Your eye can just about separate two dots a tenth of a millimetre apart. A bacterium is a hundred times smaller than that. Everything biologists know about the inside of a cell comes from instruments built to get past that limit — and from being able to work out how big the thing really is.
📚 What you need to know
Magnification is how many times bigger the image is than the real object. Resolution is the smallest gap between two points that still shows up as two points.
Light microscopes resolve down to about 200 nm; electron microscopes down to about 0.5 nm.
Light microscopes can show living, coloured specimens. Electron microscopes cannot — the sample is in a vacuum and the image is black and white.
Stains increase contrast so structures can be told apart.
Freeze fracture splits a frozen sample along the middle of membranes, showing membrane proteins.
Cryogenic electron microscopy freezes a sample so fast that molecules keep their natural shape.
Fluorescent stains and immunofluorescence make one specific molecule glow, so you can see where it is in the cell.
Magnification = image size ÷ actual size, and a scale bar tells you the real length the bar represents.
Magnification is not resolution
These two get mixed up constantly, so pin them down now.
Two different thingsMagnification = how many times larger the image is Resolution = the smallest distance between two points that can still be seen as separate
Think about zooming into a photo on your phone. Keep zooming and the picture gets bigger, but past a certain point it stops getting clearer — you just get bigger blur. You increased the magnification and gained nothing, because you ran out of resolution.
Microscopes work the same way. Magnifying beyond the resolution limit is called empty magnification: a larger image with no extra detail in it. This is why an electron microscope is genuinely better rather than just stronger. It has a much smaller resolution limit, so magnifying more actually reveals more.
The reason is the wavelength. A light microscope uses visible light, and you cannot resolve two points much closer together than about half a wavelength of what you are using to look at them. Electrons behave as waves with a far shorter wavelength, so the limit drops by a factor of several hundred.
The bars start at each resolution limit. Notice the light microscope stops well short of a ribosome — that is why organelles were only described properly once electron microscopes arrived.
Light and electron microscopes
Each type buys you something and costs you something. Exam questions almost always want a trade-off, not a winner.
Feature
Light microscope
Electron microscope
What passes through the sample
Visible light
A beam of electrons
Resolution
About 200 nm
About 0.5 nm
Useful magnification
Up to roughly ×2000
Hundreds of thousands of times
Living specimens
Yes — you can watch cells move
No — the sample sits in a vacuum
Colour
Natural colour, or stained
Black and white; colour is added afterwards
Preparing the sample
Quick and simple
Slow, skilled, and can create artefacts
Cost and size
Cheap, sits on a bench
Very expensive, fills a room
Two kinds of electron microscope
Transmission (TEM) — electrons pass through an extremely thin slice of the specimen. Best resolution, shows internal structure, but the image is flat and two-dimensional.
Scanning (SEM) — electrons bounce off the surface. Lower resolution than TEM, but gives a three-dimensional view of the outside of the specimen.
🧠
TEM or SEM?
T for through and thin slices, so you see inside. S for surface and scanning, so you see the outside in 3D.
Getting more out of a sample
A raw cell is mostly transparent and looks like grey mush. Several techniques fix that, and each one is on the syllabus for a reason.
Technique
What is done
What it lets you see
Staining
A dye is added that binds to particular structures
Increases contrast, so structures such as nuclei stand out from the background
Freeze fracture
The sample is frozen very fast, then cracked open. The crack runs along the weak middle of membranes
The inside face of a membrane, including the proteins embedded in it
Cryogenic electron microscopy
The sample is frozen so quickly that ice crystals never form, then imaged with electrons
Molecules in close to their natural shape, without the chemicals normally needed to prepare a sample
Fluorescent stains
A dye that glows under a certain wavelength of light is attached to a molecule
Where that molecule sits inside a living cell
Immunofluorescence
A fluorescent marker is attached to an antibody that binds one specific protein
The exact location of one named protein, because antibodies are so specific
Why immunofluorescence is such a big deal. Ordinary stains colour a whole type of structure. An antibody binds to one protein and nothing else, so attaching a glowing tag to it lets you follow that single protein around a living cell. That is the difference between seeing that a cell has a skeleton and seeing which fibre is which.
