3.1.8Unit 3: Practical Skills in Biology I

Core Practical 5 – Light Microscopy — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 27 Unit quiz

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Part 1 of 5Preparing a Slide

A temporary mount lets you view fresh cells quickly with a light microscope.

Cut a small piece of tissue and use forceps to peel off a very thin layer, so that light can pass through it.

Place the thin layer on a clean slide in a drop of water, which prevents the cells from drying out.

Add a drop of a suitable stain, such as iodine solution, to make the structures easier to see.

Lower a coverslip gently at an angle with a mounted needle to avoid trapping air bubbles.

Blot away any excess liquid with a paper towel before placing the slide on the stage.

Start at low power

Always begin with the lowest-power objective lens, which gives a wide field of view and helps prevent damage to the lens or slide.

Focusing

Use the coarse focus to find the specimen and then the fine focus to sharpen the image before changing lens.

Eyepiece graticule

A small scale in the eyepiece has arbitrary divisions, and so it must be calibrated for each objective lens.

Stage micrometer

A slide with a scale of known size, often divided into 10 μm units, is used to calibrate the graticule.

Calibration

Line up the two scales and count how many graticule divisions match a known number of stage divisions.

Measuring cells

Measure a cell in graticule divisions and multiply by the calibration value to get the real size.

Graticule divisions	Stage micrometer length	One graticule division
40	250 μm	6.25 μm

Biological Drawing and Limitations

Drawing rules

Use a sharp pencil on plain paper, draw clear single lines, with no shading or sketching, and add a title and magnification.

Labels

Use ruled label lines that do not cross, ending at the structure they identify, with no arrowheads.

Size

The drawing should fill at least half the space available and show the proportions of the structures accurately.

Limitations

Tissue sections are cut at different planes, and preparation may change the appearance of cells.

Question

On a ×40 objective, 40 eyepiece graticule divisions line up with 25 divisions of a stage micrometer, each 10 μm.
A cell measures 12 graticule divisions.
Calculate its length.

Answer The cell is 75 μm long.

Coverslip A thin piece of glass placed over a specimen on a slide.
Eyepiece graticule A scale inside the eyepiece used to measure specimens.
Stage micrometer A microscope slide with a precise scale, used to calibrate the graticule.
Stain A dye that makes structures more visible.

Using the high-power lens first, which can crush the slide and scratch the lens.
Trapping air bubbles under the coverslip, which can be mistaken for cells.
Drawing cells with shading or overlapping lines instead of clear single lines.
Using the same graticule calibration for different objective lenses.

Last updated: 28 October 2026