Scope it out!
My lab exceeds the memory limit (as an attached document) so I am partially putting it on this page but here is link to my lab page for my seventh grade life science class site. Unfortunately the pictures do not copy-paste from the word doc to this wiki format and therefore are absent...thus you need to download the lab from my class site if you really want to take a gander or use it.
Scoping it Out Lab Name: _______________________
Period:_____ Date: ____________
Life scientists use microscopes to study things that are too small to be seen with the unaided eye.
Why is called a "compound" light microscope ? "Compound" just refers to the fact that there are two lenses magnifying the specimen at the same time, the ocular (eyepiece) & one of the three objective lenses (that you can rotate above the stage).
Magnification is a measure of how big an object looks to the eye compared to its actual size. Magnification is usually written by a number followed by “x”, which stands for times life size.
A magnification of "100x" means that the image is 100 times bigger than the actual object.
If two lenses are always magnifying the specimen how do you figure out the total magnification being used ?
You multiply the power of the ocular and the power of the objective being used.
Total Magnification = ocular lens mag. x objective lens mag.
For example, if the ocular is 10x and the low power objective is 4x, then the total magnification under low power is 10 x 4 = 40x.
FILL IN THIS TABLE:
Resolution
Microscopes allow you to see fine details. Spaces between objects that are closer together than 0.1 mm can be seen. The ability of a microscope to separate very small distances is called resolving power or resolution. If the resolving power of a microscope is not good, the image will appear blurred. Once you can resolve fine details you can then magnify them. So, the actual purpose of a microscope is to see small things clearly.
Procedure:
1. Obtain a slide of a newspaper and a magazine page.
2. Click the low-power objective lens into place. Place the slide on the stage of the microscope and clip it into place using the stage clips.
3. Observe the samples under low power. Are you able to see the tiny ink dots from which the pictures are constructed? ________________________________________
4. Viewing the stage from the side, switch to medium power. Use the fine adjustment knob to clearly bring the images into focus. Are the individual ink dots still visible? _____________________________________________________________________
5. Make a drawing of one of the views. Be sure to write down the magnification power used.
Magnification: _________ X
Depth of Field
Another desirable feature of a microscope is depth of field, which is the range of depth that an object is in acceptable focus. Depth of field is the area in front of and behind the specimen that will be in acceptable focus. For example, when you take a photograph of a close up of a person the background will often be out of focus. A microscope that has a thin depth of field will have to be continuously focused up and down to view a thick specimen to focus on the layer that you want
to see.
1. Obtain a slide of thread pieces. Notice that the colored threads crisscross at on point.
2. Place the slide on the stage and locate the point at which the threads cross using the low-power objective lens. Which color thread is on top? __________________________________________
3. Switch to high power. Use the find adjustment to bring the threads into sharp focus.
4. Use the diaphragm under the stage to adjust the light until you can see fine details. What do you notice about the threads that you can’t see without the microscope? __________________________________________
__________________________________________
__________________________________________
Microscopic Measurements
Life scientists often need to know the actual size of objects they are studying with a microscope. One way to find out is to measure the size of the field of view of the microscope and estimate how much of the field is covered by the object. This procedure is like guessing the length of a whale by seeing it stretched out on a football field. If the whale stretches across the field, you would estimate the whale’s length to be about 50 meters.
The circular area of the slide that you see when you look through a microscope is called the "Field of View". If you know how wide your field of view is, you can estimate the size of things you see in the field of view. Figuring out the width of the field of view is easy --- all you need is a thin metric ruler.
Procedure
Carefully place a thin transparent millimeter ruler on the microscope stage so that the millimeter scale is over the stage opening. Look through the low-power objective lens and we can measure the field of view in millimeters.
Use the course adjustment knob to focus on one of the millimeter line markings in the middle of the ruler –look for the label “cm.”
Move the ruler until one of these markings is just visible at the edge of the field of view, as shown.
The total width of the field of view in this example is less than 1.5 mm. A fair estimate would be 1.3 or 1.4 mm. (Relax, it's an estimate).
