Page 56 - Introduction to Colloid and Surface Chemistry

3 Optical properties

Optical and electron microscopy

The optical microscope- resolving power

Colloidal particles are often too small to permit direct microscopic
observation. The resolving power of an optical microscope (i.e. the
smallest distance by which two objects may be separated and yet
remain distinguishable from each other) is limited mainly by the
wavelength A of the light used for illumination. The limit of
resolution 8 is given by the expression

a = A / 2 n s i n a (3.1)

where a is the angular aperture (half the angle subtended at the
object by the objective lens), n is the refractive index of the medium
between the object and the objective lens, and n sin a is the
numerical aperture of the objective lens for a given immersion
medium.
The numerical aperture of an optical microscope is generally less
than unity. With oil-immersion objectives numerical apertures up to
about 1.5 are attainable, so that, for light of wavelength 600 nm, this
would permit a resolution limit of about 200 nm (0.2 /u,m). Since the
human eye can readily distinguish objects some 0.2 mm (200 ^tm)
apart, there is little advantage in using an optical microscope,
however well constructed, which magnifies more than about 1000
times. Further magnification increases the size but not the definition
of the image.
Owing to its large numerical aperture, the depth of focus of an
optical microscope is relatively small (c. 10 /u-m at x 100 magnification
and c. 1 /urn at x 1000 magnification). This is not always a