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How do we use a light microscope to see cells, and how do we calculate magnification?

Use a light microscope to observe cells and calculate magnification and actual size

A focused answer to the O-Level Biology outcome on microscopy. Parts of the light microscope, how to prepare and view a slide, and how to calculate magnification, actual size and image size with worked numbers.

Generated by Claude Opus 4.88 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
  4. Try this

What this dot point is asking

SEAB wants you to know the main parts of a light microscope and how to use it to view cells, to prepare a simple slide, and to calculate magnification, actual size or image size using the magnification equation. The calculation is one of the few pieces of maths in O-Level Biology, so the method must be secure.

The answer

Parts of the light microscope

The main parts are the eyepiece lens (which you look through), the objective lenses (of different powers, on a rotating turret), the stage (where the slide sits, held by clips), the focusing knobs (coarse and fine), and the light source or mirror (which lights the specimen from below). Light passes up through the thin specimen and through the two lenses to give a magnified image.

Preparing a slide

To view cells you need a thin specimen so light can pass through. Place the specimen flat on a clean slide in a drop of water, then lower a cover slip gently at an angle to avoid trapping air bubbles. A stain such as iodine is often added because cell parts are colourless; the stain makes structures such as the nucleus stand out.

Focusing safely

Start with the lowest-power objective. Use the coarse focusing knob first, watching from the side as you bring the lens close, then look through the eyepiece and turn the knob to move the lens away until the image is sharp. Switch to higher power and use the fine knob only. Starting low and watching from the side avoids cracking the slide.

The magnification equation

The total magnification is the eyepiece magnification multiplied by the objective magnification. The key working equation links three quantities:

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

Rearranging gives the two forms you will need:

actual size=image sizemagnification,image size=magnification×actual size\text{actual size} = \frac{\text{image size}}{\text{magnification}}, \qquad \text{image size} = \text{magnification} \times \text{actual size}

Both sizes must be in the same units before you start. Remember 1 mm=1000 μm1\ \text{mm} = 1000\ \mu\text{m}.

Examples in context

Example 1. A scale bar. Many textbook photographs show a scale bar, for example a line labelled 10 μm10\ \mu\text{m}. Measuring the bar with a ruler and comparing it to the labelled length lets you work out the magnification of the whole image using the same equation.

Example 2. Choosing the objective. To scan a whole onion slide you use the low-power objective for a wide view, then switch to high power to study a single cell in detail. The trade-off between field of view and detail is part of using the instrument well.

Try this

Q1. State the equation linking magnification, image size and actual size. [1 mark]

  • Cue. magnification=image sizeactual size\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}.

Q2. A structure is 0.05 mm0.05\ \text{mm} wide and is drawn 30 mm30\ \text{mm} wide. Calculate the magnification. [2 marks]

  • Cue. magnification=300.05=×600\text{magnification} = \dfrac{30}{0.05} = \times 600.

Q3. Explain why a stain is added when preparing a slide of cells. [2 marks]

  • Cue. Cell structures are largely colourless and hard to see; the stain colours parts such as the nucleus so they stand out and the cells can be identified.

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.

Original4 marksA cell appears 40 mm40\ \text{mm} wide in a drawing made under a microscope. The magnification of the drawing is ×200\times 200. (a) Calculate the actual width of the cell in millimetres. (b) Give this width in micrometres (1 mm=1000 μm1\ \text{mm} = 1000\ \mu\text{m}).
Show worked answer →

(a) Actual size =image sizemagnification=40200=0.2 mm= \dfrac{\text{image size}}{\text{magnification}} = \dfrac{40}{200} = 0.2\ \text{mm}.

(b) Converting: 0.2 mm×1000=200 μm0.2\ \text{mm} \times 1000 = 200\ \mu\text{m}.

Markers reward the correct rearrangement (actual size = image size divided by magnification), the correct value 0.2 mm0.2\ \text{mm}, and the correct conversion to 200 μm200\ \mu\text{m}. A common slip is to multiply instead of divide.

Original3 marksDescribe how you would prepare a slide of onion epidermis to view cells under a light microscope, and state why a stain such as iodine is added.
Show worked answer →

Peel a thin layer of onion epidermis and place it flat on a clean glass slide in a drop of water. Lower a cover slip gently, at an angle, to avoid trapping air bubbles. Add a drop of iodine solution at the edge of the cover slip so it spreads across the specimen.

The stain is added because the cell parts are otherwise colourless and hard to see. Iodine stains structures such as the nucleus and starch, making them stand out so the cells can be identified clearly.

Markers reward a thin specimen, the use of water and a cover slip lowered to avoid bubbles, and a reason for staining (to make colourless structures visible).

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