All subjects · Biology · WBI12 (Unit 2)

Magnification, resolution and staining

Microscopes make cells look bigger, but seeing detail depends on resolution. Stains make transparent structures visible.

Magnification

Magnification = image size / actual size.

Rearranged: actual size = image size / magnification.

Always convert to the same units first: 1 mm = 1000 μm, and 1 μm = 1000 nm.

Resolution

Resolution is the ability to distinguish two points close together as separate. Magnifying beyond the resolution limit only gives a bigger blurred image.

Light microscope: uses light; resolution about 0.2 μm (200 nm); useful magnification up to about x1500. Living cells can be viewed, in colour.
Electron microscope: uses a beam of electrons, which have a much shorter wavelength, so resolution is far higher (well under 1 nm) and magnification can exceed x500 000. Specimens must be in a vacuum, so they are dead. A TEM passes electrons through thin sections to show internal ultrastructure; a SEM scans the surface to give 3D images.

Why stain?

Most cell structures are transparent and colourless, so there is little contrast. Stains bind to particular structures, making them visible: for example methylene blue for animal cell nuclei, iodine for starch, toluidine blue or acetic orcein for chromosomes. Electron microscopy uses heavy metal stains that block electrons.

Worked example

A mitochondrion is 3 μm long. In a micrograph its image is 45 mm long. Calculate the magnification.

Convert to the same units: 45 mm = 45 000 μm.
Magnification = 45 000 / 3

Answer: x15 000

Key idea: Magnification = image size / actual size (same units). Resolution is the smallest separation at which two points can be seen as separate: about 200 nm for light, far smaller for electron microscopes. Stains add contrast.

Check yourself

A cell is 20 μm wide. Its image in a drawing is 40 mm wide. What is the magnification?
Show answer
x2000. 40 mm = 40 000 μm; 40 000 / 20 = 2000.
An image of a cell is 30 mm long at a magnification of x1500. What is the actual length in μm?
Show answer
20 μm. 30 000 μm / 1500 = 20 μm.
Convert 0.2 μm into nanometres.
Show answer
200 nm. 1 μm = 1000 nm, so 0.2 μm = 200 nm.
What is resolution?
- How many times larger the image is than the object
- The brightness of the image
- The ability to distinguish two points close together as separate
- The size of the field of view
Show answer
The ability to distinguish two points close together as separate. Resolution is about detail, not size.
Why does an electron microscope have a higher resolution than a light microscope?
- Electrons have a much shorter wavelength than light
- It uses brighter light
- It uses stains
- Specimens are alive
Show answer
Electrons have a much shorter wavelength than light. Shorter wavelength allows finer detail to be resolved.
Why are specimens often stained for light microscopy?
- Stains make cells larger
- Stains kill bacteria
- Most cell structures are transparent, so stains add contrast
- Stains increase resolution beyond 200 nm
Show answer
Most cell structures are transparent, so stains add contrast. Stains bind to structures, making them visible.
Which is a limitation of electron microscopes?
- Specimens must be dead because they are in a vacuum
- They cannot show mitochondria
- Their resolution is lower than light microscopes
- They only show colour images
Show answer
Specimens must be dead because they are in a vacuum. The vacuum means living cells cannot be viewed.

Practise with instant feedback

Sources

Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.