When scientists look at very small objects, they use a system of measurement known as the International System of Units, or SI, based on the metric system. Microscopic images are usually measured in millimeters (mm) or micrometers (m m).
Now millimeters is a nice metric unit, but when we use a MICROscope we tend to use MICROmeters (one millionth of a meter). To convert from millimeters to micrometers, move the decimal 3 places to the right-which is like multiplying by a 1000. Our 1.5 mm estimate becomes 1500 micrometers.
4. Count the number of millimeters you see across the diameter, or center, of the field of view. Estimate any additional fraction of a millimeter you see. Notice the diameter of the field of view in the picture is about 1.5 mm.
5. Record this number, which is the diameter of your field of view under low power, in data table below.
6. Find the number of micrometers in your field of view by multiplying the millimeters by 1000. Record the micrometers in the table below.
7. Repeat steps 2 - 5 using the high-power objective lens. CAUTION: Never use the coarse adjustment knob with the high-power objective lens.
· Prepare a wet mount of a strand of your hair. Your teacher can help with this procedure. CAUTION: Use caution when handling microscopes and glass slides. Measure the width of your hair strand while viewing it under low and then high power. Estimate how many hairs would fit across the field of view and do the math to find the width of one.
The estimate the width of a human (your) hair in __________ mm and
_________ m m (micrometers)
First of all memorize this:
When switching from low to high power, the area in the field of view gets smaller & darker. (You see a smaller area of the slide under high power.) This is why centering what you want to see prior to switching to high power is so important.
The fraction of the area seen under high power is the same as the ratio of the low & high power magnifications.
Um, huh?
For example : if the medium power objective is 10x and the high power objective is 40x, then under high power we will see 10/40 or 1/4 of the area of the slide we saw under low power.
Now we can get the ruler out of the way, prepare a slide, focus, and estimate the size of things we see ! (Exciting, ain't it?)
For example, if something we were looking at a specimen that took up half of the field of view, its size would be approximately 1/2 x 1500 micrometers = 750 micrometers. If something appeared to be 1/5 of the field of view, we would estimate its size to be 1/5 x 1500 = 300 micrometers.
Challenge #1: ocular power = 10x
low power objective = 4x
The diagram shows the edge of a millimeter ruler viewed under the microscope with the lenses listed above. The field shown is the low power field of view.
What is the approximate width of the field of view in micrometers? _________ m m
Challenge #2: ocular = 10x
low power objective = 4x
The picture shows the low power field of view for the microscope with the lenses listed above.
a) What is the approximate size of the cell in micrometers? ________
c) How many cells like the one in the picture could fit in this low power field of view ? _______
Challenge #3:
If the pointer shown to the right in the picture with the cell is 10 micrometers in length, then about how wide is this cell?
20 micrometers
10 micrometers
5 micrometers
0.1 micrometers
Challenge #4:
Look at diagram A that shows an object viewed under low power. Knowing the circle diameter to be 1500 um, the estimated length of object (a) is 400 um.
What is the estimated length of object (b)? __________
Challenge #5:
Look at diagram B that shows an object viewed under high power.
Knowing the circle diameter to be 375 um, the estimated length of object (c) is 100 um.
What is the estimated length of object (d)?
__________
Review Questions
1. What is the purpose of a microscope?
2. What is magnification?
3. How do you find the total magnification power of a microscope?
4. How is magnification expressed?
5. What is resolving power?
6. What would cause an image to appear blurred?
7. What is the circular area that you see when you look into a microscope?
8. The ocular lens of a microscope has 6x marked on it and the low-power objective lens has 10x. What is the total low-power magnification?
9. A magazine shows a photograph of a computer chip that is magnified 100x. What does this mean?
10. If the field of view of a microscope is 1.5 mm, what is its width in micrometers?
11. An object can be magnified 100, 200, or 1000 times when viewed under a microscope. Does the object's actual size change with each magnification? Explain.