Unit 1: Introduction to Biology

Module 2: The Microscope

Learning Objective(s): 2.5

Lesson 2: Calculations Related to the Microscope

Lesson Objectives:

  • To become familiar with calculating total magnification and field of view of a microscope.

Evaluation:

  • Activity; Calculations Practice Sheet - Self Check

The following 3 steps are the calculations you will use when doing work related to the microscope. Review each calculation; then click on the link at the bottom of the page to proceed to the practice worksheet.

Calculations Related to the Microscope:

1) Calculating the magnification of a microscope:

Total magnification = magnification of eyepiece x magnification of objective lens

Ocular Lens - W10XD (Look for a whole number or a number followed by an X to determine the power.

Low Power Objective - DIN4

10X ocular x 4X objective = 40X total

Medium Power Objective - DIN10

10X ocular x 10X objective = 100X total

High Power Objective - DIN40

10X ocular x 40X objective = 400X total

Microscope images used with permission.

The images to the right illustrate how the appearance of an image changes with magnification. As the magnification increases, the size of the image increases. However, because less of the image can be seen, the 'field of view' decreases.

higher magnification = smaller field of view

alfalfa stem @ 40X
alfalfa stem @ 100X
alfalfa stem @ 400X

2)       Measuring the microscope field of view on lowest power:

Place a clear plastic ruler with mm markings on top of the stage of your microscope.  Looking through the lowest power objective, focus your image.  Count how many divisions of the ruler fit across the diameter of the field of view.  Multiply the number of divisions by 1000 to obtain the field of view in micrometers (µm). Record this in µm (1mm = 1000 µm ).

Magnified at 40X, the lines of the ruler are clearly visible. However, in order to accurately measure the field of view, one line should be moved to the edge of the field of view, as shown in the image to the right.

Now that one of the lines has been moved to the extreme left, it is easier to determine the field of view:

2 mm x 1000 = 2000 µm

(image @ 40X)

In this example, the field of view would be calculated:

3 mm x 1000 = 3000 µm

3)       Calculating the microscope field of view on other powers:

i. Using the above procedure, determine the field of view in micrometers (µm) for the lowest power on your microscope.

ii. After you have determined the field of view for low power, use the equation below to mathematically calculate the field of view on higher powers:

Click on the link below to check your understanding of the above calculations:

PRACTICE CALCULATIONS for Total Magnification and Field of View

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