Exercises that will help you understand how your microscope works
1.
Calculating magnification
One of the most basic things you need to know is how much the microscope is magnifying the
specimen you are looking at. Each lens in the microscope enlarges the image and the total
magnification is given by this formulae:
Magnification of
eyepiece (ocular) lens
Magnification of
objective lens
X
=
Total
Magnification
Most school microscopes have an eyepiece lens that magnifies 10 times (x10). Objective lenses
are usually x4, x10 and x40. All microscope lenses will have magnifications etched into their metal
casings.
The table below gives the magnifications of the most common microscope configurations:
Eyepiece
(ocular) lens
magnification
x5
x10
(most
common)
x15
Objective lens magnification
Low power
x4
Medium
Power x10
High power
x40
x20
x50
x200
x40
x100
x400
Oil immersion
lenses x100
x500
x1000
Most common magnifications
x60
x150
x600
x1500
Check the microscope you are using and fill in the table below. You will need to know the
magnifications of your microscope when you examine specimens and draw diagrams.
All microscope observations and diagrams must be accompanied by an indication of the
magnification you used.
Eyepiece lens
magnification
X
Objective lens
magnifications
Low power
Total
magnifications
X
=
Medium
power
X
=
High power
X
=
Oil immersion
(if your
microscope
has one)
X
=
2.
“e” inversion
For this exercise you will need a small piece of paper ripped from a glossy magazine. Make sure
the piece has a letter ‘e’ on part of it. It is best to choose a portion where the font size is small or
you will not be able to see the entire letter in your ‘field-of-view’.
e
slide
cover slip
Place this on a microscope slide, there is no need for water but a cover slip will keep the paper flat.
Focus under Low Power. What do you notice about the orientation of your letter ‘e’?
________________________________________
Draw what you see here:
________________________________________
________________________________________
Try moving the slide left/right, and up/down. What happens when you move the slide?
______________________________________________________________________________
Now use Medium Power and see if the same thing occurs. What about High Power?
(Of course you will probably not be able to see the entire letter).
______________________________________________________________________________
This exercise illustrates how difficult it can be when manipulating slides on the microscope stage. It
is particularly difficult when following moving organisms such as Paramecium because you must
move the slide in the opposite direction to the direction the organism moves.
Below are some pictures showing the type of images you should see:
3.
Indirect measurement under the microscope
In this exercise you will learn how to estimate the sizes of cells and other objects under the
microscope. This method of measurement is ‘indirect’ because the object itself is not measured,
size is estimated by knowing the diameter of the microscope’s ‘field-of-view’ (diam. f.o.v.).
The f.o.v. of the microscope is the circular area or image you can see when you focus on a
specimen.
All you will need for this exercise is a clear plastic ruler with millimetre marks on it. Place the ruler
on the stage so that the edge of the ruler cuts the f.o.v. in half (i.e. follows the diameter of the f.o.v.
Focus under Low Power.
You will see something similar to this:
Field-of-view (lit area)
You will not be
able to see this area
mm marks of ruler
Adjust the ruler so that the left hand mm mark is half in and half out of your f.o.v. (see red arrow).
You can now simply measure the diameter of the f.o.v. by counting the mm marks. In the example
above the diam. f.o.v. is 4.5 mm
Millimetres however are not a small enough unit to use when measuring cells as they are very
small. Instead we use micrometers (usually called microns, symbol µm).
A micron is one millionth of a meter, remember that one millimetre is one thousandth of one meter.
This means that there are 1000 microns in one millimetre.
1 µm = 10-6 m ( 1/1,000,000th m)
or
1,000,000 µm = 1 m
1 mm = 10-3 m (1/1,000th m)
or
1,000 mm
= 1 m.
1 µm = 10-3 mm ( 1/1,000th mm)
or
1,000 µm
= 1 mm.
This means that the Low Power diam f.o.v. = 4.5 mm
= 4,500 µm (microns)
How to estimate the size of
cells:
Imagine a row of cells as shown
(right). There are about 14 cells
in one diameter of this L.P. f.o.v.
That means 14 cells in
4,500 µm (microns)
The average length of each cell
= 4,500 / 14
= 320 µm
Note that cells are not directly measured but their size is estimated indirectly.
It is now time to do the same exercise using Medium Power and if possible using High Power
This is similar to what you will see using Medium Power.
As before you should adjust the ruler so that the left hand mm mark is half in and half out of your
f.o.v. (see red arrow). Now simply measure the diameter of the f.o.v. by counting the mm marks. In
the example above the Medium Power diam. f.o.v. is 1.8 mm or 1,800 µm
It might not be possible to do this exercise using High Power as often there is insufficient room to
get the plastic ruler under the high power objective.
Your microscope:
Of course using different microscopes the measurements obtained in this exercise might be
different. Use the table below to add the values you obtained.
Objective
lens
Magnification
(eyepiece x
objective)
L.P.
x10 x4 = x40
M.P.
x10 x10 = x100
H.P.
x10 x40 = x400
Diameter f.o.v.
millimetre
micron
(mm)
(µm)
If you cannot estimate the H.P. diameter f.o.v. by observing the ruler you can calculate it using this
formulae:
H.P. diam f.o.v.
= L.P. diam f.o.v. X
L.P. magnification
H.P. magnification
For the microscope values used in this exercise:
H.P. diam f.o.v.
= 4,500 X
= 450 µm
40
400
(less than ½ of one mm)
4.
Resolution exercise
For this exercise you will need a small piece of paper ripped from a glossy magazine that includes
part of a coloured illustration.
slide
cover slip
piece of coloured picture
Place this on a microscope slide, there is no need for water but a cover slip will keep the paper flat.
Focus under low power. What do you notice about the picture?
______________________________________________________________________________
Below are some pictures showing the type of image you should see:
Low power x40
Medium power x100
Low power x40
Low power x40
Can you see what this is a picture of?
It is in fact someone’s eye and eyebrow.
What this exercise illustrates is the important difference between magnification and resolution.
Magnification simply means enlarging the image so that it appears bigger than the specimen.
Resolution refers to the increased detail you are able to see. It is the ability to distinguish two
points or objects that are very close together.
In the example above our eyes are not able to resolve the dots of ink that make up the coloured
picture in a glossy magazine. However the microscope not only enlarges the image it also
increases resolution so that we can now resolve them.