IB Practical 1: Microscope – Estimating Size and Calculating Magnification
IB Assessment Objectives:
 Use a light microscope to investigate the structure of cells and tissues, with drawing of cells.
 Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in
drawings or micrographs.
Assessment for Lab:
 Analysis
 Evaluation
Part 1 – Estimating Size of Specimens under the Microscope
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 / size by comparing
it to the field of view that you are using.
For example, if the doorway below is 10 units, how high is the stick person?
Procedure:
1. Place the data tables into your lab notebook.
2. 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 each lens is noted on the side of the lens. In
our school, the low power is also indicated by a different color – red – than the other objective lenses. You will
also notice it is the shortest objective.
3. Record the magnification of all the lenses for your microscope in the data table.
4. Place your information on the Smart Board for the first data collected. Calculate the class average and record it.
5. Set up a microscope with the 4x objective lens in place and focus it on the ruler scale. See the diagram below for
guidance.
6. Move the ruler until the 0.0mm of it touches the left-most point of the field of view at the widest point.
7. Read the diameter (in mm) and record your result in the data table. Note your accuracy – i.e. +/- 0.5mm
8. Change the objective to the 10x lens and repeat steps 5 & 6 with the ruler. The field of view will be MUCH
smaller.
9. Estimate the 40x objective’s field of view by dividing the 4x diameter by 10 – use mm units.
10. Place your measurements for your group on the Smart Board data table. You will then need to calculate the
class average. Again, do not forget to do the uncertainty with your measurement.
You should also be able to estimate the size of an organism or cells viewed under a microscope, as well as draw it and
give an estimate of the size of your drawing.
For example, you have drawn this organism:
You have calculate the size of the organism as 1,125 μm long. Your drawing is
6.2 cm long (note – the diagram below is NOT 6.2 cm long – it is just an
example).
On your drawing, 1,125 μm is represented by 6.2 cm = 62 mm = 62,000 μm.
The MAGNIFICATION OF THE DRAWING is 62,000 μm / 325 μm = about 55
times. You can describe this clearly by stating “drawn at 55x actual size.” If you
viewed this on low power, you would state “viewed at 40x magnification.” You
should be careful in your terminology!
Now, you need a scale bar on your diagram. The easiest would be 500 μm
here. This is simply going to be a ratio.
Actual size of drawing = Actual size of specimen
Scale bar of drawing
Desired size of scale bar
So, 6.2 cm / x = 1125μm / 500μm, so the scale bar would be approximately
2.75cm long next to your drawing.
Guidelines for Drawings:
 Use a sharp pencil on white, unlined paper – glue this into your lab notebook.
 Pen is not acceptable since errors cannot be corrected.
 Be sure lines are clear and un-smudged.
 Center the drawing on the page.
 Use a ruler to draw straight lines.
 Scale bars should be at the bottom right if doing a more horizontal orientation, and to the right (as indicated
above) if doing a more vertical orientation.
 Labels must be PRINTED – do NOT use cursive.
 Draw only what you see – not what you think you should be seeing. Only draw what is requested.
 Title your drawing with what has been drawn, what total magnification was used, and what is the drawn size
(see above for descriptions on how to write this).
 If there is an organism where you know the specific, scientific name, you should indicate that an underline it. For
example, Paramecium.
 Whenever you are doing the math for the magnification, exception of doing the scale bar line in the equation
above, be sure that you are using the same units!
 See the example on the next page for an exemplar of how to do a drawing.
Example 1
Record the answers in your lab notebook.
If you have a drawing and you want to calculate the magnification or the actual size of the object, you can use this
formula:
Magnification = Measured Size of Diagram / Actual Size of Object
Assume that the “x 90,000” is μm. This is an error that you should NOT make on your drawings!
Determine the actual length of this bacterial cell. Keep in mind that bacterial cells are SMALL. Give your answer in μm.
Example 2
You should also be able to use a scale bar to determine the actual size of an object that is either drawn or is a picture –
often called a “micrograph.”
 Measure the size of the drawing using a ruler or straight edge of some kind.
 Calculate the actual size using the scale bar – measure the scale bar first.
o
o
o

Use a ruler to measure the length of the specimen in the picture in mm.
Measure the length of the scale bar in mm.
Calculate how many scale bar lengths make the specimen. (Divide the length of the specimen by the
length of the scale bar.)
o Calculate the size. Multiply the scale bar label by your calculation.
Calculate the actual size of the prokaryotic cell in the micrograph below using the scale bar.
