In most cases, we are interested in the relative refractive indexm, i.e.,
the ratio of the refractive indices of the materials on either side of a phase
boundary. The higher the value ofjm 1 j, the stronger the refraction of a
light beam at that boundary. Whether we can see a boundary depends on
the contrast with the environment, which follows from the fraction of light
reflected.
Reflection. For perpendicular incidence of light on a plane surface,
the fraction reflected is given byðm 1 Þ^2 =ðmþ 1 Þ^2 ; form¼1.5 this fraction
equals 0.04, form¼1.1 it is 0.002, which is very little. At an oil–air
boundarymequals about 1.45. For foods containing no air cells and no
crystals,mis smaller than 1.1 for most structural elements, and it is often
much closer to unity.
For oblique incidence, the reflection is stronger, and moreover
refraction occurs, further enhancing contrast. Consequently, oil droplets
in water (m¼1.09) can readily be observed in a simple light microscope.
Figure 9.5 gives the minimum size needed for particles to be visible with
various microscopic techniques. However, measures to enhance contrast are
often needed ifmis close to unity.
Scattering. If the particles are small, which means of the order of
the wavelength of light (about 0.5mm) or smaller, the scattering of light can
no longer be separated into reflection, refraction, and diffraction. If the size
FIGURE9.7 Illustration of reflection, refraction, and diffraction of light that is
incident upon a particle of transparent material (1) of a refractive index that is higher
than that of the surrounding medium (2). I is incident light, R, reflected light, B
broken or refracted light, and D diffracted light. Part of the light passing near the
edge of a particle shows diffraction, and the angle by which it is diffracted is
appreciable only if the particle is not very large compared to the wavelength.