Part 2. Microscopes
1. Ask, What could we use to magnify the materials further? Distribute the microscopes and allow groups to examine them for a few minutes. Then ask, Where is the lens? Is there more than one lens? (yes, in the eyepiece and at the bases of the objectives) Ask, What do you notice about the lenses? Students should note the curvature of the lens and the “X” markings on the sides of the eyepiece and objective. Ask, What does “X” usually mean in mathematics? (multiplication or “times”) Explain that the bottom lens number (on the objective in use) and the top lens number (on the eyepiece) are multiplied to indicate the total number of times a specimen is magnified when observed. For example, an eyepiece of 10x with an objective of 4x will magnify an image 40 times (10 x 4 = 40).
2. If students are not familiar with microscopes, help them locate the basic parts. For example, tell students, One part of the microscope is called the stage. It is similar to a stage for a performance. Can you find it? What about the arm? Have students use “The Compound Microscope” sheet to find the eyepiece, objectives, coarse and fine focus knobs, arm, stage, and light source of their microscopes.
Many microscopes also have a condenser to intensify the light and a diaphragm aperture to adjust the amount of light passing from the light source up through the object. Encourage students to examine the microscope and propose the function of each part.
3. Finally, have students create a temporary slide, called a “wet mount.” Instruct students to cut out a 1-cm x 1-cm piece of newsprint, and to put the piece of newsprint in the center of a clean microscope slide. Have students place a drop of water on the paper, cover the drop gently with a cover slip and then place the slide on the microscope stage. If the stage has clips, have students place the clips over the slide to hold it in place.
If the microscope has a light source, make sure the light is aimed up through the paper. Initially, the diaphragm should be adjusted to its largest opening. If the image is too bright (seems “washed out”) when viewed, help students reduce the amount of light by partially closing the diaphragm.
Note: Low-power “dissecting-type” microscopes may not have a light source below the stage.
4. Direct students to move the lowest power objective into place above the print material (not all microscopes have multiple objectives) and to use the coarse focus knob to lower the tip of the objective until it is just above the coverslip. Students should look through the eyepiece and use the coarse focus knob (depending on the microscope) to move the objective gradually upwards until the printing on the paper comes into view.
Remind students that the object sample will come into focus when the objective is very close to the stage. Tell students to use the fine focus knob to sharpen the appearance of the image, and use caution not to break the coverslip. Each student should have an opportunity to adjust and focus the microscope.
5. Have students draw their observations of the newsprint on the “Magnification Observations” sheet. Some students may wish to study the newsprint at a higher magnification by first centering the object in the field of view, then gently rotating the middle objective into position and adjusting the focus using the fine focus knob only.
6. Discuss students’ observations or have them answer the following questions in their science notebooks. Ask, Which tool provided the greatest magnification? What did all of the tools have in common? What were the differences between each of the tools?
Keywords: compound microscope | concave lens | convex lens | focal point | how to use a microscope | light microscope | microbiology | microscope parts | microscope
- Illustration by M.S. Young © Baylor College of Medicine.
- Moreno, N., Tharp, B., Erdmann, D, Rahmati Clayton, S., and Denk, J. (2012) The Science of Microbes Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-54-5
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Grant Number: 5R25RR018605