Required practical - Using light microscope (AQA GCSE Biology Combined Science): Revision Notes
Required practical - Using light microscope
What is this practical about?
This practical teaches you how to use a light microscope to look at plant and animal cells. You'll learn to prepare slides with onion cells and observe them clearly under the microscope. This helps you understand what cells look like and how to identify their different parts.
The main goal is to make a slide containing onion cells, then use a microscope to observe, draw and label these cells along with other types of cells.
Equipment you'll need
You'll use several important pieces of equipment for this practical:
- Small piece of onion - this provides the cells you'll examine
- Forceps - special tweezers to handle delicate materials
- Microscope slide with coverslip - thin glass pieces to hold your specimen
- Iodine solution - a chemical that makes cell parts easier to see
- Prepared slides - ready-made slides of different plant and animal cells
- Light microscope - the main instrument for magnifying cells
Key fact: Onion skin works well because it peels away easily in single layers of cells, making it perfect for microscope work.
Method - preparing a specimen
Follow these steps carefully to make your onion cell slide:
Step 1: Use forceps to peel off a thin layer of skin from the inside of an onion piece. This layer should be very thin and almost transparent.
Step 2: Place this thin layer flat on your microscope slide. Try to avoid wrinkles or folds.
Step 3: Add one or two drops of iodine solution on top of the onion layer. This stain helps you see the cell structures more clearly.
Step 4: Lower the coverslip over the specimen. Place one edge down first at a 45-degree angle, then gently lower the other edge. This technique helps push out air bubbles and excess liquid.
Top tip: If you add a drop of water first before the coverslip, it can help the tissue lay flat and prevent curling.
Method - observing a specimen
Now you're ready to use the microscope properly:
Step 5: Place your prepared slide on the microscope stage and secure it with the clips.
Step 6: Turn the focusing wheel to move the objective lens to its lowest power setting. This gives you the widest view to start with.
Step 7: Look through the eyepiece and use the coarse focusing wheel to bring the specimen into rough focus. Lower the lens until it almost touches the slide, then slowly raise it.
Step 8: Once you can see the specimen through the eyepiece, use the coarse wheel to get a clearer image. The cells should start to become visible.
Step 9: Switch to a higher power objective lens for more detail. Use the fine focusing wheel to get a sharp, clear image.
Step 10: Repeat these focusing steps with other prepared slides to compare different cell types.
Remember: Always start with low power and work up to high power. This makes it much easier to find and focus on your specimen.
Recording your results
When drawing what you observe, follow these important rules:
- Draw clear outlines of what you actually see - don't guess or add details that aren't there
- Use shading sparingly - only shade if it helps show the difference between structures
- Add labels with straight lines pointing to specific cell parts
- Write the magnification you used somewhere on your drawing
This helps create accurate scientific records of your observations.
Key parts of a light microscope
Understanding the microscope parts helps you use it properly:
- Eyepiece - the lens you look through at the top
- Objective lenses - different power lenses that magnify the specimen
- Stage with clips - the platform that holds your slide in place
- Coarse focusing wheel - for rough focusing when starting
- Fine focusing wheel - for sharp, detailed focusing
- Mirror or lamp - provides light to illuminate the specimen from below
Modern fact: Many newer microscopes use electric lamps instead of mirrors to provide consistent lighting.
Safety tips
When working with microscopes and chemicals, safety is paramount. Handle iodine solution with particular care as it can cause staining and damage.
Safety Guidelines:
- Wear eye protection when using iodine solution as it can stain skin and damage clothes
- Wash off spills immediately - iodine leaves permanent stains
- Work carefully to avoid splashing the solution
Always treat the microscope gently as it contains delicate glass lenses and moving parts.
Key Points to Remember:
- Start with low power objective lens, then increase magnification
- Onion skin peels easily in single cell layers, making it ideal for this practical
- Iodine solution stains cells to make structures visible, but handle it safely
- Draw only what you see - accurate observation is key to good science
- Label your drawings clearly with straight lines and correct magnification noted