Size Under the Microscope
SUMMARY: In this lab, students will calibrate the objectives on their microscope by determining the field width for each objective. Using this information, students can then determine the sizes of unknown organisms.
  1. 1. Minimalist or "cartoon" drawings to just illustrate the points being made without distracting information
  2. 2. Detailed drawings to show as much information as possible
Medium Difficulty



Microorganisms are important. Recent news topics have focused on a variety of microorganisms: E. coli K12 in food contamination, life on Mars, Ebola virus in Africa, AIDS, and the recent rise in Tuberculosis infections. Size is an important characteristic that students might investigate as they attempt to put these organisms into perspective. An understanding of the relative size of eggs and sperm can help students comprehend the relevance of structure and function in the design of each.

When studying cells it is useful to be able to measure them. We can measure cells then make comparisons between different types of cells. Students don't relate to cell size because they don't have something to compare it to. The following exercise is designed to help students gain a key skill in the study of microbiology.

Size is important. Any one who has gone to one of these "warehouse" stores has had the experience of wondering what they were going to do with five gallons of peanut butter. At the same time five gallons of peanut butter is a fairly easy thing to imagine and relate to. A sea urchin egg has a volume of about 500 femptoliters or looked at another way 40,000,000,000,000 eggs fit in that 5 gallon peanut butter jar! (40 trillion!). And for sperm, multiply that number by about 200,000! It is no wonder that students have trouble relating to the size of organisms under the microscope.


Procedure & Math

  1. Click here for the graphic of the microscope view to print an overhead transparency. (laser and ink-jet printers can print onto transparencies directly or Xerox onto one).

  2. Cut out the entire 100X view (black area, clear areas, etc. as ONE unit). Do the same for the 400X view, the 100X, 400X & 400X super rulers. You should have 5 and only 5 pieces at this point.

  3. Place the 100X view on your overhead and focus on your screen. This is representative of a typical view through a 10X objective coupled with a 10X eyepiece. Microscopes will vary as to the field of view that they show.

  4. Field of view can be measured by using the 100X ruler(a 'student' millimeter ruler as seen under 100 power magnification) and counting and estimating the number of mm's in the view at the widest point, the diameter (app 1.5 mm here, individual microscopes will vary). Ignore the 400X circle for the moment.

  5. Remember that 1 mm is 1000 microns. So 1.5mm equals 1500 microns

  6. Knowing that the diameter of the field of view is 1.5 mm or 1500 microns wide, a rough estimate can be made of the size of the singled celled organism in view. It takes approximately 10 lengths of the organism to fill the diameter of the screen. So

    So the approximate length of the organism is 150 microns. It is hard, however, to accurately estimate 10 lengths.

  7. Now, switching to the 40X objective with the 10X eyepiece gives 400X magnification. The field of view is

    This is illustrated by the 400X circle on the 100X view. This circle is ¼ the diameter of the 100X view.

  8. Switch to the 400X view and 400X ruler (a millimeter ruler as seen under 400 power magnification). Note that the mm ruler that the students have available cannot be used to measure the field of view for the 400X. (attempting to use the 400X ruler shows this point nicely).

  9. But we know that the 400X has ¼ the field of view of the 100X.

  10. OK, now we can use the higher 400X magnification to make a better estimate of the size of the organism. Under the 400X view it takes 3 lengths of the organism to span the diameter of the field of view.

  11. So…

    The approximate length of the organism is 125 microns.

    We have a better estimate as to the size of the organism: 3 lengths is much easier to estimate than 10 lengths.

  12. This is confirmed by using the 400X Super Ruler which has units of measure down to 25 microns. Though there are "real" rulers with this accuracy, they are very expensive ($500 up) and as we see, unnecessary for this level of study.

  13. Have students determine the field of view of their 10x and 40x objectives on their own microscopes.

  14. Have students determine the size of each cell or organism on their own microscopes using prepared slides or living organisms.


  1. Have the students figure out the field of view for any other objectives they may have. (4x, 20x, 100x?)
  2. After determining field sizes, give sizes of some organisms and have students decide which microscope objective and/or type of microscope would be the best for viewing. (protozoa, bacteria, viruses, insects, etc.)
  3. Ask students how speed of movement or interactions with other organisms would influence their choice of microscope objective. [see animation]


Introduction To Microscopy