Using a Microscope to Estimate Size

1. Using a Microscope to Estimate Size Problem: How can you use a microscope to estimate the size of an object? Introduction: When you view objects with a microscope, the objects appear to be much larger then they really are. When an object is magnified, you can get a false sense of its actual size . You can estimate the size of magnified objects if you know the size of the field of view. The field of view is the brightly lit circle you see as you look through the eyepiece. The size of the field depends on which objective lens you are using. The field of view decreases as you go from low to high power. In this lab , you will determine the diameter of the filed of view for each objective lens. Then you will use your results to compare the sizes of a variety of cells. Skills Focus: Observe, Measure, Calculate, Predict. Materials: Compound microscope transparent ruler prepared slides of a variety of cells Pre-lab: 1. Review: Put the field of view provided by each objective in order of small to large. ( 4x, 10x, 40x) 2. An analogy: A photographer may take a wide view and close up shot of the same scene. How does this compare to low and high powered objectives of a microscope? What is the advantage of each view? 3.Calcualte: Eight cells fit across a filed of view of 160 um (micrometers). What is the width of each cell? 4.Predict: Which cell do you think will be larger, the plant cell or the bacterial cell? Explain. Procedure: Part A: Determine a Field of View for Each Objective Lens Move the low power objective in place. Place a ruler on the microscope stage so that the millimeter marks are lined up on the diameter of the lit circle. As you look through the eyepiece, focus on the millimeter marks and line one up with the left edge of the circle. Count the number of whole mm you see and estimate what fraction of the partial mm is visible. Record the data in the table and convert to micrometers ( 1000 micrometers in a mm). Record the power of each objective in the table. Divide the power of the low power lens by the power of the medium. Multiply the result by the diameter of the low power field of view to calculate the diameter of the medium field of view. Repeat step 5 for the high power lens. Data Table Low Power Medium Power High Power

2. Part B: Comparing the size of cells. When you examine the cells from the prepared slide, decide whether to estimate the sizes based on low, medium, or high power. Using the diameter of the field of view you choose, estimate and record and record in the table of each cell. List the cells you use: Predict: Put in order of size from small to large. ( this is your prediction) Data: Analyze and Conclude: Infer: Why didn’t you use the ruler to measure the field of view of the medium and high power lenses? How did you decide which lens to use to estimate the size of the cells? 3. Predict: Some diseases are caused by one cell damaging another ( like bacterial and viral infections). Is it possible that a bacterial cell can injure a plant or animal cell by surrounding and eating them? What about the bacterial cell entering the other cells? Explain your reasoning. 4. Conclude: What is the correct order for the size of the cells you viewed? How does this compare to your prediction?