What is the difference between light microscopy and electron microscopy?
A typical optical microscope uses visible light to illuminate a sample and a series of glass lenses to magnify the image of the sample. Since you are using light, you can place the specimen under the microscope in ambient air, or for some applications, in a small amount of water or oil. For compound light microscopy, we usually need the specimen to be thin because we want light to pass through it so that we can see internal details. This usually means cutting sections of the sample, but depending on the sample, the thickness of the sections may be about 1 to 20 microns. With stereo or dissecting light microscopy, there is no such requirement because you are usually just looking at the surface of the sample. Observe the magnified image in an optical microscope through the eyepieces,
Electron microscopes use a carefully controlled beam of electrons as a form of illumination. The beam is controlled and focused by a series of electromagnetic lenses, which are essentially powerful electromagnetic coils with a central hole through which the electrons pass. The lens controls the light beam hitting the sample and also magnifies the image of the sample. Since you are working with an electron beam, the entire electron optical system needs to be in a high vacuum, which means the sample must be suitable for the vacuum environment. In a transmission electron microscope (TEM), electrons must pass through the sample, so the sample must be very thin, less than 0.1 microns. Magnified images are viewed on a fluorescent screen but can be recorded with a CCD camera mounted below or above the screen.
Scanning electron microscopy (SEM) is very similar to an optical dissecting microscope in a way, in that you are looking very carefully at the surface of the specimen, so it doesn't have to be thin. In SEM, the sample is scanned with a finely focused electron beam, so the sample must be able to withstand high vacuum and must be reasonably conductive. (This is because you are dumping a stream of electrons into the sample, and the current must be conducted away.) SEM samples are often coated with a very thin coating of carbon or metal (such as gold or chromium) to make them conductive.
The comments above describe the differences in physical instrumentation, and I didn't even mention that electron microscopes are larger and more complex than light microscopes. But the main difference between light and electron microscopy is resolution – the ability to resolve very small details. Resolution is ultimately limited by the wavelength of light in optical microscopy and the effective wavelength of the electron beam in electron microscopy. Since the wavelength of visible light is approximately in the range of 400-700 nanometers, the optimal resolution of optical microscopy is approximately 200 nanometers (0.2 micrometers). For a TEM operating at 200 kilovolts, the wavelength of the electron beam is 0.0025 nanometers, the actual resolution of such an instrument is about 0.2 nanometers, or a thousand times better than an optical microscope. Advanced TEMs may have resolutions close to 0.1 nanometers, and Many TEMs can image atoms in regular structures.
Since magnification is simply the ratio of how an object appears to the eye or screen compared to its actual size, this means that a very good optical microscope has a maximum magnification of 1000-2000x and the maximum available magnification of a high quality TEM is 1-2 million times. For SEM, there are many other factors that affect resolution, and the maximum available magnification is probably around 300,000x.
As you can see, there are indeed many differences between light and electron microscopy, with resolution issues being the main one. For practical applications, the choice of which type of instrument to use will ultimately depend on the resolution and magnification required and the ease of sample preparation.
