Resolution

Resolution

The human eye can recognize two objects if they are not closer than

0.1 mm at a normal viewing distance of 25 cm. This ability to

optically separate two objects is called resolving power.

Resolution in the light microscope

[IMAGE]In the light microscope, the quality of the objective lens

plays a major role in determining the resolving power of the

apparatus.

The resolving power of the optical system can be expressed as

where

* R is the distance between distinguishable points (in nm),

* [IMAGE]is the wavelength of the illumination source (in nm),

* NA is the numerical aperture of the objective lens (in best lenses

approx. 1.4)

Light source

Green

Blue

Ultraviolet

Wavelength (nm)

546

436

365

Resolution (nm)

190

160

130

Due to this limitation of resolving power by light microscopy, other

sources of illumination, with shorter wavelengths than visible light,

have been investigated.

Resolution in the electron microscope

The resolving power of the TEM is approximately 10,000 times greater

than that of the light microscope. The very short wavelength of

electrons and modern technology makes it easy to obtain a resolution

of 3 nm on a TEM. As electrons are accelerated through a potential

difference of V volts, they have a wavelength ( [IMAGE]) equal to

[IMAGE]

For V= 50,000 Volts (50keV) [IMAGE]=0.0055nm

The numerical aperture, however, of the electromagnetic lenses is

extremely narrow ( [IMAGE]= 1 degree).