A light microscope can magnify a cell 1500 times, yet it will never show you a ribosome. To understand why, you need two separate ideas, magnification and resolution, and a way to measure what you see in micrometres.
Magnification is how many times larger the image is than the real object: magnification = image size ÷ actual size, with both in the same unit (1 mm = 1000 μm, 1 μm = 1000 nm). Resolution is the smallest distance between two points that can still be seen as separate. It is limited by wavelength: a light microscope resolves about 200 nm, an electron microscope about 0.5 nm, so only the electron microscope shows organelle detail. To measure with a light microscope, calibrate the eyepiece graticule against a stage micrometer for each objective.
Fig. 1: calibrating an eyepiece graticule against a stage micrometer with the ×40 objective
Section 01
Light and electron microscopes.
Click down the size ladder from a person to a carbon atom. Each step is ten times smaller. Watch where the eye gives up (about 0.1 mm), where the light microscope gives up (200 nm), and what is left for the electron microscope.
Light microscope
Transmission EM
Scanning EM
radiation
light (wavelength 400 to 700 nm)
electron beam through a thin section
electron beam scanned over the surface
resolution
about 200 nm
about 0.5 nm
about 3 to 10 nm
useful magnification
up to about ×1500
up to ×500 000 or more
up to about ×100 000
image
colour, living or dead cells
2D section, black and white; specimen dead, in a vacuum
3D surface view, black and white; specimen dead, in a vacuum
Units: 1 mm = 1000 μm; 1 μm = 1000 nm. Cells are measured in μm, organelles in μm or nm, ribosomes and membranes in nm.
Why electrons win: resolution is roughly half the wavelength used. Electrons have a far shorter wavelength than light, so points much closer together can be distinguished.
Examiner trap
Increasing the magnification does not improve the resolution. Beyond about ×1500 a light microscope just gives a larger blur, because two points closer than 200 nm still merge. Define both terms separately: magnification is about size, resolution is about detail.
Section 02
Measuring with a graticule.
Switch on the stage micrometer, then change the objective. The stage micrometer is a slide ruled with 10 μm divisions; the eyepiece graticule is a scale with no units inside the eyepiece. Line them up and the graticule gets its value. Then switch back to the onion epidermis and click both ends of a cell.
Calibrate: line up the two scales and find a point further along where they coincide again. Divide the micrometer length by the number of graticule divisions (Fig. 1: 180 μm ÷ 40 = 4.5 μm per division).
Measure: rotate the eyepiece so the graticule lies across the cell, count divisions and multiply. In the sim at ×10, one division is 18 μm, so an onion cell 7.5 divisions long is 135 μm.
Recalibrate for every objective: the graticule stays the same size in the eyepiece but the specimen is magnified more, so each division covers less of it (18 μm at ×10, 4.5 μm at ×40).
Section 03
Magnification and scale bars.
Electron micrographs rarely state a magnification; they carry a scale bar. Drag the ruler onto the scale bar to find the magnification, then onto the cell to find its actual size.
Magnification from a scale bar: measure the bar with a ruler (say 20 mm), convert to the bar's unit (20 000 μm), divide by what it represents (5 μm): magnification = ×4000.
Actual size: actual = image ÷ magnification. A cell image of 48 mm at ×4000 is 48 000 μm ÷ 4000 = 12 μm.
Drawing a scale bar: work backwards. At ×4000, a 10 μm bar must be 10 × 4000 = 40 000 μm = 40 mm long.
Examiner trap
Convert before you divide. The most common slip is dividing a length in mm by a length in μm, which puts the answer out by a factor of 1000. Write both measurements in μm first, and check the answer is a sensible size for that structure.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
On the size ladder, which is the largest object that can only be seen with an electron microscope?
Quick check+10 XP
In the virtual microscope one graticule division is 18 μm with the ×10 objective. What is it with the ×40 objective?
Quick check+10 XP
Which equation is correct?
Quick check+10 XP
How many nanometres are there in one micrometre?
Section 04
Slides and drawings.
The practical side of 1.1: making a temporary preparation and turning what you see into a useful drawing.
Temporary preparation: peel a thin layer (for example onion epidermis) so light passes through, lay it flat in a drop of water or stain (iodine in potassium iodide for plant cells, methylene blue for cheek cells), and lower a coverslip at an angle with a mounted needle to avoid trapping air bubbles.
Drawing cells: sharp pencil, clear continuous lines, no shading, correct proportions, and a drawing that fills most of the space available. Draw only what you see.
Labels: ruled label lines that touch the structure and do not cross; add a title and the magnification or a scale bar.
Low-power plan diagrams show the outlines of tissues only, with no individual cells; high-power drawings show a few cells in detail.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
A mitochondrion is 36 mm long in an electron micrograph magnified ×12 000. What is its actual length?
Exam style+20 XP
A scale bar on a micrograph is 25 mm long and is labelled 5 μm. What is the magnification of the micrograph?
Exam style+20 XP
With a ×40 objective, 100 eyepiece graticule divisions match 0.25 mm on a stage micrometer. A cell spans 32 graticule divisions. How long is the cell?
Exam style+20 XP
Ribosomes cannot be seen with a light microscope, even at ×1500. Which statement explains this?
Skill unlocked
Microscopes and measurement, mastered.
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