Download Stage 2 Physics- Prathan Ingniwat Directed Investigation/ Research

Stage 2 PhysicsDirected Investigation/ Research Project
Prathan Ingniwat
Application, Ideas, and Concepts of Microscopes
Directed Investigation
The purpose of this investigation is to present the ideas and concepts revolving around
the use of microscopes, both light (or compound) and electron, and discuss, in some
detail, the finer points or specifics concerned in the application of these concepts. We will
briefly look at the normal light based microscope, its effectiveness and limitations, and
compare that with the electron microscopes, which are: the Transmission Electron
Microscope, The Scanning Electron Microscope, and The Reflection Electron
Microscope
The conventional light or compound microscope functions with the use of a minimum of
two converging lenses. These lenses are used to ‘bend’ light in such a way as to create
intermediate and virtual images, which, when passed into the eye, are turned into real
images in the back of the eye. The two lenses are commonly known as the objective lens,
which is closest to the object, and the ocular lens, which is closer to the eye. There is also
a third lens usually known as the condenser, which is uses to provide even illumination
from a lamp or light source positioned at the base of the microscope. The focal length of
the Objective lens and its distance are calibrated so that the lens is able to form a real
image within the barrel of the microscope, which is what we call the intermediate image.
A diagram from the Applications for senior secondary physics book describes this affect
with the use of rays.
The first known ray starts initially
parallel to the optic axis, and as
it travels through the objective lens,
is turned ‘inwards’ and passes through
point F. The second ray, passing through
the centre of the lens where the surfaces
are parallel, is not deviated and continues
on its initial path. The point where
these to ‘rays’ intersect or join is
where the intermediate image is created.
The second lens is basically just
a magnifier, however instead of
directly magnifying the original
object, the lens magnifies the
intermediate image and creates
what can be described as an enlarged,
inverted ‘imaginary image’ which is
then projected into the eye which in turn,
forms a real image.
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Stage 2 PhysicsDirected Investigation/ Research Project
Prathan Ingniwat
One thing that all microscopes have in common is that they all
have a maximum useful magnification, this is due to diffraction.
A microscope is no longer able to distinguish between to fine
points where light from each point diffracts upon passing
through the lens. These diffractions can be seen as multiple
discs of varying intensity, and any attempt to magnify further
only produces more enlarged, fuzzy, and confusing images.
Since diffraction of a wave depends on the waves’ wavelength,
smaller wavelengths mean less diffraction, thus smaller
wavelengths can provide improved magnification. With normal
light microscopes, the typical wavelength is about 500 nm, and
the minimum distance distinguishable is approximately 2x10-7
metres or 200 nm, which gives a maximum useful
magnification by 1000 times. One suggested substitute by the
Applications for senior secondary physics is Ultra-Violet Light,
which increases useful magnification to about 3000 times,
however, to do so, the lenses must be adjusted to the right
wavelength, and the lens material must be transparent to ultraviolet light, and photographic recordings of the image must be
taken. Because of the effort and hassles of a ultra-violet
microscope, they and other ultra-violet techniques are used to
study objects that fluoresce. Also, since lens have minimal
refractive effects against X-rays, X-rays are considered
ineffective for conventional microscopy, thus, we venture into
the vast, tiny world of electrons.
Once the wavelike nature of electrons was discovered,
scientists realised that they could apply electrons in the use of
microscopes because electrons have a very small wavelength
and can be focused with electric of magnetic fields.
An old transmission
electron microscope
The first, original form of electron microscopy invented (first
prototype being produced in 1931) was the Transmission
Electron Microscope. The TEM works by feeding a high
voltage into a cathode, which is then formed by ‘magnetic
lenses’, or in some case electric lenses, which are more like
tubes or hollow cylinders. These lenses diffract the beam or ray
through the use of magnetic or electric fields, much like the
glass lens would diffract light in the normal compound
microscope. Because the electron beam must pass through the
object, the object must be very thin so that it is semi-transparent
for electrons, however because of this; the resulting image
comes up much the same way as the compound microscope
(flat, magnified image).
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Stage 2 PhysicsDirected Investigation/ Research Project
Prathan Ingniwat
The TEM is able to distinguish points roughly 0.1nm apart, and can produce a useful
magnification of over 1 million times. Whereas the compound microscope produces an
image in the back of the observer’s eye, the TEM produces an image on a fluorescent
screen, which can be recorded photographically or digitally, displayed on a monitor via
video camera, or viewed by a low magnification binocular. The electron wavelength is
approximately 100 000 times smaller than that of light given above, being around
0.005nm.
Bibliography – Sources of Material
Lawrence, N and Olesnicky, A. (1994) Physics Essentials Adelaide Greg Eather and
Associates, Publication Division in conjunction with Adelaide Tuition Centre (pg113)
Duncan, T. (1983) Physics For Today and Tomorrow Second Edition London, John
Murray Ltd (pg10)
Storen, A and Martine, R. (1987) Physics for Senior Students Melbourne Australia,
Thomas Nelson.
Richard, P (1998) Applications for senior secondary physics Adelaide, South Australia,
Hyde Park Press
Wikipedia (unknown date) ‘Electron Microscope’, in Wikipedia
<http://en.wikipedia.org/wiki/Electron_Microscope>
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