SKILL BUILDER: How to Use a Microscope
Lesson 15 of 17
This lesson is fairly straightforward in intent and implementation. Students learn the names and functions of the parts of a microscope and practice the technical operations of the microscope to make qualitative and quantitative observations.
I teach this before the first examination of the aquatic chamber in the Modeling Human Impact lesson sequence, but it could be covered earlier depending on the flow of your schedule.
Connection to Standard:
While so much of the focus on "technology" in the standards seems to mean computer technology, a microscope is inarguably an important piece of technology that broadens the horizons of inquiry for scientists, and as such, is an important tool to gather more information that can be integrated into student writing. The tool itself is complex, so it's proper use requires technical operation and following multi-step instructions.
In this lesson we more or less dive right into it. I reinforce the etymology behind the term “microscope” (micro- = small, -scop = to view), but only very briefly. This being a high school class, most students already know what a microscope is used for. The difference here is beginning to develop sufficient skills to enable using the microscope in a way where the focus is on data collection.
Since my classroom has traditional desks in the center and lab tables around the perimeter of the room, I begin by asking students to sit at lab tables that have electrical outlets. Depending on the set-up of your room, it may not be necessary to move at all, or you may have to arrange desks in front of outlets (I end up having to do this in larger classes when there aren’t enough seats at lab tables).
Once students have found the location they’ll be working, I have them get one microscope for each pair of students. It’s possible to do this lesson with fewer microscopes, but it can get fairly tedious for students that have to wait for more than one other person to use the microscope.
I am very formal with my description of how to prepare to use the microscope. I have them first remove the dust cover and have them fold it and place it under the base of the microscope. Then I have them carefully uncoil the power cord and plug it in. Once all students have their microscopes, I distribute the "parts of a microscope" worksheet and we begin the orientation.
Once everyone has a working microscope in front of them, we go over the worksheet showing the parts of the microscope. I randomly choose a student to read a definition of a part off of the back of the worksheet and then solicit answers from volunteers to which part that might be. Once a correct answer has been offered, I direct the rest of the class to write the name of the part on the appropriate line of the diagram on the front page (Please note that I made the worksheet by taking a picture of the specific microscopes available in my classroom, you may wish to do the same with your microscopes if they are of a significantly different design).
When we get to the lenses, I make sure to point out a few things.
I ask students to look through the eyepiece as they rotate the objective lenses and ask them what they see. I hope they see that moving from one objective to another has a space of darkness in between.
I ask them to listen for a “clicking” sound when the objective lenses are in proper position. I then ask them to look through the eyepiece to observe that when the objective lens has been “clicked” into place, they can observe a full circular field of view.
I ask them to look at the numbers on the lenses to get a sense of magnification power. I have them note that the ocular lens of the eyepiece has a listed magnification of 10X. I then ask them to look at the objective lenses. On my microscopes, the objective lenses come in 4X, 10X, and 40X. I ask students to calculate total magnification by multiplying the ocular lens magnification by the objective lenses’ magnification, resulting in total magnification of 40X, 100X and 400X.
As for the coarse adjustment knob, I ask students to turn it and observe any observable effects to the exterior of the microscope. They usually quickly point out that turning this knob rises and lowers the stage. I then ask them to do the same with the fine adjustment knob, which usually leaves them befuddled as there is no observable effect on the microscope. I assure them that it is also rising and lowering the stage, but at such a small rate that we cannot easily observe it. I tell them that after using the coarse adjustment knob to bring an object into relative focus, the fine adjustment knob is for bringing it into "HD" (high definition)... they usually understand it when explained that way.
Finally, I ask them to examine the x and y axis knobs under the stage. By turning these knobs, they determine that the top and bottom knobs move the stage (along the y axis) or the stage clips (along the x axis), respectively.
After students have been oriented with the parts and functions of the different parts of the microscope, I ask each group to send a representative to pick up a prepared slide. This link has a good starter set of slides appropriate for any biology or environmental science class.
I prefer to use prepared slides for this lesson because when students are just learning to use a microscope’s basic functionality, they really don’t need to be trying to track fast moving microorganisms rapidly swimming through three dimensions. In this case, prepared slides provide an opportunity to focus on a two-dimensional, static image.
I allow students to explore a while first, and then after a few minutes ask them who is ready to take the “focus test”. The “focus test” consists of my walking over to a student, checking to see how focused I can make the image (to check that their microscope is capable of high definition focus), then intentionally unfocusing the image. Students then have to focus the image as best as they can in a short time (usually under a minute, though I’m not actually timing them). If the image looks fairly close to the resolution I was able to achieve, the students pass the “test” and can move on to the independent activity section of the lesson.
For those students that do not pass the “test”, I simply go back and unfocus the image again and allow them another chance to focus the image. I repeat this until all students have passed the test and they are all ready to go to the independent activity.
Once students are finished with the focus test, I hand them the microscope observation sheet and allow them to begin their independent activity. This activity consists of observing the same point on a slide at low (40X or 100X) and high (100X or 400x) magnification. Please note that many students have difficulty focusing at 400x, so I often recommend they do 40X and 100x. At both powers of magnification, students must make a detailed, color illustration of the image they see on the slide. Students can use the circular shape on the worksheet to approximate the circular field of view offered by the microscope, which often makes working out the relative size of objects within their field of view easier. In addition to the drawing, I ask students to include the title of the slide and detailed qualitative and/or quantitative observations of what they observe. Without a gridded slide or hemacytometer, it may be hard to make actual measurements so I usually accept basic quantitative data based on counting and describing sets. When comparing the same slide at different magnifications, I ask students to focus on describing what differs between their low power and high power observations rather than repeating the same observations, “only bigger”.
When we have about 5 minutes left in class, I ask all students to stop whatever they're doing and ask students to send a partner to the front of the class with the slide. I remind them that the slides are glass and to be very careful in handling them and walking with them.
During this time, I ask the other partner to wrap up the extension cord neatly and secure it with a plastic tie. Once the power cord has been neatly secured, I ask them to unfold the dust cover from under the microscope and then place it over the microscope.
Again stressing the need to be careful with the microscopes, I ask students to pick up the microscopes using use one hand on the arm and the other hand under the base of the microscope. I then ask them to return the microscopes from where they took them. Once that is done and I've accounted for all microscopes, class is dismissed.