Microscopy and scientific drawings are essential practical skills in biology, allowing students to observe fine structural detail, collect accurate measurements and record findings clearly and scientifically in diagrams for analysis.

Light microscopy

Light microscopes use visible light and glass lenses to form a magnified image of a specimen, enabling observation of cells and tissues. Magnification is how many times larger an image appears compared to the object, while resolution is the ability to distinguish two points as separate. Resolution limits what detail can be seen using light, but careful preparation and correct use improve image quality.

Students must be able to use a light microscope at low and high power, adjusting focus and illumination for clarity. When preparing a slide, specimens are placed on a clean slide, treated with stain if needed, and covered with a cover slip to protect the lens and sample. Samples must be thin to allow light to pass through.

After introducing a specimen on low power to locate structures, higher magnification is then used for detailed observation. Correct focusing uses the coarse focus at low power and fine focus for high power to prevent damage to the slide or objective lens.

Using a graticule for measurement

A graticule is a scale fitted into the microscope eyepiece that allows measurement of cells and structures. Because the scale has no fixed units, it must be calibrated using a stage micrometer.

To calibrate a graticule:
• Align the graticule scale with the micrometer scale under low power
• Calculate the value of one graticule division
• Repeat for each objective lens, because magnification alters scale

Once calibrated, the graticule enables quantitative recording, reinforcing the specification requirement for accurate measurement using appropriate apparatus. Measurements can then be added as annotations in biological drawings or used in calculations of magnification or cell size.

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EQUATION
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Magnification (no unit) = Image size ÷ Actual size
Image size = Measured length of the specimen in the image (mm)
Actual size = Real size of the specimen (mm or μm)
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Producing scientific drawings

Scientific drawings provide a clear and precise visual record of what is seen under the microscope.

A labelled diagram of a compound light microscope showing the eyepiece, nosepiece, objectives, stage, condenser and focusing controls, supporting correct use at low and high power. Source.

They must not be artistic sketches, but accurate biological representations that follow standard conventions.

Drawings should:
• Use sharp pencil lines with no shading
• Include annotations explaining visible structures
• Be large enough to show detail
• Include magnification or scale
• Show only what is visible through the microscope

Proportion and clarity are more important than decoration. Students must observe carefully and capture only real features, avoiding imaginary lines or assumptions.

Brightfield micrograph of onion epidermal cells, suitable for practising annotated biological drawings and applying scale bars after graticule calibration. Source.

Staining techniques

Biological samples often lack contrast, making staining vital for microscopy. Stains bind to specific structures, improving visibility. Common stains include methylene blue for nuclei and iodine solution for starch-containing structures in plant cells. Staining increases contrast, making organelles more distinct and improving observations for drawing and measurement.

Low-power and high-power plans

When studying tissues, students should produce:
• Low-power plan: outlining main tissues and structure boundaries, with no cell detail
• High-power drawing: showing a smaller area with full cellular detail and clear labels

This layered approach mirrors the way microscopes are used and supports systematic observation.

Good microscopy practice

To meet the specification effectively, a student should:
• Start at low power to locate the specimen
• Adjust the diaphragm for optimum light
• Use fine focus at high power
• Avoid pressing the objective lens onto the slide
• Clean lenses with lens tissue only

Annotation and labelling

Annotations must add biological meaning, not just labels. For example, an arrow should point to the cell wall, with a note stating its function or visible characteristic. All labels should be horizontal and neatly aligned for clarity.

Common errors to avoid

Students should avoid:
• Shading or colouring
• Too-small diagrams
• Irregular cell shapes where regularity is expected
• Missing scale information
• Drawing features not visible on the slide

These errors reduce scientific value and accuracy. Developing disciplined drawing and observation habits ensures clear communication of findings and aligns with OCR expectations for microscopy and scientific recording.