Cells were invisible for most of human history. The microscope didn't just let us see them — it forced us to completely rethink what life is made of.
Before you read on, make a prediction:
A light microscope and a transmission electron microscope are both pointed at the same cell. Predict two specific differences you would expect to see between the two images produced.
Come back to this at the end of the lesson.
Core Content
For most of human history, no one knew what living things were made of at a structural level — not because they hadn't thought about it, but because there was no technology to look. The typical cell is 10–100 micrometres across. The human eye resolves detail down to about 100 micrometres at best. Cells sit right at — or below — the limit of naked-eye vision.
The microscope didn't just magnify. It opened an entirely new domain of scientific inquiry. Each generation of microscope technology unlocked a different layer of biological structure — and with it, questions that the previous generation couldn't even formulate.
The HSC requires you to compare three types of microscope and understand what each one reveals about cell structure.
| Type | How it works | Max magnification | Resolution | What it reveals | Limitations |
|---|---|---|---|---|---|
| Light microscope (LM) | Visible light through specimen; glass lenses focus image | ~1,500× | ~200 nm | Cell shape, nucleus, large organelles, cell division stages | Cannot resolve most organelle internal structure; limited by wavelength of visible light |
| Scanning Electron Microscope (SEM) | Electron beam scans specimen surface; detectors collect scattered electrons | ~100,000× | ~1–20 nm | 3D surface structure of cells and organelles; virus shape | Specimen must be dead and metal-coated; no internal detail; greyscale only |
| Transmission Electron Microscope (TEM) | Electrons pass through ultra-thin specimen sections | ~500,000× | ~0.1 nm | Internal ultrastructure — membrane layers, ribosomes, organelle membranes, virus internal structure | Specimen must be dead, fixed and ultra-thinly sliced; 2D image only |
The relationship between magnification, image size and actual size is always:
Magnification = Image size ÷ Actual size
Actual size = Image size ÷ Magnification | Image size = Actual size × Magnification
Units matter — always convert to the same unit before calculating.
A cell in a micrograph measures 45 mm. Magnification is ×500. What is the actual size in µm?
Actual size = Image size ÷ Magnification = 45 mm ÷ 500 = 0.09 mm 0.09 mm × 1000 = 90 µmThe cell is 90 µm — within typical eukaryotic range (10–100 µm).
A mitochondrion is 2 µm long. In a TEM image it appears 40 mm. What is the magnification?
Convert: 2 µm = 0.002 mm Magnification = 40 mm ÷ 0.002 mm = ×20,000In an exam, justify your choice using three criteria: internal vs surface detail, living vs dead specimen, resolution required.
| Investigation | Best choice | Why |
|---|---|---|
| Observing mitosis in a living root tip | Light microscope | Specimen can be alive; resolution sufficient for chromosomes; coloured stains improve contrast |
| Examining 3D surface texture of a pollen grain | SEM | High-resolution 3D surface image; no internal detail needed |
| Determining internal structure of a virus | TEM | Highest resolution; penetrates specimen to show internal protein coat and nucleic acid arrangement |
| Counting and classifying blood cell types | Light microscope | Cells can be stained; resolution sufficient; fast and accessible |
Misconception: Higher magnification always gives a better image.
Magnification without resolution gives a larger blur. Resolution is the limiting factor. A light microscope at ×1500 cannot show ribosome detail because the wavelength of visible light (~400–700 nm) is physically too large to resolve structures below 200 nm.
Misconception: Electron microscopes produce colour images.
Electron microscopes produce greyscale images. Any colour in published micrographs is false-colour added digitally after capture to highlight different structures.
Misconception: SEM shows internal cell structure.
SEM produces a surface image only — it cannot penetrate the specimen. TEM is required for internal organelle ultrastructure.
Magnification = Image size ÷ Actual size
Always convert units first. 1 mm = 1,000 µm.
Activities
In your book, draw and label a simple diagram of each of the three microscope types. For each diagram include:
Then write two sentences explaining why electron microscopes have higher resolution than light microscopes. Use the word "wavelength."
Draft your summary sentences here.
A marine biologist is studying a newly discovered deep-sea microorganism. She has access to all three microscope types.
Write your responses here or in your book.
Assessment
1. A scientist needs to observe the internal membrane structure of the endoplasmic reticulum. Which microscope is most appropriate?
2. A cell in a micrograph measures 30 mm. The actual size of the cell is 15 µm. What is the magnification?
3. Which best explains why a scanning electron microscope cannot be used to observe living cells?
4. TEM resolution is ~0.1 nm. Light microscope resolution is ~200 nm. What does this mean in practical terms?
5. A published micrograph shows a cell with bright red mitochondria and a blue nucleus. Which is correct?
1. Distinguish between magnification and resolution. Explain why increasing magnification on a light microscope beyond a certain point does not improve image quality. (3 marks)
1 mark each: magnification definition, resolution definition, wavelength explanation
2. A student measures a chloroplast in a light micrograph as 18 mm. The magnification is ×600. Calculate the actual size of the chloroplast in micrometres. Show all working. (3 marks)
1 mark formula, 1 mark conversion, 1 mark correct answer with units
3. In January 2020, scientists used TEM to image SARS-CoV-2 and reveal the structure of its spike proteins. Explain why a light microscope could not have been used, and outline how this structural information contributed to vaccine development. (3 marks)
1 mark LM limitation; 1 mark TEM capability; 1 mark vaccine connection
Answers
SA1: Magnification is the ratio of the image size to the actual size — how many times larger the image appears. Resolution is the minimum distance between two points that can be distinguished as separate structures. Increasing magnification on a light microscope beyond approximately ×1,500 produces "empty magnification" — a larger but no more detailed image — because the wavelength of visible light (~400–700 nm) physically limits resolution to ~200 nm. No amount of magnification can resolve structures closer than this limit using visible light.
SA2: Actual size = Image size ÷ Magnification = 18 mm ÷ 600 = 0.03 mm = 30 µm. The chloroplast is 30 micrometres long.
SA3: A light microscope could not be used because SARS-CoV-2 is approximately 100 nm in diameter and its spike proteins are ~10–15 nm — both below the ~200 nm resolution limit of a light microscope. TEM, with resolution of ~0.1 nm, revealed the three-dimensional arrangement of spike proteins on the viral surface. This structural data allowed researchers to identify the exact shape of the spike protein's receptor-binding domain, which was used as the target antigen in mRNA vaccine design — enabling the immune system to produce antibodies that recognise and neutralise the virus.
You predicted differences between a light microscope and TEM image of the same cell. How close were you?
The key differences are: resolution (TEM reveals internal membranes, ribosomes, organelle ultrastructure invisible by LM), colour (TEM greyscale vs LM staining), and dimensionality (TEM gives a 2D cross-section; SEM gives a 3D surface view). If you said "more detail" — that's correct, but practice naming exactly which structures become visible at each level of resolution.