Part 2 – Observation of Living Cells
For this portion of the lab, you will use two different types of plant cells to make some drawings and estimations of sizes
of cells. For estimating, you will need to know the field of view diameter from your first table. For example, you see the
following cells in a line across your field of view:
There are four cells in the field of view at 4000x total magnification.
 Estimate how much of the field of view is taken by the length of an
individual cell.
 Take that estimate (i.e. 0.33 for 33% - NOT correct for this example) and
multiply it by the field of view.
 That should give you the approximate length of one cell.
1. Pick off an entire healthy leaf from the Elodea plant. Use forceps (tweezers) and gently place it on the
microscope slide.
2. Add a drop of water, then holding the cover slip at a 45° angle, drag until it touches the water and the water
forms a film along the edge. Gently lower the cover slip. This is to minimize air bubbles on your slide.
3. Place your slide on the stage – follow the instructions from the virtual lab and go to the highest power.
4. You may use a petri dish to trace a circle if you like, then draw at least four cells (they can be adjacent to each
other) or portions of those cells. Be sure to follow the complete labeling instructions found in part 1 of the lab.
1. Obtain a slice of onion. Carefully peel a thin layer from the outer part of the onion using the forceps – it should
be translucent, not transparent. If needed, you can use a drop of stain on the slide to help visualize the cells
prior to putting on the coverslip.
2. Use the medium power and count the number of cells that fit in the field of view lengthwise and widthwise.
Create a data table in your lab notebook to track your data. Repeat this in THREE DIFFERENT AREAS.
3.
4.
5.
6.
7.
Determine the mean length and width of an individual cell using suitable units for each trial / area.
Place your average on the Smart Board. Calculate the class average.
Using the same slice of onion, peel a similar section from the INSIDE of the onion.
Repeat the procedure to obtain an estimate of the length and width of an individual cell.
Think back to the Beak of the Finch lab – you will want to determine if the means of the cells are significantly
different or not. For this, you will want to use a t-test. You can also include other statistical tests as well as
graphing of your data.
 To be most accurate with a t-test, it is best to have at least 10 replicates for your data, not just 3.
 It is advised that you ask other groups and exchange data – you can also contact students from the other
class.
 An online t-test calculator is on the class website.
 Even when using an online calculator, it is still advisable to show a calculation within your lab. For
example, if I were to simply calculate the means of the data, I would indicate it as follows in my lab
write-up:
Where X = sample measurements, and N = number of samples, and Ʃ is the sum.
For this lab, the mean of the length of cells would be:
is the mean of these values.
So, the mean for the length is 60 μm.
Note: There is an equation editor that you can use in Microsoft Word, or you may hand write the calculations.
Any other test should have the same format:
 Show the equation
 Explain what the variables represent (you can do this as a bulleted list instead of paragraph form)
 Show ONE sample calculation from your actual data.
 Include a data table that has ALL of your calculations clearly identified. For the rest, you can use the online
calculator, then just state the values.
Student t-test
 Null hypothesis would be there is NO DIFFERENCE in the means of the values.
 Alternate hypothesis would be there IS A DIFFERENCE in the means of the values.
 You would then need to look back at what your independent (manipulated) variable is for the data.
 If there is a statistically significant difference, then it would be due to the manipulated variable as long as all
other variables were held constant.
 The online t-test calculator will also give you other statistical data, as well as provide an explanation as for the ttest. You should be familiar with the explanation as you will need to be able to explain what this data means in
your evaluation section.
Assessment for Lab
 Analysis section that includes
o All your data tables and lab drawings.
o Any manipulated data including formulas used for any statistical tests, descriptions of formula variables
and a sample, worked calculation for any calculations you are using.
 Evaluation section that includes
o Description of your data – literally state the data in words
o Explanation of your data – relate this to what you have learned – what does the data mean? If you did a
statistical test, explain the test and interpret the results as they relate to the lab.
 What NOT to do: The null hypothesis is supported by the t-test. This is not an explanation, this
is a description and has no value if not accompanied by an explanation.
You are strongly encouraged to use the rubric given to you with the checklist and questions for these sections!!
Table 1: Calculations of Microscope Objectives
Magnification of
Field Diameter (mm)
Microscope
Low (_______ X)
Field Diameter (μm)
Calculated Constant
(FD x Magnification)
Average Constant for
Microscopes
Medium (______ X)
High (_______ X)
Table 2: Class Means of Field Diameters
Magnification of Microscope
Low (_______ X)
Medium (_______ X)
High (________X )
Field Diameter (mm)
Class Mean
Field Diameter (μm)
Class Mean