ESTIMATING SIZE AND CALCULATING MAGNIFICATION. Purpose: To determine an approximate field diameter for each of the objective lenses on our microscopes. To estimate the cell six Background Information: When viewing a small organism through the microscope, it’s usually necessary to have some idea of its size. Therefore, you need to have some means of estimating the size. When someone is standing near a doorway, you can estimate their height by comparing them to the doorway. In the same way, you can estimate an organism’s length by comparing it to the field of view that you are using. Example: If the “doorway is 10 units, how high is the stick person? Answer: Approximately ___ units high. Learning to calculate total magnification Calculate the total magnification of the low power objective lens by multiplying the magnification of the ocular lens by the magnification of the objective lens. The magnification of the lenses is etched on the sides of the actual lens holders. Record the Magnification of all power levels for your microscope in table 1. Example: Low power: Objective lens = 4X Ocular lens = 10X Total magnification at Low Power = 4X(10X) = 40X Total magnification at Medium Power = 10X(10X) = ______X Total magnification at High Power = 40X(10X) = ______X 1 Determining Diameter of Field of View 1. Take a clear plastic ruler and examine the millimetre scale under low power. 2. Place the centre of one of the scale marks along the edge of the field as shown below. 3. Count the whole number of millimetre spaces. If there is part of a spacing, estimate (in decimals) the size of the millimetre portion that shows. Record the field diameter in millimetres in your data table 1. Example: The distance across this field of view is 4.2 mm. To calculate the field diameter for high power The field diameter for high power cannot be measure directly using your millimetre ruler because this field diameter is LESS than one millimetre. Therefore, we must calculate the field diameter a different way. high power magnification low power magnification 400x 4x = = low power field diameter high power field diameter 4.2mm ? 40x X 4.2mm=16.8 400x X ? = 400? 400? 400 ? = = 16.8mm 400 0.042mm Therefore the high power field diameter is 0.042mm 2 To calculate the field diameter for medium power Now we could follow the same steps as we did under the low power but since we are not actually conducting the experiment with a microscope we are going to practice using the formula above medium power magnification = low power magnification 100x 4x = low power field diameter medium power field diameter 4.2mm ? 40x X 4.2mm=16.8 100x X ? = 100? 100? 100 ? = = 16.8mm 100 _______mm Therefore the medium power field diameter is ______mm Estimating Cell Size 1. Place the object to be studied in the middle of the field of view. 2. Estimate the number of objects that could fit across the field of view 3. Divide the diameter for the field of view by the number of objects that could fit across it to estimate the size of the object. Size of object = diameter of field of view number of objects Using the picture beside you determine how many objects would fit across the field of view Size of object = 4.2mm 1mm=1000m 3

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# To calculate the field diameter for high power