In this video, we are going to look at the parts of the light microscope, as well as
how to determine magnification and the Field of View .
Light microscopes use a combination of lenses and light to magnify objects.
The light microscope lets one be able to view objects in their natural colour, as well as
view stains used to highlight certain structures.
The parts of the light microscope are; The eyepiece, also known as the ocular lens,
which is where you look into the microscope.
The eyepiece tube.
The revolving nosepiece, which the objectives are attached to.
The arm.
The stage, which is where you put your slide.
The Stage clips that hold your slide in place.
Underneath the stage is the diaphragm, also known as the condenser.
There is a coarse adjustment knob and a fine adjustment knob.
At the bottom, you have the base as well as the light source.
How do you work out the magnification of the object you are looking at through the microscope?
Magnification is the amount in which something has been magnified compared to its real size,
in other words how much bigger it appears.
To determine the total magnification you times the magnification of the ocular lens by the
magnification of the objective lens used.
The magnification of the ocular lens should be written on the side of it, or in the user
manual.
The objective lenses have the magnification written on the side of the lenses.
If you have an ocular lens with a magnification of 10 times and you are using the objective
with the magnifying power of 4 times, the total magnification would equal 40 times.
If you have the same ocular lens as this doesn’t change but instead were looking at the object
now with a 10 times objective lens.
It would be 10 times by 10 times, with a total magnification of 100 times.
When looking through the microscope, there is going to be a circular area you can see.
This is called the field of view.
The Field of View diameter is simply the diameter of this Field of View.
When on low power magnification, you can measure the field of view diameter directly by placing
a transparent ruler under the objective.
You then need to focus the microscope to the lines on the ruler and measure the diameter
in mm.
So here we would have a diameter of 2.5mm, rounded to one significant digit.
When the power of magnification gets higher, the smaller the field of view diameter, and
using direct measurement is not suitable.
The field of view diameter can still be determined at higher power magnifications using an equation
that uses the measurement taken at the lower power magnification.
You times the diameter measured at the lower power by the magnification of the lower power
objective then divide this by the magnification of the higher power objective.
So if our measurement was 2.5mm for the low power objective lens 4x, and we were trying
to work out the field of view diameter with the objective lens 100x we would times 2.5
by 4, then divide by 100, which gives a field of view of 0.1mm, which we can convert to
a more useful measurement of micrometers, of 100um.
How then do you estimate the size of a specimen or other sample in the field of view?
If we look at this paramecium under high magnification of 100x.
The first thing you need to be able to do this is to find out the field of view diameter
as previously explained, which gave us a Field of View diameter of 100 micrometers.
You then need to estimate the fraction of this field of view diameter the sample takes
up, this paramecium takes up about around 1/10th of the Field of View diameter.
You then need to multiply the Field of View diameter by this estimated fraction to get
the estimated size of the specimen.
If the Field of View diameter was 100 micrometers, and the estimated length to be around 1/10th
so 0.1 of the Field of View diameter it would be 100 micrometers times 0.1 which gives an
estimated size of 10 micrometers.
Another way to estimate the size of a sample is to count how many times the object would
fit across the FOV and then divide the Field of View diameter by this count.
If the Field of View diameter was still 100 micrometers, and around 10 paramecium would
fit across the Field of View diameter then it would be 100 micrometers divided by 10
which gives us an estimated size of around 10 micrometers.
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