IB BIOLOGY
MICROMETRY
Micrometry: the use of a microscope for measurement
I. Measuring the Diameter of the Low Power Field (LPD)
A. do this by using a transparent plastic ruler which is placed
directly on the stage of the microscope
1. the position of the ruler is important
Q: Which of the following examples shows the correct placement
of the ruler? If the ruler is incorrectly placed, write down what
is wrong with the ruler’s position.
Example 1: _________________
Example 2: __________________
_________________________
___________________________
Example 3: __________________
Example 4: _________________
___________________________
___________________________
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II. Calculating the Diameters of the Medium and High Power
Fields of View (MPD and HPD)
A. The size of the field of view is inversely proportional to the
microscope magnification. Therefore, multiplying the microscope
magnification by 2 results in the size of the field of view becoming
smaller by a factor of 2 (i.e. the size of the field is halved).
B. The general equation relating the microscope magnification and
the diameter of the field of view of each objective lens is as
follows: L=low, M= medium, H=high, P=power, D=diameter and
M= magnification
LPD X LPM = MPD X MPM = HPD X HPM
C. Mathematically, this can be broken down into three separate
equations as shown below:
1. Equation 1: LPD X LPM = MPD X MPM
2. Equation 2: LPD X LPM = HPD X HPM
3. Equation 3: MPD X MPM = HPD X HPM
D. just as in math, if you know all the variables except one, you can
solve the equation for the missing variable
1. usually, we know the low power magnification (LPM) and
diameter of field as well as the medium (MPM) and high power
magnification (HPM) so that we can calculate the medium and
high power diameters of field. The equations used are given
below:
a. MPD = LPD X LPM
MPM
b. HPD = LPD X LPM
HPM
III. Calculating the Actual Size (AS) of a Specimen
A. do this by comparing the size of the specimen (as seen through
the microscope) to the known size of the field of view (LPD, MPD,
or HPD)
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B. there are three ways of doing this (but they are all really the
same thing)
C. For the following examples, imagine that you are using a
microscope that has an ocular magnification of 10X, a low power
objective that magnifies 4X, a medium power objective that
magnifies 10X and a high power objective that magnifies 40X.
Also, the diameter of the low power field has been measured and
is 5.0 mm. As a general rule, try to find the longest dimension
(length) of the specimen.
Example 1: The picture below represents what was seen using the low
power objective lens.
Method 1: Counting Method (Estimating)
1. ________ specimens could fit across the diameter of the field
2. diameter of field / # specimens = size of specimen
Method 2: Fraction Method (Estimating)
1. the specimen takes up ________ (fraction) of the field
2. fraction X diameter of field = size of specimen
Method 3: Measuring Fraction Method
1. using a ruler, the length of the specimen on the diagram/photograph
is _______________
2. using a ruler, the diameter of the field on the diagram/photograph is
_______________
3. make a fraction that is your measurement 1/measurement 2
4. fraction from 3 X diameter of field = size of specimen
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Q: Which of the three methods above should give the most accurate
value for actual size? Explain your answer.
A:
Example 2: Use any of the three methods above. This time, the picture
below represents what was seen using the high power
objective lens.
IV. Calculating the Drawing Magnification (DM)
*Note that this is usually not equal to the microscope magnification.
A. The formula used is as follows: DM = DS/AS**
**Note that the actual size (AS) and the drawing size (DS) must be in
the same units.
A. Now calculate the drawing magnifications of the pictures from
Examples 1 and 2.
Example 1:
Example 2:
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