ESTIMATING SIZE AND CALCULATING
MAGNIFICATION.
Purpose: To determine an approximate field diameter for each of the objective lenses
on our microscopes. To estimate the cell six
Background Information:
When viewing a small organism through the microscope, it’s usually necessary to have
some idea of its size. Therefore, you need to have some means of estimating the size.
When someone is standing near a doorway, you can estimate their height by comparing
them to the doorway. In the same way, you can estimate an organism’s length by
comparing it to the field of view that you are using.
Example: If the “doorway is 10 units, how high is the stick person?
Answer:
Approximately ___
units high.
Learning to calculate total magnification
Calculate the total magnification of the low power objective lens by multiplying
the magnification of the ocular lens by the magnification of the objective lens.
The magnification of the lenses is etched on the sides of the actual lens holders.
Record the Magnification of all power levels for your microscope in table 1.
Example: Low power: Objective lens = 4X
Ocular lens = 10X
Total magnification at Low Power = 4X(10X)
= 40X
Total magnification at Medium Power = 10X(10X)
= ______X
Total magnification at High Power = 40X(10X)
= ______X
1
Determining Diameter of Field of View
1. Take a clear plastic ruler and examine the millimetre scale under low power.
2. Place the centre of one of the scale marks along the edge of the field as shown
below.
3. Count the whole number of millimetre spaces. If there is part of a spacing,
estimate (in decimals) the size of the millimetre portion that shows. Record the
field diameter in millimetres in your data table 1.
Example:
The distance across this field of view is 4.2 mm.
To calculate the field diameter for high power
The field diameter for high power cannot be measure directly using your millimetre ruler
because this field diameter is LESS than one millimetre. Therefore, we must calculate the
field diameter a different way.
high power magnification
low power magnification
400x
4x
=
=
low power field diameter
high power field diameter
4.2mm
?
40x X 4.2mm=16.8
400x X ? = 400?
400?
400
?
=
=
16.8mm
400
0.042mm
Therefore the high power field diameter is 0.042mm
2
To calculate the field diameter for medium power
Now we could follow the same steps as we did under the low power but since we are not
actually conducting the experiment with a microscope we are going to practice using the
formula above
medium power magnification =
low power magnification
100x
4x
=
low power field diameter
medium power field diameter
4.2mm
?
40x X 4.2mm=16.8
100x X ? = 100?
100?
100
?
=
=
16.8mm
100
_______mm
Therefore the medium power field diameter is ______mm
Estimating Cell Size
1. Place the object to be studied in the middle of the field of view.
2. Estimate the number of objects that could fit across the field of view
3. Divide the diameter for the field of view by the number of objects that could fit
across it to estimate the size of the object.
Size of object = diameter of field of view
number of objects
Using the picture beside you determine how many
objects would fit across the field of view
Size of object = 4.2mm
1mm=1000m
3