How Do We Know? Microscopy, Magnification and Scale

Everything on this page — the cristae inside mitochondria, the grana inside chloroplasts, even the existence of ribosomes — was discovered using microscopes. To use microscope images properly, you need to understand resolution, magnification, units, and when to estimate rather than count exactly.

Resolution vs Magnification

Magnification is how much bigger an image has been made. Resolution is the smallest distance at which two points can still be seen as separate — it determines how much real detail is visible. Light microscopes use visible light and have a resolution of about 200 nanometres (nm). Electron microscopes use electrons, which have a much shorter wavelength, giving a resolution of about 0.2 nm — around 1000 times better. Because of this, increasing the magnification of a light microscope image cannot reveal detail that its resolution cannot separate — the image just becomes bigger and blurrier, sometimes called "empty magnification".

This is why electron microscopy has increased our understanding of sub-cellular structures so much: only its far higher resolution can reveal internal ultrastructure such as the cristae of mitochondria or the double membranes of chloroplasts.

The Magnification Formula

Magnification = image size ÷ real size

This can be rearranged to find either of the other two values:

• Real size = image size ÷ magnification — used to find the true size of an organelle from a micrograph
• Image size = real size × magnification — used to predict how big a structure will appear once magnified

Converting Between Units

Organelles are measured in units far smaller than a millimetre:

• 1 millimetre (mm) = 1000 micrometres (μm)
• 1 micrometre (μm) = 1000 nanometres (nm)
• So 1 mm = 1 000 000 nm

Very small measurements are often written in standard form (A × 10ⁿ, where 1 ≤ A < 10). For example, a ribosome with a diameter of 0.00002 mm is written as 2 × 10⁻⁵ mm.

Using a Scale Bar

Grade 7-9

Micrographs (microscope images) often include a scale bar — a line labelled with a real length (e.g. "1 μm"). To find the magnification of the whole image: measure the scale bar's length on the page, convert it to the same units as its label, then divide. For example, if a scale bar labelled 1 μm measures 20 mm on the page: convert 20 mm to 20 000 μm, then magnification = 20 000 μm ÷ 1 μm = ×20 000. This same magnification then applies to every other structure in that image.

When to Estimate Instead of Count

Grade 7-9

Some structures — like the number of mitochondria in a single cell — are far too numerous to count exactly in a reasonable time. Estimating is appropriate whenever an exact count would be impractical and a slightly less precise answer is still good enough for the purpose of the investigation. A sensible method is to count the number of structures within one small, measured section of the image, then scale this figure up to estimate the total for the whole area.