Electron vs. Light Microscopes: Basic Differences
There are not many things that these two microscope types have in common. Both electron and light microscopes are technical devices which are used for visualizing structures that are too small to see with the unaided eye, and both types have relevant areas of applications in biology and the materials sciences. And this is pretty much it. The method of visualizing the structures is very different. Electron Microscopes use electrons and not photons (light rays) for visualization. The first electron microscope was constructed in 1931, compared to optical microscopes they are a very recent invention.
Electron microscopes have certain advantages over optical microscopes:
- The biggest advantage is that they have a higher resolution and are therefore also able of a higher magnification (up to 2 million times). Light microscopes can show a useful magnification only up to 1000-2000 times. This is a physical limit imposed by the wavelength of the light. Electron microscopes therefore allow for the visualization of structures that would normally be not visible by optical microscopy.
- Depending on the type of electron microscope, it is possible to view the three dimensional external shape of an object (Scanning Electron Microscope, SEM).
- In scanning electron microscopy (SEM), due to the nature of electrons, electron microscopes have a greater depth of field compared to light microscopes. The higher resolution may also give the human eye the subjective impression of a higher depth of field.
Electron microscopes have a range of disadvantages as well:
- They are extremely expensive.
- Sample preparation is often much more elaborate. It is often necessary to coat the specimen with a very thin layer of metal (such as gold). The metal is able to reflect the electrons.
- The sample must be completely dry. This makes it impossible to observe living specimens.
- It is not possible to observe moving specimens (they are dead).
- It is not possible to observe color. Electrons do not possess a color. The image is only black/white. Sometimes the image is colored artificially to give a better visual impression.
- They require more training and experience in identifying artifacts that may have been introduced during the sample preparation process.
- The energy of the electron beam is very high. The sample is therefore exposed to high radiation, and therefore not able to live.
- The space requirements are high. They may need a whole room.
- Maintenance costs are high.
When should one use optical (light) microscopes?
One big advantage of light microscopes is the ability to observe living cells. It is possible to observe a wide range of biological activity, such as the uptake of food, cell division and movement. Additionally, it is possible to use in-vivo staining techniques to observe the uptake of colored pigments by the cells. These processes can not be observed in real time using electron microscopes, as the specimen has to be fixed, and completely dehydrated (and is therefore dead). The low cost of optical microscopes makes them useful in a wide range of different areas, such as education, the medical sector or for hobbyists. Generally, optical and electron microscopes have different areas of application and they complement each other.
Different types of electron microscopes
There are two different types of electron microscopes, scanning electron microscopes (SEM) and transmission electron microscopes (TEM). In the TEM method, an electron beam is passed through an extremely thin section of the specimen. You will get a two-dimensional cross-section of the specimen. SEMs, in contrast, visualize the surface structure of the specimen, providing a 3-D impression. The image above was produced by a SEM.
Different types of light microscopes
The two most common types of microscopes are compound microscopes and stereo microscopes (dissecting microscopes). Stereo microscopes are frequently used to observe larger, opaque specimens. They generally do not magnify as much as compound microscopes (around 40x-70x maximum) but give a truly stereoscopic view. This is because the image delivered to each eye is slightly different. Stereo microscopes do not necessarily require elaborate sample preparation.
Compound microscopes magnify up to about 1000x. The specimen has to be sufficiently thin and bright for the microscope light to pass through. The specimen is mounted on a glass slide. Compound microscopes are not capable of producing a 3D (stereoscopic) view, even if they possess two eye pieces. This is because each one of the eyes receives the same image from the objective. The light beam is simply split in two.
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Electron microscopes illuminate their specimen using a beam of electrons. Magnetic fields are used to bend beams of electrons, in much the same way as optical lenses are used to bend beams of light in light microscopes. Two types of electron microscopes are widely in use: transmission electron microscope (TEM) and scanning electron microscope (SEM). In transmission electron microscopes, the electron beam passes throughthe specimen. An objective “lens” (which is really a magnet) is used to first produce an image and using a projection “lens” a magnified image can be produced on a fluorescent screen. In scanning electron microscopes, a beam of electrons is fired at the specimen, which causes secondary electrons to be released from the surface of the specimen. Using an anode, these surface electrons can be collected and the surface could be “mapped”.
Typically, the resolution of SEM images are not as high as those from TEM. However, since electrons are not required to pass through the sample in SEM, they can be used to investigate thicker specimen. Furthermore, images produced by SEM reveal more depth details of the surface.
The resolution of an image describes the ability to distinguish between two different points in an image. An image with a higher resolution is sharper and more detailed. Since light waves undergo diffraction, the ability to distinguish between two points on an object is intimately related to the wavelength of light used to view the object. This is explained in the Rayleigh criterion. A wave also cannot reveal details with a spatial separation smaller than its wavelength. This means that the smaller the wavelength used to view an object, the sharper is the image.
Electron microscopes make use of the wave nature of electrons. The deBroglie wavelength (i.e. the wavelength associated with an electron) for electrons accelerated to typical voltages used in TEMs is about 0.01 nm whereas visible light has wavelengths between 400-700 nm. Clearly, then, electron beams are able to reveal much more detail than beams of visible light. In reality, the resolutions of TEMs tend to be of the order of 0.1 nm rather than 0.01 nm due to effects of the magnetic field, but the resolution is still about a 100 times better than the resolution of a light microscope. Resolutions of SEMs are a little lower, of the order of 10 nm.
Difference Between Light Microscope and Electron Microscope
Source of Illumination
Light microscope uses beams of visible light (wavelength 400-700 nm) to illuminate the specimen.
Electron microscope uses electron beams (wavelength ~0.01 nm) to illuminate the specimen.
Magnifying Technique
Light microscope uses optical lenses to bend rays of light and magnify images.
Electron microscope uses magnets to bend rays of electrons and magnify images.
Resolution
Light microscope has lower resolutions compared to electron microscopes, about 200 nm.
Electron microscope can have resolutions of the order 0.1 nm.
Magnification
Light microscopes could have magnifications of around ~×1000.
Electron microscopes can have magnifications of up to~×500000 (SEM).
Operation
Light microscope does not necessarily need a source of electricity to operate.
Electron microscope requires electricity to accelerate electrons. It also requires the samples to be placed in vacuums (otherwise electrons may scatter off air molecules), unlike light microscopes.
Price
Light microscope is much cheaper compared to electron microscopes.
Electron microscope is comparatively more expensive.
Size
Light microscope is small and could be used on a desktop.
Electron microscope is quite large, and could be as tall as a person.
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