Working out magnification
This is the calculation that appears every year. It is one equation with two rearrangements.
Learn this one
magnification = image size ÷ actual size
actual size = image size ÷ magnification
image size = actual size × magnification
Magnification never has a unit, because the two lengths cancel. If your answer comes out in mm or µm, you have divided the wrong way round.
Units, and how to move between them
Unit
Symbol
In metres
Typical use
millimetre
mm
1 × 10–3 m
What you measure on the page with a ruler
micrometre
µm
1 × 10–6 m
Cells and most organelles
nanometre
nm
1 × 10–9 m
Membranes, ribosomes, molecules
Two conversions do everything: 1 mm = 1000 µm and 1 µm = 1000 nm. Going to a smaller unit means multiplying by 1000; going to a bigger one means dividing by 1000.
Measure in millimetres, then convert to micrometres straight away by multiplying by 1000. Doing the conversion first, before any dividing, removes almost all the mistakes I see in this calculation.
Scale bars
A micrograph in a book might be printed at any size, so a stated magnification would be useless. Instead the image carries a scale bar: a short line labelled with the real length it stands for.
With a magnification of ×2000, the cell above is 60 mm ÷ 2000 = 0.03 mm across in real life, which is 30 µm.
Worked examples
WE 1
Calculate a magnification
A cell in a micrograph measures 45 mm across. The cell is actually 30 µm across. Calculate the magnification. (2 marks)
Step 1: same units first
45 mm × 1000 = 45 000 µm
Step 2: divide image by actual
45 000 µm ÷ 30 µm = 1500
Step 3: write it properly
Magnification has no unit, so it is written with a multiplication sign in front.
×1500converting to µm first is safer than converting the cell size to mm
WE 2
Calculate an actual size
An organelle measures 8 mm on an image taken at a magnification of ×400. Calculate its actual size in µm. (2 marks)
Step 1: rearrange
actual size = image size ÷ magnification
Step 2: substitute
8 mm ÷ 400 = 0.02 mmStep 3: convert to the unit asked for
0.02 mm × 1000 = 20 µm
20 µmcheck the question for the unit it wants — a right number in the wrong unit loses the mark
WE 3
Use a scale bar
A micrograph carries a scale bar labelled 10 µm, which measures 20 mm when you put a ruler on it. A cell in the same image measures 60 mm across. Calculate the magnification and the real width of the cell. (3 marks)
Step 1: magnification from the bar
20 mm = 20 000 µm, so magnification = 20 000 µm ÷ 10 µm = ×2000Step 2: real width of the cell
60 mm ÷ 2000 = 0.03 mm
Step 3: convert
0.03 mm × 1000 = 30 µm
×2000, and the cell is 30 µm widethe scale bar and the cell must be measured on the same copy of the image
💡 Exam tips
Convert units before you divide. It is the single biggest source of lost marks in this topic.
Write magnification as ×1500, with no unit after it.
Show the equation, then the numbers, then the answer. Working earns marks even if the final number slips.
Learn the two resolution figures: 200 nm for light, 0.5 nm for electron.
If asked to compare microscopes, give a trade-off: better resolution but no living specimens.
Know one sentence for each of freeze fracture, cryo-EM, fluorescent stains and immunofluorescence.
⚠ Common mistakes
Saying an electron microscope has higher magnification and stopping there. The real advantage is resolution.
Dividing actual by image. The image is the big one, so magnification should come out larger than 1.
Putting a unit on magnification. The units cancel.
Multiplying instead of dividing when converting µm to mm. Smaller unit, bigger number.
Claiming electron microscopes show living cells in colour. Vacuum kills the sample, and the image is black and white.
Measuring the scale bar on one printout and the cell on another. Different sizes, meaningless answer.
Up next: General Cell Structure — what all those instruments actually reveal, and the parts every cell shares before we split them into prokaryotes and eukaryotes.
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