Video Transcript
When looking at a slice of onion
under the light microscope using a 40 times magnification lens, you can observe the
following. What are you looking at? (A) Bacteria that have infected the
onion, (B) onion cells, or (C) viruses that have infected the onion.
This question is asking us to
recognize what cell, or particle, is being seen down a microscope. So let’s review some key facts
about microscopes and the images they form.
In school, it is the common light
microscope that is used to observe specimens. The specimen to be examined is
mounted on a glass slide and placed on the stage. Light from a light source under the
stage passes through the specimen and into the first of two lenses, called the
objective lens. This forms a magnified image of the
specimen. The light from this first image
then passes through the eyepiece lens. This results in the final image,
which has been magnified further. The eyepiece lens usually magnifies
the image by 10 times. The objective lens has various
magnifications, usually from four times to 100 times. The total magnification is the
magnification of the objective lens multiplied by the magnification of the eyepiece
lens.
The question says this onion was
looked at with a total magnification of 40 times. This means that in the photograph
the onion tissue is being observed under the lowest power objective lens, four
times. We know this because the eyepiece
lens has a power of 10 times. And four multiplied by 10 gives us
the total magnification of 40 times. This is important because this will
help us work out what we are looking at.
Out of the three answer options,
the viruses are by far the smallest. They range from about 20 nanometers
to 200 nanometers in size. A nanometer is one one thousandth
of a micrometer, and a micrometer is one one thousandth of a millimeter. Therefore, there are one million
nanometers in a millimeter. This gives you an idea of just how
small viruses are. The smallest thing you can see with
a light microscope is about 500 to 1000 nanometers. This means viruses are so small
that they cannot be seen using a light microscope, only by using more powerful
microscopes, such as the electron microscope. We can therefore rule out option
(C).
Bacteria are on average about one
micrometer, so one one thousandth of a millimeter. Onion cells are approximately two
hundred micrometers long, which is 0.2 millimeters. If we look at the cells in the
photograph from the question, we can see that they have a nucleus, here stained
blue, and a cell wall, stained purple. Bacteria are prokaryotes, so they
do not have a nucleus. You would also not be able to see
the detail in cells as small as bacteria under 40 times magnification. We can therefore rule out answer
option (A).
This leaves us with the correct
answer, option (B). When looking at the onion tissue
under the light microscope using a 40 times magnification lens, you can observe
onion cells.
