Cambridge Notes, Past Papers, Revision Questions

Cambridge A-Level Biology 9700 – The Microscope in Cell Studies

The Microscope in Cell Studies

Learning Objective

Use an eyepiece graticule and a stage micrometer to make measurements and express them in the appropriate units: millimetre (mm), micrometre (µm) and nanometre (nm).

Key Components

Eyepiece Graticule – a glass plate with a fine scale etched into the eyepiece.

Stage Micrometer – a slide with a precisely known scale (usually 1 mm divided into 100 µm divisions).

Objective Lens – provides the primary magnification.

Ocular Lens (Eyepiece) – usually 10× magnification.

Procedure for Calibrating the Microscope

Place the stage micrometer on the stage and bring it into focus using the lowest power objective.

Rotate the nosepiece to the objective you will use for measurements (e.g., 40×).

Focus the micrometer scale sharply.

Look through the eyepiece and align the eyepiece graticule divisions with the micrometer divisions.

Count the number of graticule divisions that span a known length on the micrometer (e.g., 10 µm).

Calculate the value of one graticule division using the formula:
\$\text{Size of one graticule division} = \frac{\text{Known length on micrometer}}{\text{Number of graticule divisions counted}}\$

Record the calibration for each objective used.

Units and Conversions

The three units commonly used in microscopy are related as follows:

Unit	Symbol	Equivalent in metres (m)	Equivalent in µm	Equivalent in nm
Millimetre	mm	1 × 10⁻³ m	1 000 µm	1 000 000 nm
Micrometre	µm	1 × 10⁻⁶ m	1 µm	1 000 nm
Nanometre	nm	1 × 10⁻⁹ m	0.001 µm	1 nm

Example Calculation

Suppose under a 40× objective you find that 25 graticule divisions span 10 µm on the stage micrometer.

First determine the size of one division:

\$\text{One division} = \frac{10\ \mu\text{m}}{25} = 0.40\ \mu\text{m}\$

Now measure a cell that occupies 120 graticule divisions:

\$\text{Cell size} = 120 \times 0.40\ \mu\text{m} = 48\ \mu\text{m}\$

Convert to other units if required:

\$48\ \mu\text{m} = 0.048\ \text{mm} = 48\,000\ \text{nm}\$

Practical Tips

Always calibrate each objective separately; magnification changes the apparent size of the graticule.

Record calibration values in a table for quick reference during experiments.

When measuring, count whole graticule divisions and estimate any fraction of a division for greater accuracy.

Use the same eye (dominant eye) for all measurements to minimise parallax error.

Common Sources of Error

Parallax – not looking straight through the eyepiece.

Incorrect focus – a blurred scale leads to inaccurate division counts.

Temperature expansion of the stage micrometer (rare but possible in high‑precision work).

Assuming the eyepiece graticule is perfectly calibrated without verification.

Suggested diagram: A schematic showing the eyepiece graticule aligned with the stage micrometer, with arrows indicating the measurement process.

Summary Checklist

Calibrate each objective using the stage micrometer.

Record the size of one graticule division in µm.

Measure specimens by counting graticule divisions.

Convert the measured value to the required unit (mm, µm, nm) using the conversion table.

Document any sources of error and steps taken to minimise them.

use an eyepiece graticule and stage micrometer scale to make measurements and use the appropriate units, millimetre (mm), micrometre (µm) and nanometre (nm)