Phase-contrast microscopes are very useful for observing unstained specimens. This may include living and moving protists, such as amoebae. The microscopes have special phase-contrast objectives and a condenser that make different cellular components, which each have slight differences in their refractive indices, visible. As light passes through a specimen, some of this light is refracted, due to slight differences in density and thickness of the various parts of the specimen’s cells; thats, the refractive index of the specimen is different from that of the surrounding medium. The special optics of the microscope convert the differences between the transmitted light and refracted rays to a significant variation in light intensity, giving a clear image that appears dark against a light background.
Electron microscopes use a beam of fast-travelling electrons, which has a wavelength of about 0.004nm. As this is a much shorter wavelength than that of visible light these microscopes have a much higher resolution that optical microscopes (so organelles can be seen). Electron microscopes produce very clear, highly magnified images. In an electron microscope, electrons are fired from a cathode and focused by magnets, rather than glass lenses, onto a screen or photographic plate. There are two types; transmission electron microscope and scanning electron microscopes. Transmission electron microscopes were developed in the 1930s. The specimen, which can be e very thin section of a larger object, has to be chemically fixed by being dehydrated and stained with metal salts. The beam of electrons passes through the stained specimen. Some electrons pass through and are focussed on the screen or photographic plate. The electrons form a two dimensional grey-scale image. When photographed, this is called an electron micrograph. Transmission electron microscopes can currently produce a magnification of up to x2,000,000. A new generation of microscopes is being developed and those microscopes can magnify a specimen up to x50,000,000. Scanning electron microscopes were developed in the 1960s. Electrons do not pass through the specimen, which is whole, but cause secondary electrons to bounce off the specimen’s surface and be focusses on a screen. This results in a 3D image, with a magnification from x15 up to x200,000. The image is black and white, but computer software programmes can add false colour. Both types of electron microscopes are large and very expensive and need a great deal of skill and training to use.