Classroom Explorations: What’s the Size of What You See?
Materials & Equipment
Materials Per Pair
  • compound microscope
  • prepared microscope slides or slides and specimens
  • lens paper
  • clear metric ruler with millimeter divisions
Group Size
  • whole class & pairs
  • Preview the images and movie listed under Materials & Equipment.
  • Download the student pages and provide them to the class. If you don’t have a tech center, print and duplicate the student pages.
  • You can make extra rulers by placing clear rulers on a copy machine, copying them onto an overhead transparency, and cutting the transparency into strips.
Alternative Approach
  • Students may follow the links and instructions on the student pages to complete Part One independently.
  • To calculate the size of microscopic specimens using a scale bar.
  • To determine the field diameters for different objective lenses in a compound microscope, and to use this number to calculate the size of microscopic specimens.
  • Project the image of red blood cells. How large are the cells? Can students tell?
  • Turn on the scale bar, and explain that scale bars are often superimposed on images to help the viewer understand the size of what they see.
Part One: Using scale bars
  1. Have students read the first problem on the student pages. Ask several students how many red blood cells they think would fit, end to end, along the scale bar in the image. Take the average of their estimates (which should be about six cells), and tell them to use this number for the denominator of the fraction in the equation. Then have them calculate the diameter of one red blood cell (which is about 0.008 mm) by dividing the length of the scale bar by the number of cells.

    The remaining images may be projected for the entire class, or students can work independently, following the links and instructions on the student pages.

  2. Project the image of Volvox globator, and give students time to read about and briefly discuss this organism if it’s unfamiliar to them. Have them do the second problem.
  3. Project the image of sea urchin sperm and have students do the third problem.
  4. Open the sea urchin embryo cell division page, and play the movie. (You’ll need to replay it several times.) Have students do the fourth problem. Tell students that they should use the external membrane as the embryonic boundary for their calculations. (In the video, a single fertilized egg completes two rounds of cell division, becoming two and then four cells. Students will see that the size of the embryo itself does not change from the one-cell size, but that with successive divisions, the individual cells become smaller. Prior to fertilization, eggs stockpile all of the materials that an embryo needs for its early development, and early cell divisions divide the materials among the progeny cells. The embryo relies on these materials until it begins to synthesize materials on its own.)
Part Two: Finding the size of microscope specimens
  1. Explain that the second part of this activity will help them determine the actual size of the specimens they observe with their own microscopes, and distribute the materials.
  2. Have the students determine the field diameters of their microscope objectives by completing Part Two of the student data sheets. You may wish to lead them through this process, answering questions as they go. If your students are unfamiliar with mathematical proportions, show them an example first.
  3. Have students use their microscope field diameters to calculate the size of the specimens or prepared slides you’ve provided.
After students are familiar with determining the size of a cell or other specimen on their microscopes, you might challenge them to determine the rate of movement of a specimen. Possible specimens include any of the motile protozoa, or even the streaming chloroplasts in Elodea cells.
The magnifying power of most ocular lenses on student microscopes is 10X. Objective lens magnifying power may vary depending on the brand of microscope. In general, most student compound microscopes are equipped with low power (4X), medium power (10X), and high power (40X) objective lenses. The higher the magnification, the longer the barrel of the objective lens.

Total magnification

The total magnification of the image that reaches the eye through the microscope ocular is the product of both the ocular magnification and the objective magnification. Using the example above, the total magnification of low power is 40X, medium power is 100X, and high power is 400X.

Field diameter
Field diameter is determined by the number of millimeters observed to fit across the diameter of the field of vision. The lower the magnification is, the larger the field of view. The field of view can vary significantly based on the brand of microscope. For example, the field diameter of a “typical” 10X objective (100X total magnification) can vary from about 1.0 millimeter to 2.0 millimeters. As the magnification increases, the amount of surface area in the image decreases: Magnification and field diameter are inversely related. Students can easily see this by looking at the ruler with different objectives, and they can now apply their knowledge to determine the size of genuine specimens.