationship; recalibrate instead of reusing the old conversion.
Begin focusing at low power, centre the specimen, then increase magnification and use fine focus. A thin mount and carefully lowered coverslip reduce overlap and air bubbles. Select appropriate staining, record observations accurately and distinguish what is visible from what a textbook says should be present.
Magnification equals image size divided by actual size, in the same units. A scale bar remains useful when an image is resized because the specimen and bar scale together. Greater magnification without greater resolution merely enlarges an unresolved image; it does not reveal a ribosome.
Step by step
- 1
Align the scales
Use matching marks sufficiently far apart to reduce proportional reading error.
- 2
Calculate one division
Keep the physical unit throughout the conversion.
- 3
Measure the specimen
Multiply its eyepiece divisions by the calibrated length per division.
Worked example
Work through the evidence
25 eyepiece divisions align with 0.10 mm. A cell spans 18 eyepiece divisions. Find its length.
One way to explain it
0.10 mm = 100 micrometres. One division = 100/25 = 4 micrometres. The cell length = 18 x 4 = 72 micrometres.
Why this answer works
- Convert mm to micrometres before dividing.
- Use the objective at which the calibration was made.
Watch out for this
A student says: "Every eyepiece division always equals one micrometre." What is wrong with this?
Show the answer
Eyepiece divisions have no fixed specimen length until calibrated at the selected objective.
Check your understanding
50 divisions span 100 micrometres. What length is 15 divisions?
Show the answer
30 micrometres
Each division is 2 micrometres, so 15 x 2 = 30.
Quick recall
What does one eyepiece division represent at this objective?