AQA GCSE FOUNDATION Biology: Using a Light Microscope to Observe Cells

Using a Light Microscope to Observe Cells

This tutorial will guide you through the process of using a light microscope to observe cells, focusing on preparing a slide, adjusting magnification, calibrating the eyepiece graticule, and calculating cell size.

Preparing a Slide

1. Gather your materials: You will need a microscope slide, a coverslip, a specimen (e.g., onion skin, cheek cells), a dropper, water, and iodine solution.
2. Prepare the specimen: If using onion skin, carefully peel a thin layer from the inner surface of an onion. For cheek cells, gently scrape the inside of your cheek with a clean toothpick.
3. Place the specimen on the slide: Using tweezers, place the specimen in the center of the slide. Add a drop of water to the specimen to prevent it from drying out.
4. Add iodine solution: Iodine is a stain that helps make the cell structures more visible. Carefully add a drop of iodine solution to the specimen.
5. Apply the coverslip: Gently lower a coverslip onto the specimen, avoiding air bubbles.

Adjusting Magnification

1. Start with the lowest magnification: Begin by using the lowest magnification objective lens (usually 4x).
2. Focus using the coarse adjustment knob: Slowly turn the coarse adjustment knob until the specimen is in focus.
3. Increase magnification: Once the specimen is in focus at the lowest magnification, you can increase the magnification by rotating the objective lens to a higher power (e.g., 10x or 40x).
4. Refine focus using the fine adjustment knob: When you switch to a higher magnification, you will need to use the fine adjustment knob to bring the specimen into sharp focus.

Calibrating the Eyepiece Graticule

1. Understand the eyepiece graticule: The eyepiece graticule is a small, ruled disc that sits within the eyepiece of the microscope. It is marked with a scale that allows you to measure the size of objects under the microscope.
2. Calibrate using a stage micrometer: A stage micrometer is a slide with a known scale (usually in millimeters or micrometers).
3. Align the scales: Place the stage micrometer on the stage of the microscope. Adjust the focus and align the eyepiece graticule with the scale on the stage micrometer.
4. Determine the conversion factor: Count the number of divisions on the eyepiece graticule that correspond to a known distance on the stage micrometer. This gives you the conversion factor, which is the size of one division on the eyepiece graticule in micrometers.

Calculating Cell Size

1. Measure the image size: Using the calibrated eyepiece graticule, measure the length or width of the cell you are observing.

2. Apply the formula: Use the following formula to calculate the actual size of the cell:

   Actual Size = Image Size / Magnification

   • Image Size: The size of the cell measured using the eyepiece graticule.
   • Magnification: The total magnification of the microscope (objective lens magnification x eyepiece magnification).

3. Express the size in micrometers: The calculated actual size of the cell will be in micrometers (µm).

Example

Let's say you measure the length of a cell to be 10 divisions on the eyepiece graticule, and your microscope has a total magnification of 400x (40x objective lens x 10x eyepiece). The conversion factor for your eyepiece graticule is 10 µm per division.

To calculate the actual size of the cell:

1. Image Size: 10 divisions
2. Magnification: 400x
3. Actual Size = 10 divisions x 10 µm/division / 400x = 0.25 µm

Therefore, the actual length of the cell is 0.25 µm.

Conclusion

By following these steps, you can confidently use a light microscope to observe cells, measure their size, and gain a deeper understanding of their structure and function. Remember to practice your technique and always be careful when handling the microscope and its parts.