HOW SMALL IS SMALL?
Grades 9 - 11
Young science students very often have difficulty conceptualizing just
how small various microscopic objects and organisms are. For example, many
students continually to confuse atoms with cells. They don't seem to have
an understanding that it takes millions, even billions of atoms to make
a typical cell. By clarifying the immense difference in size between such
structures, students should be better able to conceptualize this rather
abstract idea of microscopic objects. This lesson is designed to teach students
the difference between macroscopic and microscopic, and that even things
viewed within the microscopic world are vastly different in size. The lesson
serves as an introduction to microbiology topics. Note: Keep in mind that
students must have already had some previous experience with the microscope,
and a good understanding of metric measurements is helpful.
"National Geographic: The Invisible World. (Available from
PBS video @ 1-800-328-7271. $14.99)"
Students will be able to:
- identify objects as macroscopic, visible with an optical microscope,
or visible with an electron scanning microscope.
- calculate field of view size in an optical microscope.
- approximate sizes of objects seen under optical microscopes.
(Per group of 2 students)
1 transparent plastic millimeter ruler.
Macroscopic: Large enough to be seen or examined by the
Microscopic: Too small to be seen or examined by the unaided eye.
Micrometer: a thousandth of a millimeter.
Magnification: The factor by which an object is enlarged. Note: when
using the term magnification in reference to the enlargement caused by a
microscope, the factor refers to how much larger the object appears than
if it were viewed at a distance of 25.4 cm (10 in.).
Administer a pretest. Simply ask the students to list the following
objects in order from largest to smallest. You may have the students work
in groups or individually. By administering the pretest the students will
be introduced to the items that they will be seeing in the video. The test
will also serve as a diagnostic tool for the teacher to see how much the
class already knows about the microworld.
virus, atoms, molecules, grain of sand, mites, typical animal cell, typical
plant cell, a pollen grain, width of a hair, etc.
Have the students share answers for comparison. This will allow the students
to see what others are thinking, and allow the teacher to see how much the
students already know.
We're now going to view a short segment of a video called The
Invisible World. You will be seeing a variety of objects that are magnified
at less than 100x to over 20,000x. To give the students a specific focus
for viewing introduce the video by saying,"I would like you to copy
the table I have provided on the board (table 1), and then as you watch
the video fill in the table as best you can with objects, organisms, and
details of objects that you see in the video."
Fill out the table with things seen in the video by putting them under the
category of how they can be viewed.
scanning electron microscope
BEGIN tape at the segment with a couple and a child walking
on the beach and the narrator says "We delight in exploring the world
we can see..."
PAUSE tape after he says ...our eyes can barely resolve objects that
are 1/300 of an inch in diameter. Ask for a student volunteer to express
this number in millimeters. If they don't have enough information to do
so, provide them with the fact that 1 inch = 25.4 mm. 25.4 mm/300 = 085
mm (which you may want to round to 1 mm). Point out that this means that
anything over 1 mm should be seen by the average human eye, while objects
or details smaller than this may require the aide of a microscope.
PAUSE tape after seeing cell divide to check for understanding of
focus for viewing responsibility. Ask the students to share what they have
gathered in the table so far, and under what categories they have placed
these objects. You only need to correct mistakes at this point if some students
are really struggling. They should have: Unaided eye - sand; optical microscope
- plankton, bacteria, plant cells, blood cells, dividing cell.
PAUSE tape after the narrator says "...a powerful new instrument
called the scanning electron microscope has penetrated a uncharted level
of detail and size. Take a moment to highlight what was just said in the
video about the difference between an optical microscope and a scanning
PAUSE tape when narrator says "This microscope has a power of
100,000x." Have the students write on their table the magnification
abilities of optical microscopes (2000x) vs. scanning electron microscope
(up to 1,000,000x). Segue back into the tape by reminding them of their
focus for viewing.
PAUSE tape when narrator says... we are never completely alone. Take
time now to review all of the items that students have listed on their table.
It is important to point out that all of the microscopic, multicellular
organisms seen in the segment that they just viewed are visible under an
optical microscope, but the detail with which they were just seen can only
be seen with a scanning electron microscope.
STOP tape after the segment on viewing individual atoms, immediately
after the scientist says...."and every time you have a new way of looking
at things you find out something new." This concludes the video portion
of the lesson.
We have now seen a variety of things that can only be seen with
the assistance of magnification equipment. Now let's see if we can get a
better understanding of how big these things are as we view them under our
1) Students work with a partner.
2) Students set their microscope on the lowest power.
3) Record the magnification of the ocular and the objective.
4) Multiply these two numbers to get the total magnification for this setting.
5) Repeat this procedure for remaining objectives.
6) Now return the microscope to the low-power position.
7) Place a transparent millimeter ruler on the stage.
8) Focus the microscope so that a clear image of the divisions on the ruler
can be seen.
9) Move the ruler until the edge of one of the markings is on the edge of
the field of view.
10) Measure the field of view across the center in millimeters, and record
11) Calculate the diameter of the field of in micrometers by multiplying
your answer by 1000.
12) Find the diameter of the medium-power objective's field of view. This
can be accomplished by dividing the magnification of the medium-power objective
by the magnification of the low-power objective. Then divide the diameter
for the low-power objective by the quotient. The result is the diameter
of the medium-power objective's field of view.
13) Repeat step 12 for the high power objective.
14) Now you are prepared to estimate the size of various objects viewed
under the microscope. For example, if an object has a height of about half
of the field of view, and you calculated the diameter of that field of view
to be 1000um, then the height of the object is about 500um.
15) If time permits select 3 or 4 prepared slides. Estimate and record the
size of the objects viewed in those slides.
Take a trip to a University or other institution that has a
scanning electron microscope, or invite a researcher to your school to share
information on the usefulness of such technology.
Draw or paint detailed color sketches of organisms, objects and patterns
seen under the microscope. Develop a complete display of these pieces.
Using a microscope with the ability to video tape, record the viewing of
some living organisms. Then set the tape to video.
Investigate the origins of the names for various microorganisms.
Continue with the theme of the metric system. Create wheels or other instruments
which make for easy shifts from meters to centimeters to millimeters and
Master Teacher: Robert C. Lopez
Lesson Plan Database
Thirteen Ed Online