What is resolution?
It is the ability to distinguish closely spaced points as separate. Resolution can also be understood as the least distance between two closely opposed points, at which they may be recognized as two separate entities. The smallest distance we can see between points in a light microscope (LM) is about 200 nm [There are 1000000 nm (= nanometers) in 1 mm] whereas a typical scanning electron microscope (SEM) can distinguish gaps smaller than 10 nm.
Resolution is dependent on the wavelength of the beam we use to see the material, and this explains why the electron beam, with its much shorter (smaller) wavelengths is able to provide better detail than the light microscope. Visible light has a longer wavelength than an electron beam. In this image we see the waves in blue. As tip-to-tip wavelength gets smaller, the ability to resolve the spots gets better.
In more detail
The first breakthrough in the development of the electron microscope came when Louis de Broglie advanced his theory that the electron had a dual nature, with characteristics of a particle or a wave. De Broglie combined some of the principles of classical physics with quantum theory and developed an equation to calculate the very small wavelengths of these particles (electrons).
Table 1 shows the resolving power for the light microscope. Note that it improves as the wavelength of the illuminating light decreases. Abbe (1893) showed, using the Abbe equation (see below), that the smallest resolvable distance is about half the wavelength of the illumination used. Thus, this distance is the ultimate resolving power of any instrument. This is called the Abbe Criteria of Resolution.
Table 1. Resolution achieved with visible light
To explain this more fully, the resolving power of the optical (light) system can be expressed as Abbe's equation:
R = 0.61 λ/NA where:
R is the (minimal resolvable) distance between distinguishable points (in nm),
λ is the wavelength of the illumination source (in nm),
The optimal resolving power for a light microscope is obtained with ultraviolet illumination (λ = 365) if a system with the optimal NA is used (1.4). In this example:
R = 0.61x365/1.4
R = 159 nm
The wavelengths of electrons in an SEM are much smaller than the wavelength of ultraviolet light (see Table 2). For example UV light has a wavelength of 365nm where as an electron beam has a wavelength of around 0.005nm: an impressive difference.
Table 2. Wavelengths of electron beams generated at different accelerating voltages (compare with Table 1)
|Accelerating Voltage (kV)||Wavelength (nm) (symbol = λ)|