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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. 1/3 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). 2/3 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> 3/3