Cells and organisation · GCSE Biology
Microscopy and magnification
Calculate magnification for GCSE Biology, convert mm to µm to nm, and compare light and electron microscopes on resolution, living specimens and which organelles you can actually see.
Magnification = image size ÷ real size. Convert units first: 1 mm = 1000 µm, 1 µm = 1000 nm. Resolution is not the same as magnification.
The important bits
What you need to know
- 1
Magnification = size of image ÷ size of real object. Rearrange: real size = image size ÷ magnification; image size = real size × magnification.
- 2
Convert to the same unit before dividing. 1 mm = 1000 µm; 1 µm = 1000 nm; 1 m = 1000 mm. A nucleus drawn 8 mm at ×2000 is 8000 µm ÷ 2000 = 4 µm.
- 3
A light microscope uses light and glass lenses. Maximum useful magnification is about ×1500–×2000. Resolution is about 0.2 µm, so you can see nuclei and chloroplasts but not ribosomes or plasmids clearly.
- 4
An electron microscope uses a beam of electrons. Magnification can exceed ×1 000 000 and resolution is about 0.1 nm, so mitochondria, ribosomes, membranes and bacterial DNA loops become visible.
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Resolution is the shortest distance between two points that can still be distinguished as separate. High magnification with poor resolution gives a bigger blur.
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Light microscopes can view living cells and can use stains (iodine for starch, methylene blue for nuclei). Electron specimens are dead, in vacuum, often stained with heavy metals, and the machines are expensive.
- 7
Required practical skills: start on the lowest power objective, centre the specimen, then increase power; use the fine focus; stain to add contrast; calculate magnification from a scale bar if given.
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Order of size (largest to smallest): animal cell (~20 µm) > nucleus (~8 µm) > mitochondrion (~1–2 µm) > bacterium (~1–5 µm) > virus (~100 nm) > ribosome (~20–30 nm).
Quotations worth analysing
Short evidence. Real method.
“magnification = size of image / size of real object”
Write the triangle: image on top. Most lost marks are inverted division or mixed millimetres and micrometres.
“Resolution is the ability to distinguish two points as separate.”
Do not treat resolution as a synonym for magnification. Electron microscopes win on both, but the definition is about distinguishing points.
“1 mm = 1000 micrometres (µm).”
Convert the measured image into µm before dividing by magnification if you want the real size in µm.
Go deeper
How do I stop losing the unit-conversion mark?
Measure the image with a ruler in millimetres. If the real size is needed in micrometres, multiply the millimetre reading by 1000 first. Then divide by magnification. Example: image of a chloroplast is 12 mm, magnification ×4000. 12 mm = 12 000 µm. Real size = 12 000 ÷ 4000 = 3 µm, a sensible chloroplast width. If you forget the ×1000 you get 0.003 µm, which is smaller than a ribosome and should ring an alarm. If you multiply by magnification you get an enormous number. Write the formula, substitute with units, then box the answer with the unit. Scale-bar questions: measure the bar and the object in the same unit, then real size of object = (object length ÷ bar length) × bar’s labelled real size.
Go deeper
When must I choose an electron microscope in an explain question?
Choose it when the feature is smaller than about 0.2 µm or when you need internal membranes: ribosomes, plasmids, the detail of mitochondria’s inner membrane, viruses. Say higher resolution (and usually higher magnification) so sub-cellular structures can be distinguished. Then give a disadvantage if the question is evaluate: specimens are dead, preparation can add artefacts, the equipment is expensive, and you cannot watch living processes such as cytoplasmic streaming. Light microscopes win when the cell must stay alive, when you are counting pondweed cells in a school lab, or when colour stains matter. “Electron microscopes are more zoomed in” is weak; use resolution and a named organelle.
Go deeper
What does a microscope practical method actually need?
Thin specimen (light must pass through), coverslip lowered at an angle to avoid air bubbles, stain if the cell is transparent, lowest power first to find the cell, then high power, fine focus only on high power so you do not crack the slide. Iodine stains starch blue-black in potato or in a leaf after ethanol decolourising. Methylene blue stains nuclei in cheek cells. Safety: ethanol is flammable; iodine stains skin. To estimate size without a specialised eyepiece graticule, some papers let you compare the cell to the known field-of-view diameter. Always state repeats and that you recorded the objective lens so magnification of the drawing can be calculated later: drawing magnification = drawing size ÷ actual size.
See the idea in action
A student measures a nucleus as 8 mm on a micrograph taken at ×2000. Convert 8 mm to micrometres: 8 × 1000 = 8000 µm. Real size = image size ÷ magnification = 8000 ÷ 2000 = 4 µm. That is a sensible order of magnitude for a nucleus. If the answer were 4 mm, units were not converted. If it were 16 000 µm, the formula was upside down. A ribosome at 25 nm would not be visible as a clear dot on a school light microscope because 25 nm is far below 0.2 µm resolution.
Exam technique
Turn knowledge into marks
Write the formula, convert to one unit, then calculate. In compare tables, pair each microscope with resolution, living versus dead specimens, and one named structure only the electron microscope can show (ribosome or plasmid).
Common mistakes
Do not give these marks away
- 01
Forgetting to convert millimetres to micrometres before using the magnification formula.
- 02
Treating magnification and resolution as the same thing.
- 03
Saying light microscopes cannot see nuclei, or that electron microscopes can view living cells.
A mitochondrion is 4 mm on an image taken at ×2000. What is its real length in micrometres?
A2 µm
B4 µm
C8 µm
D2000 µm
Show the answer
2 µm. 4 mm = 4000 µm. Real size = 4000 ÷ 2000 = 2 µm. That matches the typical 1–2 µm length of a mitochondrion.
Quick questions
If this is the bit you searched
How do you calculate magnification in GCSE Biology?
Magnification = size of image ÷ size of real object. Convert both measurements to the same unit first, usually micrometres.
What is the difference between magnification and resolution?
Magnification is how many times larger the image is. Resolution is the shortest distance at which two points can still be seen as separate.
Why can electron microscopes see more than light microscopes?
They have much higher magnification and resolution, so organelles such as ribosomes and mitochondrial membranes can be distinguished. Specimens must be dead.
How many micrometres are in a millimetre?
1000. 1 mm = 1000 µm, and 1 µm = 1000 nm.