What you'll learn
The microscope is one of the most important tools in biology, and this required practical develops the skills of preparing a slide, using a light microscope, and making a scientific drawing. For AQA GCSE Biology you need to know how to use a light microscope correctly, how to prepare a slide of cells, how to calculate magnification, and how to make and label a biological drawing. This guide covers the method step by step, the magnification calculation, how to draw what you see, and the difference between light and electron microscopes. By the end you should be able to describe the practical accurately, calculate total magnification, and use the magnification formula confidently.
Key terms and definitions
Microscope — An instrument that magnifies small objects so they can be seen in detail.
Objective lens — The lens close to the specimen; microscopes have several of different powers.
Eyepiece lens — The lens you look through, usually ×10.
Magnification — How many times larger an image is than the real object.
Resolution — The ability to distinguish two points that are close together as separate.
Slide — The glass plate on which a specimen is placed for viewing.
Stain — A dye used to make cell structures show up more clearly.
Coverslip — A thin square of glass placed over the specimen to protect the lens and flatten the sample.
Core concepts
Using a light microscope
A light microscope uses light and lenses to magnify a specimen. To view a slide:
- Clip the slide onto the stage.
- Select the lowest power objective lens first.
- Turn the coarse focusing wheel to bring the stage and lens close together while watching from the side.
- Look through the eyepiece and turn the coarse wheel to move the stage away until the image comes roughly into focus.
- Use the fine focusing wheel to make the image sharp.
- To see more detail, switch to a higher power objective lens and refocus using the fine wheel only.
Always start on low power to find the specimen, because it is much easier to locate at low magnification.
Preparing a slide
To prepare a slide of cells, such as onion cells:
- Peel a thin layer of tissue (for onion, a single layer of cells from inside the onion).
- Place it flat on a clean glass slide.
- Add a drop of stain, such as iodine, to make the structures show up more clearly.
- Lower a coverslip gently onto the specimen using a mounted needle, starting at an angle, to avoid trapping air bubbles.
The stain is important because many cell structures are colourless and hard to see without it. Lowering the coverslip carefully prevents air bubbles that would obscure the view.
Calculating magnification
The total magnification of a microscope is found by multiplying the two lens powers together:
total magnification = eyepiece magnification × objective magnification
For example, a ×10 eyepiece and a ×40 objective give a total magnification of 10 × 40 = ×400.
The magnification formula
You also need to relate the size of an image to the real size of an object using:
magnification = size of image ÷ size of real object
This can be rearranged to find any of the three quantities. It is essential that the image and object sizes are in the same units before you divide, so convert millimetres to micrometres (×1000) where needed.
Making a scientific drawing
When you draw what you see down the microscope, follow the rules for a scientific drawing:
- Use a sharp pencil and draw clear, continuous lines (no sketching or shading).
- Draw only what you can actually see, in proportion.
- Add a title and label the structures you can identify, using straight, ruled label lines that do not cross.
- Include the magnification you used.
A good biological drawing is simple, accurate and clearly labelled, not artistic.
Light versus electron microscopes
A light microscope uses light and has a lower magnification and resolution. An electron microscope uses a beam of electrons instead of light, giving a much higher magnification and resolution. This means electron microscopes can reveal much smaller structures, such as the internal detail of cells, that a light microscope cannot show. However, electron microscopes are large, expensive, and cannot be used to view living specimens.
Working with units of size
Cells and their structures are very small, so biologists use small units of length. One millimetre (mm) is one thousandth of a metre, and one micrometre (µm) is one thousandth of a millimetre — so 1 mm = 1000 µm. Many cells are measured in micrometres: an onion cell might be around 100 µm across, while a bacterium is only a few micrometres. When doing magnification calculations you often need to convert between these units, multiplying by 1000 to go from mm to µm, or dividing by 1000 to go the other way. Getting comfortable with these conversions prevents the most common calculation errors in this topic.
Estimating the size of a cell
You can estimate a cell's real size using the microscope. One method is to work out how much of the field of view a row of cells fills. For example, if you can see 10 cells spanning a field of view that is 2 mm (2000 µm) wide, then each cell is roughly 2000 ÷ 10 = 200 µm across. Alternatively, if you know the magnification and measure the image size of a cell, you can rearrange the magnification formula to find the real size. Being able to estimate sizes from what you observe is an important practical skill and a frequent exam question.
Worked examples
Example 1: Calculating total magnification
A microscope has a ×10 eyepiece and a ×20 objective lens. What is the total magnification? Total magnification = 10 × 20 = ×200.
Example 2: Using the magnification formula
A cell has a real size of 0.05 mm and appears 10 mm wide in an image. Calculate the magnification. Both are in mm. Magnification = image size ÷ real size = 10 ÷ 0.05 = ×200.
Example 3: Finding the real size
An image of a cell is 30 mm long at a magnification of ×300. What is the real size of the cell? Real size = image size ÷ magnification = 30 ÷ 300 = 0.1 mm (which is 100 µm).
Example 4: Explaining the use of a stain
Explain why a stain such as iodine is added when preparing a slide. Many cell structures are colourless and difficult to see. A stain makes the structures show up more clearly against their background, so they can be seen and identified under the microscope.
Common mistakes and how to avoid them
The most common calculation error is forgetting to convert units. The image and object must be in the same units before dividing. Converting mm to µm (×1000) is often needed, and mixing units gives an answer that is out by a factor of a thousand.
Students often start on high power to find the specimen. Always start on the lowest power objective, because the specimen is far easier to locate, then increase magnification.
Another mistake is shading a scientific drawing. Biological drawings use clear pencil lines only, no shading, and should show only what is actually visible.
When lowering the coverslip, do not drop it flat — lower it at an angle with a needle to avoid trapping air bubbles, which look like dark circles and obscure the cells.
Finally, do not confuse magnification with resolution. Magnification is how much bigger the image is; resolution is the ability to see two close points as separate. Electron microscopes have higher resolution, which is why they show more detail.
Exam technique for "Required practical: microscopy"
Method questions expect specific detail: start on low power, use the coarse wheel then the fine wheel, add a stain, and lower the coverslip at an angle to avoid air bubbles. Include these points precisely.
Calculations are almost guaranteed. Learn both formulas — total magnification = eyepiece × objective, and magnification = image ÷ real size — and always check your units. Rearranging the formula to find the real size or image size is common, so practise all three versions.
For drawing questions, state the rules: sharp pencil, clear lines, no shading, ruled labels, a title and the magnification. When comparing microscopes, contrast light and electron microscopes on magnification, resolution and whether living specimens can be viewed.
Quick revision summary
- Use a light microscope by starting on the lowest power, focusing with the coarse wheel then the fine wheel.
- Prepare a slide by adding a stain (e.g. iodine) and lowering the coverslip at an angle to avoid air bubbles.
- total magnification = eyepiece × objective (e.g. ×10 × ×40 = ×400).
- magnification = image size ÷ real size — keep both in the same units.
- A scientific drawing uses clear pencil lines, no shading, ruled labels, a title and the magnification.
- Electron microscopes have much higher magnification and resolution than light microscopes but cannot view living specimens.