Microscopy Techniques Revision Notes | A Level AQA Biology

1. Why Microscopy Matters in Biology

Cells are invisible to the naked eye. The entire field of cell biology depends on our ability to see structures that are measured in micrometres and nanometres — and the tool that makes this possible is the microscope.

There are three microscope types you need to know for AQA A Level: the optical microscope, the transmission electron microscope (TEM), and the scanning electron microscope (SEM). Each works differently, reveals different things, and comes with its own set of limitations. Choosing the right one for a given investigation is itself an exam skill.

2. The Optical Microscope: Principles and Limitations

The optical microscope — sometimes called the light microscope — uses visible light focused through glass lenses to produce a magnified image. Light passes through the specimen and into the objective lens, then through the eyepiece lens, producing a final magnified image for the observer.

The total magnification of an optical microscope is calculated by multiplying the magnification of the objective lens by the magnification of the eyepiece lens. For example, a x40 objective lens combined with a x10 eyepiece gives a total magnification of x400.

This 200 nm resolution limit is the critical constraint of optical microscopy. Visible light has a wavelength of roughly 400–700 nm, and two points closer together than the wavelength of the light used cannot be resolved as separate — they blur into one. No matter how powerful the lenses, you cannot overcome this physical limit with light.

• Maximum useful magnification: approximately x1500
• Maximum resolution: approximately 200 nm
• Specimens can be living or dead
• Colour images are possible (especially with staining)
• Relatively low cost and widely available
• Subcellular structures such as ribosomes and the endoplasmic reticulum cannot be resolved

3. Electron Microscopes: TEM and SEM

To see structures smaller than 200 nm, biologists needed a different approach. The solution was to replace light with electrons. Electrons have a much shorter wavelength than visible light — around 0.004 nm under typical operating conditions — which means electron microscopes can resolve structures that are orders of magnitude smaller than anything visible with light.

There are two types of electron microscope, and they work in fundamentally different ways. Understanding the distinction between them is a common exam target.

Because electrons must pass through the specimen, TEM samples must be cut into ultra-thin sections and placed in a vacuum. This means specimens must be dead and chemically fixed before imaging — living cells cannot be viewed. The TEM can resolve structures as small as approximately 0.1 nm, making it ideal for studying organelles such as mitochondria, the endoplasmic reticulum, and ribosomes.

The SEM does not transmit electrons through the specimen — it bounces them off the surface. This produces striking three-dimensional images of external structures, such as the surface of a pollen grain or the texture of a cell membrane. However, the SEM has a lower resolution than the TEM (approximately 3–20 nm) and, like the TEM, requires specimens to be dead and placed in a vacuum.

4. Magnification vs Resolution: A Critical Distinction

Here is where many students lose marks. Magnification and resolution are not the same thing, and confusing them is one of the most common errors in this topic.

Magnification is how much bigger the image appears compared to the real object. You can magnify an image indefinitely — but if the microscope cannot resolve the detail, you will produce a larger, blurrier image. This is called empty magnification: making something bigger without revealing any new detail.

Resolution is the ability to distinguish two nearby points as separate. A microscope with high resolution can show fine detail; one with low resolution cannot, regardless of how much it magnifies. Resolution is ultimately limited by the wavelength of the radiation used — shorter wavelengths give better resolution.

5. The Magnification Formula and Measuring Cell Size

One of the most directly assessed skills in this topic is using the magnification formula. The relationship is straightforward once you are clear about which measurement is which.

The most important practical point: both measurements must be in the same units before you divide. Exam questions frequently give the image size in millimetres and the real object size in micrometres — if you do not convert first, your answer will be wrong by a factor of 1000.

A useful conversion to memorise: 1 mm = 1000 micrometres (µm), and 1 µm = 1000 nanometres (nm). When measuring a cell on a micrograph, you measure the image size with a ruler, then use the scale bar or stated magnification to work backwards to the real size.

Worked example: Calculating Magnification from a Micrograph

A student views a cell under an optical microscope. The image of the cell measures 24 mm across. The actual diameter of the cell is 30 µm. Calculate the magnification. [2 marks]

1. $$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$$ — [M1] Stating the correct formula earns the method mark.
2. Image size = 24 mm = 24 × 1000 µm = 24,000 µm Real object size = 30 µm — Both values must be in the same unit before dividing. Converting mm to µm: multiply by 1000.
3. $$\text{magnification} = \frac{24{,}000 \text{ µm}}{30 \text{ µm}}$$ — The µm units cancel, leaving a dimensionless ratio.
4. $$\text{magnification} = 24{,}000 \div 30 = 800$$ — [A1] The magnification is x800. No units are needed — magnification is a ratio.

Answer: Magnification = x800

Worked example: Calculating Real Object Size from a Micrograph

An electron micrograph shows a mitochondrion with an image length of 45 mm. The magnification is x30,000. Calculate the actual length of the mitochondrion in micrometres. [3 marks]

1. $$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$$ $$\text{size of real object} = \frac{\text{size of image}}{\text{magnification}}$$ — [M1] Rearranging correctly to isolate the real object size.
2. Image size = 45 mm = 45 × 1000 µm = 45,000 µm — Convert mm to µm by multiplying by 1000, so units are consistent with the final answer in µm.
3. $$\text{size of real object} = \frac{45{,}000 \text{ µm}}{30{,}000}$$ — [M1] Correct substitution of image size and magnification.
4. $$\text{size of real object} = 45{,}000 \div 30{,}000 = 1.5 \text{ µm}$$ — [A1] The mitochondrion is 1.5 µm long — a biologically plausible value for a mitochondrion (typical range: 1–10 µm).

Answer: Actual length = 1.5 µm

6. Measuring Objects with an Optical Microscope

In a practical setting, measuring the size of a cell or structure viewed through an optical microscope requires a calibrated scale. The standard tool for this is an eyepiece graticule — a small transparent ruler fitted inside the eyepiece — used alongside a stage micrometer, which is a microscope slide with a scale of known length etched onto it.

The stage micrometer is used to calibrate the eyepiece graticule at each objective lens magnification. Once calibrated, you can measure the image of a cell in eyepiece units and convert to real micrometres. This calibration must be repeated every time you change the objective lens, because the relationship between eyepiece units and real distance changes with magnification.

Alternatively, when working from a printed micrograph or photomicrograph, you measure the image size directly with a ruler (in mm), then apply the magnification formula using the stated magnification or the scale bar provided on the image.

7. Exam Technique and Common Mistakes

Magnification calculations are reliable mark-winners — but only if you avoid the traps that examiners consistently report. The guidance below consolidates the highest-value exam technique for this topic.

For extended-response questions comparing microscope types, structure your answer around four key features: the source of radiation used, the maximum resolution achievable, the type of image produced (2D or 3D, internal or surface), and whether living specimens can be viewed. Covering all four features with precise scientific language will consistently earn full marks on 'compare' and 'evaluate' questions.

Every spec point in this topic — the principles and limitations of all three microscope types, the distinction between magnification and resolution, and the magnification formula — can appear in calculation, short-answer, or extended-response format. Practising the formula with unit conversions and being able to justify microscope choice with reference to resolution and specimen requirements are the two skills that most reliably separate high-scoring responses from average ones.