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How have microscopes improved our understanding of cells, and how do you calculate magnification and real size?

The differences between light and electron microscopes, how electron microscopy has increased understanding of sub-cellular structures, the magnification equation, and converting between units when calculating real size.

A focused answer to AQA GCSE Biology 4.1.1.5, covering the differences between light and electron microscopes, how electron microscopy has improved understanding of cells, and how to use the magnification equation with correct units.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Light and electron microscopes
  3. The magnification equation
  4. Required practical: using a light microscope
  5. Try this

What this dot point is asking

AQA wants you to explain how electron microscopy has increased our understanding of sub-cellular structures, compare light and electron microscopes in terms of magnification and resolution, and confidently use the magnification equation, including converting between millimetres and micrometres.

Light and electron microscopes

  • Light microscopes use light and glass lenses. They are cheap, portable and can view living cells, but have lower magnification (up to about ×2000\times 2000) and lower resolution because the wavelength of light limits the detail.
  • Electron microscopes use a beam of electrons instead of light. Electrons have a much shorter wavelength, so the microscope has much higher magnification and much higher resolution and reveals far more detail. They are large, expensive and cannot view living cells (the specimen must be in a vacuum).

The magnification equation

magnification=image sizereal size\text{magnification} = \frac{\text{image size}}{\text{real size}}

You can rearrange this to find any of the three values:

real size=image sizemagnificationimage size=magnification×real size\text{real size} = \frac{\text{image size}}{\text{magnification}} \qquad \text{image size} = \text{magnification} \times \text{real size}

The most common mistake is mixing units, so always convert first. Remember that 1 mm=1000 μm1\ mm = 1000\ \mu m and 1 μm=1000 nm1\ \mu m = 1000\ nm (nanometres). Standard form is useful for very small sizes: a ribosome of 20 nm20\ nm is 2×10−5 mm2 \times 10^{-5}\ mm.

Required practical: using a light microscope

In the required practical you prepare a slide (for example onion epidermis stained with iodine), focus with the low-power objective first, then switch to higher power, and produce a clear, labelled biological drawing with a magnification stated. Good technique includes using a coverslip lowered at an angle to avoid air bubbles, and drawing with clean continuous lines without shading.

Try this

Q1. State one advantage of an electron microscope over a light microscope. [1 mark]

  • Cue. Higher resolution (or higher magnification), so more detail of sub-cellular structures is visible.

Q2. An image is 30 mm30\ mm wide and the magnification is ×1500\times 1500. Calculate the real size in micrometres. [2 marks]

  • Cue. Real size =30/1500=0.02 mm=20 μm= 30 / 1500 = 0.02\ mm = 20\ \mu m.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20193 marksAn image of a cell measures 40 mm across. The magnification is x 2000. Calculate the real diameter of the cell. Give your answer in micrometres. Show your working.
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A 3-mark calculation question rewards rearranging the equation and converting units.

Rearrange magnification equals image size divided by real size to get real size equals image size divided by magnification. Real size equals 402000=0.02 mm\dfrac{40}{2000} = 0.02\ mm. Convert to micrometres: 0.02×1000=20 μm0.02 \times 1000 = 20\ \mu m.

Markers reward the correct rearrangement, the value 0.02 mm0.02\ mm, and the correct conversion to 20 μm20\ \mu m using 1 mm=1000 μm1\ mm = 1000\ \mu m.

AQA 20214 marksExplain why the development of the electron microscope has increased our understanding of sub-cellular structures, compared with using a light microscope.
Show worked answer →

A 4-mark explain question rewards linked points about resolution and detail.

An electron microscope uses a beam of electrons rather than light. It has a much higher magnification and, more importantly, a much higher resolving power (resolution), so two points very close together can still be seen as separate. This allows biologists to see tiny sub-cellular structures, such as ribosomes and the internal membranes of mitochondria and chloroplasts, that are too small to be resolved by a light microscope. As a result our understanding of how cells work has improved.

Markers reward higher resolution as the key reason, plus an example of a structure now visible, and the idea that this advanced scientific understanding.

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