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 microscope

1 transparent plastic millimeter ruler.

Macroscopic: Large enough to be seen or examined by the unaided eye.

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.

Objects:

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."

Table 1
Fill out the table with things seen in the video by putting them under the category of how they can be viewed.

unaided eye
optical microscope
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.

RESUME tape.

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.

RESUME tape.

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 electron microscope.

RESUME tape.

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.

RESUME tape.

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.

RESUME tape.

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 microscopes.

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 this number.

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.

Art:
Draw or paint detailed color sketches of organisms, objects and patterns seen under the microscope. Develop a complete display of these pieces.

Music:
Using a microscope with the ability to video tape, record the viewing of some living organisms. Then set the tape to video.

Language Arts:
Investigate the origins of the names for various microorganisms.

Math:
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 so on.

Master Teacher: Robert C. Lopez