Required Practical: Microscopy — AQA GCSE Biology
This required practical involves using a light microscope to observe cells and making a labelled scientific drawing.
Method
- Prepare a slide — for example, peel a thin layer of onion epidermis, place it on a slide, add a drop of iodine stain (to make structures visible), and lower a coverslip at an angle to avoid trapping air bubbles.
- Place the slide on the stage and select the lowest power objective lens first.
- Use the coarse focus to bring the cells into view, then the fine focus to sharpen the image.
- Switch to a higher power objective to see more detail.
Making a scientific drawing
- Draw with a sharp pencil, using clear, unbroken lines and no shading.
- Draw only what you see, with structures in proportion.
- Add a title, the magnification, and label lines (drawn with a ruler, not crossing).
Magnification
$$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$$ Total magnification = eyepiece lens × objective lens. Remember to convert units (1 mm = 1000 µm).
Safety and accuracy
- Handle glass slides and coverslips carefully.
- Use a stain to improve contrast.
- Start on low power so you don't crack the slide with the objective.
Exam tips
- Know why iodine is added (to stain/contrast structures).
- Lower the coverslip at an angle to avoid air bubbles.
- Start with the lowest power objective.
- A good biological drawing uses clean pencil lines, no shading, a title and labelled magnification.