Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
Resolving Power
The human eye can recognize two objects if they are 0.2mm apart at a normal viewing
distance of 25 cm. This ability to optically separate two objects is called resolving power. Any finer
detail than this can be resolved by the eye only if the object is enlarged. This enlargement can be
achieved by the use of optical instruments such as hand lenses, compound light microscopes and
electron microscopes.
Resolution in the light microscope
In the light microscope, the quality of the objective lens plays a major role in determining
the resolving power of the apparatus. The ability to make fine structural detail distinct is expressed
in terms of numerical aperture (NA). The numerical aperture can be expressed as n sinα where n
is the refractive index for the medium through which the light passes (n air =1.00; n water = 1.33; n
oil = 1.4), and α is the angle of one half of the angular aperture of the lens. Light microscope
objective and condenser lenses are usually designated by this NA value.
In a light microscope, a beam of light is directed through a thin object and a combination of
glass lenses provide an image, which can be viewed by our eyes through an eye piece. The image
formed is realistic, because it uses visible multicolor light. Visible light has wave like nature with a
wavelength (λ) of 400-800 nm. Since the resolution cannot be less than half the wavelength (λ),
the ultimate resolution attainable by using the light microscope is 200nm. This corresponds to a
magnification of 1000 times as compared to an eye. Any magnification higher than this will not
resolve more detail but will only give “empty magnification”.
( 1mm = 1000 µm; 1 µm = 1000nm; 1nm = 10 A 0 )
Changes in resolution with wavelength (light microscope)
Light source Green Blue Ultraviolet
Wavelength (nm) 546 436 365
Resolution (nm) 190 160 130
Resolution improves with shorter wavelengths of light
It can be seen from the above table that resolving power improves as the wavelength of the
illuminating light decreases. To explain this more fully, the resolving power of the optical system
can be expressed as
where
R is the distance between distinguishable points (in nm),
is the wavelength of the illumination source (in nm),
NA is the numerical aperture of the objective lens.
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