Compare and contrast the use of light and electron microscopes in biological studies. Microscopes are laboratory equipment which are used to observe any matter that is too small to be seen by the naked eye. There are several types of microscopes – the two most common being the optical microscope, also known as a light microscope, and the electron microscope, which can be either a transmission electron microscope (TEM) or a scanning electron microscope (SEM). There are more differences than similarities between light microscopes and electron microscopes.
Firstly, optical microscopes and electron microscopes both work in different ways. As the light source or condenser of an optical microscope focuses light on a tiny bright spot of the specimen, the microscope accumulates light from the well-illuminated specimen in the stage of the microscope. The small objective lens then brings the image of the object into focus within the microscope’s tube, and is then magnified by a second lens, called an ocular lens or the eyepiece. The specimen in an optical microscope can be rather thick.
In contrast, unlike the light microscope, the electron microscope visualises objects using a thin beam of rapidly moving electrons that interfere with the specimen placed in the tube. In a TEM, the electrons are emitted by the electron gun in the cathode ray tube, and then accelerated by the anode. They then pass through a small aperture, which forms them into a beam, and into the vacuum inside the tube. The electromagnetic lenses around the tube of the electron microscope maintain the electron beam and focus it within the centre of the tube with an electromagnetic field.
When the beam reaches the specimen, the electrons are either absorbed, scattered, or pass through it. Since different parts of the specimen would be variously transparent to electrons, different amounts of electrons with changed energy pass through these areas. The electrons are collected on fluorescent or photographic film once they reach the end of the tube, which gives rise to a ‘shadow image’ of the specimen. However, in a TEM the specimen has to be very thin to allow the electrons to travel through it. This is a disadvantage since it can be tedious to get a hin part of the sample, and some specimens can’t be made thin enough for it. Alternatively, to observe a three-dimensional surface of the specimen, rather than a projection through it, a SEM can be used. In an SEM, the electron beam is much more sharply focused, and the electrons which are scattered off from the specimen are collected and counted by an electronic device. Each scanned point on the sample corresponds to a pixel on a television monitor; the more electrons the counting device detects, the brighter the pixel on the monitor is.
As the electron beam scans over the entire sample, a complete image is displayed on the monitor. An SEM scans the surface of the sample bit by bit while a TEM which looks at a sample all at once. A disadvantage of electron microscopes is that the images can only be produced in greyscale, whereas in an optical microscope the natural colour of the sample is maintained, since light is available for the colour to be present. Furthermore, another disadvantage is that only images of non-living specimens can be produced in electron microscopes because there is no air present.
It has to be in vacuum because if there are air molecules there are collisions between the air molecules and the electrons in the beam. Therefore the beam would decline from its direction. However, an optical microscope does not have this issue and can produce images of living specimens. In addition, there are more advantages of using a light microscope over an electron microscope. The electron beam of electron microscopes can be expensive to produce, as well as the microscope itself, whereas optical microscopes are cheap to purchase and operate.
Optical microscopes are also small and portable, whereas electron microscopes are large and require a special room to operate. On the other hand, there are advantages of using electron microscopes over light microscopes as well, such as magnification and resolution. The magnification of the specimen is how much it has been enlarged in the image, compared to the actual size of it. It is measured by multiples, for example 50x, indicating that the sample is enlarged to fifty times as big as it was. Resolution is the finest detail that can be distinguished in an image.
The resolving power of a microscope is quite different from its magnification. You can enlarge a photograph indefinitely using more powerful lenses, but the image will blur together and be unreadable. Therefore, increasing the magnification will not improve resolution. If two cellular objects are close together, a microscope with low resolution may show them as a single object, whereas one with high resolution will clearly show them to be separate. The wavelength of the light in optical microscopes limited the resolution to approximately 0. micrometres. Since the electron beam in electron microscopes has a much smaller wavelength, the resolution is a lot higher. Because electron microscopes have a much higher resolution than optical microscopes, they are also capable of having a higher magnification as well. Optical microscopes can magnify objects up to 2000 times only, and are used to view samples such as red blood cells or a human hair, whereas electron microscopes can magnify up to 500,000 times, and can be used to view individual atoms.
Overall, the light microscope would be the better choice for most people since they are inexpensive to purchase and to use, they are small and can be carried around, they maintain the original colour of the sample, and have a high enough resolution and magnification to view cells. However, for a more professional, expensive purpose, the electron microscope would be best since it takes much more detailed images of the specimens at a higher magnification. The SEM would be best to use for three-dimensional images of specimen surfaces and the TEM would be best for an overall observation through the specimen.
