Measuring with a Microscope

Transcription

1 Measuring with a Microscope Background: As you now have discovered, a microscope, as the name implies, is used to observe very small structures. Often, however, scientists are not as interested in the structures, but rather the size of the structures. As such, we have developed a way of calculating how large individual structures are using measurement. Since using a ruler is more or less out of the question, we need to find a way to measure without one. What we must do is measure indirectly, comparing the specimen to something already known to be a certain size. Two metric units are useful when measuring small objects: 1 meter (m) = 1,000 millimeters (mm) and 1mm = 1,000 micrometers (µm)* *micrometers is the most descriptive term, but you may here it referred to as microns as well. Laboratory Safety Precautions: The following symbols represent the precautions that are required for this lab: Purpose: The purpose of this laboratory experience is: -to learn how to measure using a compound light microscope. -to use what you have learned to measure specimens on prepared slides. -to meausure the diameter of your low-power field -to calculate the diameter of your high power field Materials: The following materials are needed to complete this laboratory experience: Microscope prepared slide of corn stem (cross section) Transparent metric ruler prepared slide of prepared slide of Procedure: The following procedure is utilized to perform this experience: 1. Since plastic rulers do not fit in the slide movers of our microscopes, your teacher has made transparencies of a section of ruler and taped them to a microscope slide. Determine which marks on the ruler indicate millimeters and then put the slide onto the stage of the microscope. 2. Look through the ocular and focus on the edge of the ruler using the coarse adjustment. Adjust the ruler so that the view in the low power field is similar to what you see in the figure below.

2 Empty area of field ruler Mm marking on ruler 3. Adjust the slide so that the edge of one mark is at the left side of the field of view and the ruler cuts the field of view directly in half at its widest diameter. 4. Note that 1 millimeter is the distance from the middle of one mark to the middle of the next mark. The diameter of the low-power field measures 1 millimeter plus a fractional part of another. 5. Record the measurement of the low power field in millimeters, estimating as necessary to the nearest tenth (1/10 th ) of a millimeter. 6. Now using the skills you have learned previously, convert the measurement in millimeters to micrometers. Since we cannot directly measure the high power field of view (as previously mentioned) we are forced to use some ratio equations to determine the diameter. Simply put, viewing the ruler under high power is not feasible since so little of the ruler is visible and the focusing will be difficult. HOWEVER, since we know the power of the objectives and the diameter of the low power field, we can assume that: high-power field diameter low-power magnification = low-power field diameter high-power magnification Simply plug in the appropriate numbers and you will be able to calculate the high power field diameter. Using the above equation and the numbers you have previously recorded, show your work in the space below (remember to include units as necessary). Answer: high-power field diameter =

3 Prepared Slide Observations Observation of: Zea mays (corn monocot) slide a. Under low power, focus on the prepared cross section of Z. mays. The center of the corn stem is filled with large, thin-walled cells called pith cells. b. Observe and draw what you see below. Date: 1. What is the overall width of the stem cross section on the slide? 2. How many pith cells fit across the diameter of the low power field? 3. Estimate the diameter of a single pith cell. Record your guess below. 4. Switch to high power and focus with the fine adjustment. Determine the diameter of the cells now that you can see more clearly where the defined borders of each cell are. Record their diameter below. 5. How do the measurements of the low and high power estimates compare? Why is this so?

4 Using two more prepared slides, fill in the data on the following page. Make sure you record the name of the specimen, the approximate measurement and a more refined measurement under high power.

5 Conclusion: The following can be concluded from this experience. Write your own conclusion based upon the factors that we have looked at in previous labs. Make sure to use proper sentence structure and summarize what you learned while performing this lab. Analysis Questions: Answer the following questions in the space provided. 1. Who, other than scientists, would use the microscope to measure and why? 2. Find the diameter of the high-power field of a microscope with an ocular marked 10X, a low power objective marked 10X, a high power objective marked 40X, and a low-power field diameter of 1600 micrometers. 3. What approximate fraction of the low power filed AREA would you see if you were to change to the high-power objective, using the microscope in question 2? Bibliography of Images used: Electrical hazard Symbol: