Lab 2b Magnification
Learning Objectives
After completing the lab, the student will be able to:
- Determine the total magnification of each objective lens;
- Explain or describe, and measure, the field of view.
Introduction
Sometimes scientists want not only to view a specimen, but to measure it. This can be accomplished using a slide that has a scale bar superimposed on it, called a stage micrometer.
Safety Precautions
- Handle microscopes and lenses with care.
- Do not drop or crush slides.
Materials
- Compound microscope
- Slide with newsprint
- Slide with a stage micrometer.
For this activity, you will work in pairs.
Procedure
When you first place a specimen on the microscope’s stage, the image you see in the ocular lens will likely appear blurry. This is because light rays from the object are not reaching your eye at the same time. Focusing is used to improve the image. Focusing moves the stage up and down, which changes the point at which the light rays from the object converge. When the stage is at a position so that the light rays exactly converge on the retina of your eye, the image will appear sharp and clear.
When focusing the specimen, you must be careful! The distance between the objective and the specimen is very small, and it is possible to crush the specimen against the lens if you move the stage too far. The proper distance between the specimen and the object is called the working distance.
When you look at a specimen through the microscope’s lenses, you will likely not see the entire specimen at one time. Because the microscope enlarges the image of the object, only a small portion of the object will be visible at any given time. The field of view refers to the portion of the object that is seen through the ocular lenses (see Figure 2.3).
Step 1: Turn the revolving nose piece of the microscope so that the lowest power objective is pointed at the stage.
Step 2: Place the slide with newsprint on the stage. Make sure it is oriented properly, not upside down.
Step 3: Change the magnification by turning the revolving nose piece (on which the objective lenses are mounted). Draw your observations of a single letter using the 4x, 10x, and 40x objective lenses, in your lab notebook. Note: make sure that the center of view frame is occupied by actual print before rotating in the next lens- if (for instance) the opening within a letter “O” is in the center, the stronger lens will end up zooming into that open middle space and you will see only blank space. Neither does it automatically zoom in on the tip of the pointer.
Step 4: Place the stage micrometer on the stage, and view through the 4x objective lens. What is the width of your field of view at 4x? Repeat and 10x, and 40x. Make a table in your lab notebook, giving these field of view measurements. What is the ratio of the fields of view that result from using the 4x and 40x lenses? How is it related to the ratio of the power of the lenses (4 and 40)?
Step 5: Brainstorm and use trial and error: what is the best method you can come up with to measure the size of a newsprint letter through the microscope, using the newsprint slide and the stage micrometer? Explain it in enough detail that I (or another student) could copy your method effectively.
Discussion
- When you take a picture on your phone, does the field of view get larger or smaller as you increase the magnification? How does this compare to the microscope? Explain.
- As you increased in power on the microscope, the image became darker. Why do you suppose this is so?
