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How do we use a light microscope to see cells, and how do we work out how much bigger the image is?

Use a light microscope to observe cells, prepare a simple slide, and calculate magnification using the relationship between image size and actual size

A scaffolded answer to the N(A)-Level Biology outcome on using the light microscope and calculating magnification. How to focus, how to prepare a slide, and the magnification formula worked step by step with units.

Generated by Claude Opus 4.87 min answer

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

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome has a practical part and a calculation part. The practical part is knowing how to set up a light microscope, prepare a simple slide, and view cells. The calculation part is working out the magnification, which tells you how many times bigger the image is than the real object. You should be able to use the formula magnification equals image size divided by actual size, keeping both lengths in the same unit.

The answer

Parts of the light microscope

A light microscope uses two lenses to make a small object look much bigger. The main parts are the eyepiece lens (at the top, where you look), the objective lenses (the set of lenses near the slide, each a different power), the stage (where the slide sits), the focusing knobs, and a light source or mirror underneath that shines light up through the slide.

How to view a specimen

  1. Place the prepared slide on the stage and clip it in place.
  2. Start with the lowest-power objective lens, so you have a wide view.
  3. Turn the coarse focusing knob to bring the stage and lens close, then move them apart slowly while looking, until the image is roughly in focus.
  4. Use the fine focusing knob for a sharp image, then switch to a higher-power objective if you need more detail.

Preparing a simple slide

A good slide is thin so that light passes through it. To make a wet-mount slide: place a thin layer of the specimen on a clean glass slide, add a drop of water or a stain (such as iodine for plant cells), and lower a cover slip gently at an angle with a mounting needle. The angle stops air bubbles being trapped, which would block the view.

The magnification calculation

Magnification tells you how many times larger the image is than the real object. The formula is:

magnification=image sizeactual size\text{magnification} = \frac{\text{image size}}{\text{actual size}}

The most important rule is that both measurements must be in the same unit before you divide. Magnification has no unit, because it is a ratio of two lengths. You can rearrange the formula to find any one of the three quantities.

Examples in context

Example 1. Why we stain cells. Many cell parts are almost colourless, so they are hard to see. Adding a stain such as iodine colours structures like the nucleus and cell wall, making them stand out against the cytoplasm. This is why a slide of onion cells is treated with iodine before viewing.

Example 2. A scale bar on a textbook photo. Photographs of cells often show a small scale bar labelled, for example, 10 μm10\ \mu\text{m}. You can measure the bar on the page and the cell on the page, then use the same image size divided by actual size idea to work out the magnification of the photo, even when no magnification is printed.

Try this

Q1. State the formula used to calculate magnification. [1 mark]

  • Cue. Magnification = image size divided by actual size.

Q2. A cell measures 30 mm30\ \text{mm} in a drawing made at a magnification of ×600\times 600. Calculate the real size of the cell in mm. [2 marks]

  • Cue. Actual size = image divided by magnification = 30600=0.05 mm\dfrac{30}{600} = 0.05\ \text{mm}.

Q3. Explain why a cover slip is lowered gently at an angle when preparing a slide. [2 marks]

  • Cue. Lowering it slowly at an angle pushes out the air, so air bubbles are not trapped under the cover slip; bubbles would block light and spoil the view.

Exam-style practice questions

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

Original3 marksA student looks at a cheek cell under a microscope. In the image the cell measures 40 mm40\ \text{mm} across. The real cell is 0.05 mm0.05\ \text{mm} across. Calculate the magnification of the image. Show your working.
Show worked answer →

Use the formula: magnification = image size divided by actual size.

Both sizes are already in the same unit (mm), so no conversion is needed.

Magnification = 400.05=800\frac{40}{0.05} = 800.

The magnification is ×800\times 800 (it has no unit because it is a ratio of two lengths).

What markers reward: the correct formula, both lengths in the same unit, the correct answer of 800, and writing it as a magnification (times 800) with no unit. A common slip is to divide the actual size by the image size, which gives a tiny decimal.

Original3 marksDescribe how you would prepare a slide of onion epidermis cells to view under a light microscope.
Show worked answer →

A suitable method: peel a thin, single layer (the epidermis) from a piece of onion using forceps. Place it flat on a clean glass slide. Add a drop of iodine solution to stain the cells so the parts show up. Lower a cover slip gently at an angle, using a mounting needle, to avoid trapping air bubbles. Then place the slide on the microscope stage.

What markers reward: a thin single layer (so light can pass through), a stain such as iodine (so structures are visible), and lowering the cover slip gently or at an angle (to avoid air bubbles). The reasons in brackets often carry their own marks.

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