Introduction to the Light Microscope Laboratory
PSI Biology
Name____________________________________
Objective
Students will practice proper handling and use of the light microscope. They will make
observations under low and high power magnification and describe their observations.
Materials
Compound light microscope
Glass microscope slides
Cover slips
Prepared slide of crossed fibers/threads
Transparent 15 cm plastic ruler
Prepared slide of plant root or stem
Prepared slide of bacteria
Newspaper
Scissors
Pipet or dropper
Time Requirements:
Pre-lab prep (For teachers): approx. 30 min
Lab activity: 40 min.
Procedure
1. Obtain a microscope from the microscope cabinet. Make sure to carry the microscope with two
hands and do not tilt the microscope when carrying it. Keep it close to your body as you walk
back to your seat.
2. Identify the parts of your microscope by referring to the above picture. Depending on the model
of your microscope, some of the parts may vary slightly. The number of objectives (lenses) may
also vary. Your teacher will go over these parts with you.
3. If your microscope is equipped with a lamp, plug in the microscope and turn it on.
4. Turn and click the low power objective so that it is directly over the opening in the stage. .
5. Look through the eyepiece. You should see a circle of white light. If your microscope has one
eyepiece, try keeping both eyes open. This will help reduce eyestrain.
6. Adjust the iris diaphragm to make the light as bright as possible.
7. Locate the course focus knob and slowly turn it back and forth. Look at the side of the
microscope as you do this and notice what is happening.
8. Use a piece of lens paper to clean the eye piece and the objectives each time you use a
microscope. Always use lens paper to clean the lenses as other types of paper may scratch the
lenses.
9. Use scissors to cut out a square of printed text from a newspaper. The square should be about
1cm x 1cm. Place this square on a microscope slide.
10. Using a medicine dropper or plastic pipet, add one drop of water onto the paper square and add
a coverslip. Place the slide onto the stage of the microscope.
11. Observe the slide with the low power objective and use the focus knobs to bring the letters into
focus. Move the slide slowly from side to side and front to back using the control knobs. Notice
how the image changes and record your observations in the analysis section.
12. Obtain a prepared slide of crossed fibers and look at the pattern of the fibers before placing it on
the stage.
13. Place the slide of crossed fibers on the stage and observe it using the low-power objective. Use
the focus knobs to focus your image. Move the slide around as you did in number 11. Record
your observations in the analysis section.
14. Observe a transparent ruler through the low-power objective. Use the ruler to measure the
diameter of the field of view in mm. Record this distance and the magnification of the low-power
objective in Table 1.
15. Calculate and record the diameter of the field of view through the other objectives. For
example, if a 4x objective has a field of view of 3mm (3000 micrometers), then a 10x objective
will have a field of view of (4/10) x 3 = 1.2mm (1200 micrometers).
16. Examine a prepared slide of a plant stem or root using the low-power objective and then the
high-power objective. The small round shapes you see are cells. Use the field diameters you
calculated above to estimate and record the size of a typical plant cell. For example, if 4 cells fit
across a 1200 micrometer field, then each cell is 300 micrometers long. Record your
observation in the analysis section.
17. If a prepared slide of bacteria is available, repeat step 16 with this slide.
Analysis
Table 1
Objective Size
Diameter of Field of View
1.
When observing the text you cut from the newspaper, how did the image appear under the
microscope? In what way was it different from reading it normally without a microscope?
2.
In what way did the microscope alter the image of the crossed fibers you observed? How
did moving the slide affect the image?
3.
What is the approximate size of the plant cells you observed? How did you calculate this?
Show your work.
4.
If you observed bacteria cells, what was the approximate size of the cells you observed?
Show how you arrived at your answer.
5.
What are some advantages of using the high-power objective? What are some
disadvantages?
6.
Some plant diseases are caused by bacteria. Could a bacterium injure a plant cell by
surrounding it and consuming it? Explain your answer.
Teacher notes:
Pre-Lab Prep:
1.
Check all the microscopes prior to the lab to ensure that they all work. Remove any slides
that may have been left on the stage from previous use. Instruct the students that they
should always remove any slides from the stage and place the low-power objective into
viewing position before storing their microscope.
2. Have glass slides, cover slips, prepared slides, lens paper and any other supplies in an
accessible area for the students.
3. Instruct the students to only use the fine focus knob when using the high-power objective to
prevent the lens from hitting the slide and potentially damaging the objective lens.