This is the page where marks are won and lost on units, not on biology. Get comfortable moving between millimetres, micrometres and nanometres and the magnification questions become some of the easiest in the paper.
📚 What you need to know
A light microscope shines light through a thin specimen on a slide, then focuses it through the objective and eyepiece lenses.
Always start on the lowest power objective, focus with the coarse knob, then move up and use only the fine knob.
An eyepiece graticule acts as a ruler, but it must be calibrated against a stage micrometer for each objective lens.
Magnification = image size ÷ actual size. Magnification has no units.
1 m = 1000 mm, 1 mm = 1000 µm, 1 µm = 1000 nm.
Convert everything to the same unit before you calculate anything.
A scale bar is a line whose stated length is the real size it represents.
The parts of a light microscope
Starting on low power does two jobs: it is far easier to find your specimen in a wide field of view, and it stops the lens crashing into the slide.
Preparing a slide
🧩 Making a temporary mount
Get a thin sample. Liquid: a few drops with a pipette. Solid: peel or cut a very thin layer with a scalpel or forceps, using scissors to trim first.
Add a stain if needed. Many structures are colourless, so a stain makes them visible.
Lower a coverslip gently and press down to squeeze out air bubbles — bubbles look like dark-edged circles and get mistaken for cells.
Clip the slide onto the stage and switch the light on.
Focus on low power first, then rotate to a higher objective and sharpen with the fine focus only.
Wear gloves to avoid staining your skin and to stop your own cells contaminating the sample.
Blurry image? Work through this. Drop back to a lower power and re-focus with the coarse knob. Check the sample is thin enough for light to pass through. And check you are not looking at contamination — a stray fibre or your own skin cell — rather than your specimen.
Measuring with a graticule
An eyepiece graticule is a small glass disc with a scale engraved on it, sitting inside the eyepiece. You see the scale laid over whatever you are looking at.
The catch is that its divisions have no fixed size. Change the objective lens and the specimen changes size while the graticule stays the same, so the scale means something different. That is why you must calibrate it against a stage micrometer — a slide with a scale of known real length engraved on it.
Recalibrate every time you change objective lens. The graticule divisions never change size, but what they are worth does.
Magnification calculations
Magnification tells you how many times bigger the image is than the real thing. It is a ratio of two lengths, so it has no units — just a number, usually written with a multiplication sign.
Learn this triangle
magnification = image size ÷ actual size
actual size = image size ÷ magnification
Getting the units right
Going down a size
Do this
Example
m to mm
× 1000
0.5 m = 500 mm
mm to µm
× 1000
0.005 mm = 5 µm
µm to nm
× 1000
5 µm = 5000 nm
Going back up
÷ 1000 each time
20 000 nm = 20 µm = 0.02 mm
Do the unit conversion before you touch the calculator, and pick the smaller of the two units to convert into. Most lost marks here are not biology mistakes at all — they are a factor of a thousand in the wrong direction.
Using a scale bar
A scale bar is a line drawn on a micrograph with a real length written beside it. Measure the printed line with a ruler and you have the image size; the label is the actual size. Then it is just the same formula.
Worked examples
WORKED EXAMPLE
An image of a cell measures 45 mm across. The cell is really 15 µm across. Calculate the magnification.
Step 1: convert to the same unitthe smaller unit here is micrometres, so convert the image size45 mm × 1000 = 45 000 µmStep 2: apply the formulamagnification = 45 000 ÷ 15× 3000no units on the answer – it is a ratio
WORKED EXAMPLE
A drawing of an organelle is 60 mm long and was made at a magnification of ×1500. Calculate its actual length in µm.
Step 1: rearrange the formula
actual size = image size ÷ magnification
Step 2: substitute60 ÷ 1500 = 0.04 mmStep 3: convert to micrometres0.04 mm × 1000 = 40 µm40 µmcheck the unit the question asks for – it is rarely the one you end up with
WORKED EXAMPLE
10 eyepiece graticule divisions line up with 0.4 mm on a stage micrometer. A cell spans 6 graticule divisions. Calculate its length in µm.
Step 1: calibrate one division0.4 mm ÷ 10 = 0.04 mm per division0.04 mm × 1000 = 40 µm per divisionStep 2: multiply by the number of divisions6 × 40 = 240240 µmcalibrate first, measure second – always in that order
WORKED EXAMPLE
A micrograph has a scale bar labelled 10 µm. Measured with a ruler, the bar is 30 mm long. Calculate the magnification of the micrograph.
Step 1: identify which is whichthe measured bar is the image size; the label is the actual sizeStep 2: convert to the same unit30 mm × 1000 = 30 000 µmStep 3: divide30 000 ÷ 10 = 3000× 3000
💡 Exam tip
Write the formula down first, rearrange it, and only then put numbers in. It stops most errors.
Convert units before calculating, and convert into the smaller unit shown in the question.
Magnification never has units. Writing “×3000 µm” loses the mark.
Measure scale bars carefully with a ruler in mm — estimating is a guaranteed way to be out.
If the numbers look wrong by a factor of 1000, you have almost certainly converted the wrong way.
Measurements from a microscope are quantitative data; colour and presence of structures are qualitative.
⚠ Common mix-up
Dividing the wrong way round. Image on top, actual underneath — the image is always the bigger number.
Forgetting to convert units at all. 45 mm ÷ 15 µm is not 3.
Using the coarse focus on high power. It can crack the slide and the lens.
Assuming a graticule reading is already in µm. It is in divisions until you calibrate it.
Treating air bubbles as cells. Bubbles have thick, very dark, perfectly round edges.
Confusing magnification with resolution. Magnifying a blurry image just gives a bigger blur.
Up next: Microscopes — why a light microscope hits a wall at about 200 nm, and what electron microscopes can do about it.
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