Microscopy lets biologists see structures too small for the naked eye. Light microscopes magnify up to about ×2,000 and reveal cells and large organelles; electron microscopes reach ×2,000,000 and resolve individual proteins. The formula magnification = image size ÷ actual size underpins every calculation at GCSE.
What is the difference between magnification and resolution?
These two terms are frequently confused but describe completely different properties of a microscope.
Magnification is how many times bigger the image appears compared to the actual object. A ×400 magnification makes an object look 400 times wider than it really is. Magnification alone tells you nothing about detail.
Resolution (or resolving power) is the ability to distinguish two points as separate rather than blurred together. Higher resolution means finer detail is visible. The resolution of a light microscope is limited by the wavelength of visible light — roughly 400–700 nm — so two points closer than about 200 nm cannot be resolved, regardless of magnification.
This is why a light microscope image can be blurry even at maximum zoom: you have increased magnification without improving resolution, producing empty magnification.
How does a light microscope work?
A light microscope passes visible light through a specimen and through a series of glass lenses to produce a magnified image. The key components are:
- Eyepiece lens (ocular): typically ×10
- Objective lenses: commonly ×4 (low power), ×10 (medium), ×40 (high), and ×100 (oil-immersion)
- Stage: holds the glass slide
- Condenser and iris diaphragm: control the amount and focus of light
- Coarse and fine focus knobs: adjust the distance between objective and slide
Total magnification = eyepiece magnification × objective magnification
So a ×10 eyepiece with a ×40 objective gives ×400 total magnification.
Light microscopes are cheap, easy to use, can show living specimens, and can display colour. They are limited by resolution (200 nm) and maximum useful magnification (×2,000).
How does an electron microscope differ?
Electron microscopes use a beam of electrons rather than light. Because electrons have a much shorter wavelength than visible light (down to ~0.001 nm), their resolution is dramatically higher — around 0.1 nm for a transmission electron microscope (TEM).
| Feature | Light microscope | Electron microscope |
|---|---|---|
| Source of illumination | Visible light | Beam of electrons |
| Maximum resolution | ~200 nm | ~0.1 nm (TEM) |
| Maximum useful magnification | ~×2,000 | ~×2,000,000 |
| Can view living specimens? | Yes | No (specimen must be in a vacuum) |
| Shows colour? | Yes | No (black and white; colour added artificially) |
| Cost | Low | Very high |
| Portability | Can be portable | Large, fixed instruments |
Transmission electron microscopes (TEM) fire electrons through a thin section of specimen, showing internal ultrastructure — including ribosomes, mitochondrial cristae, and endoplasmic reticulum.
Scanning electron microscopes (SEM) bounce electrons off the specimen surface, producing detailed three-dimensional images of external structures (e.g. the surface of a pollen grain or a virus).
How do you calculate magnification and actual size?
The magnification formula triangle is essential for every GCSE calculation:
Magnification = Image size ÷ Actual size
Rearranged: Actual size = Image size ÷ Magnification; Image size = Actual size × Magnification
Worked example:
A drawing of a cell measures 45 mm. The scale bar on the diagram states that 5 mm represents 50 µm in real life.
- Find the magnification from the scale bar: magnification = image size of bar ÷ actual size of bar = 5 mm ÷ 0.05 mm = ×100
- Find the actual size of the cell: actual size = image size ÷ magnification = 45 mm ÷ 100 = 0.45 mm = 450 µm
Unit conversions you must know:
| Unit | Symbol | Equivalent |
|---|---|---|
| Millimetre | mm | 1 mm = 1,000 µm |
| Micrometre | µm | 1 µm = 1,000 nm |
| Nanometre | nm | 1 nm = 0.001 µm |
Always convert both measurements into the same unit before dividing.
How do you prepare a slide for light microscopy?
The GCSE required practical for microscopy involves preparing and observing a plant or animal cell. The standard method for a temporary wet mount:
- Place a drop of water on a clean glass slide.
- Use forceps to peel a thin, single-cell-layer section of onion epidermis (or obtain a cheek cell by gently scraping the inside of the cheek with a cotton swab).
- Lower the coverslip at a 45° angle using a mounted needle to avoid trapping air bubbles.
- Add a drop of stain (iodine solution stains starch/nuclei in plant cells; methylene blue stains nuclei in animal cells) at one edge of the coverslip and draw it under with filter paper at the opposite edge.
- Place the slide on the stage, start on low power, focus, then move to higher power.
Common errors: too much stain (obscures detail), air bubbles under coverslip (appear as dark rings), too thick a section (light cannot pass through).
What cell structures can you see under different microscopes?
| Structure | Size | Visible with light microscope? | Visible with electron microscope? |
|---|---|---|---|
| Nucleus | ~10 µm | Yes | Yes |
| Chloroplast | ~5 µm | Yes | Yes (fine structure) |
| Mitochondrion | ~2 µm | Barely (as dot) | Yes (cristae visible) |
| Ribosome | ~25 nm | No | Yes (TEM) |
| Cell membrane | ~8 nm | No | Yes (TEM) |
| DNA double helix | ~2 nm | No | Yes (special TEM) |
Understanding these scale relationships — from the whole organism down to individual molecules — is the biologist's core skill: every observation is a window into how structure enables function.
Frequently asked questions
Why can't you see ribosomes with a light microscope?
Ribosomes are approximately 25 nm in diameter. The resolution of a light microscope is limited to roughly 200 nm — the minimum distance at which two separate points can be distinguished. Because ribosomes are about eight times smaller than this limit, even at maximum magnification they blur together into the background. Electron microscopes, with resolution down to ~0.1 nm, can resolve ribosomes clearly, which is how scientists confirmed their existence and structure.
What does "×400 magnification" actually mean?
It means the image you see is 400 times larger in each linear dimension than the real object. A cell 10 µm across will appear 4,000 µm (4 mm) wide in the image. Magnification is a ratio with no units. Total magnification for a compound light microscope is the eyepiece magnification multiplied by the objective magnification in use.
Why must specimens be dead for electron microscopy?
Electron microscopes work by firing a beam of electrons at the specimen inside a vacuum chamber. Living cells cannot survive in a vacuum — they desiccate immediately. Specimens must be chemically fixed, dehydrated, resin-embedded, and (for TEM) cut into sections only ~70 nm thick using a diamond-bladed ultramicrotome. These preparation steps kill the cell. SEM specimens are typically coated in a thin layer of gold or platinum to make them electrically conductive so electrons do not build up on the surface.
How is a scale bar used on a micrograph?
A scale bar is a line on the image labelled with its real-world length (e.g. "10 µm"). To find the magnification, measure the length of the scale bar on the image in mm and divide by its stated real length in mm. To find the actual size of any structure, measure it on the image, then divide by the magnification. This method works regardless of whether the image has been printed at a different size, because the scale bar shrinks or grows with the image.
For Socratic GCSE biology with Professor Darwin — tracing cell structure from the nanometre to the organism — visit aitutors.me.