Features evident at the interface between the particle and the adjacent medium.

The relative refractive index referes to the difference in the real part of the refractive index, "n" of the particle minus "n" of the medium (npart - nmedium). The greater the difference, either positive or negative, the darker the bondary of the particle. This is presented in more detail under "Relief".

Relative Absorption (kpart - kmedium): The refractive index is a complex number having both a real and an imaginary parts. The imaginary part is the absorption coefficient and is designated as "k". The higher the imaginary part of the refractive index the darker the particle.

Relief, simply speaking, is the contrast at the edge of a particle as a result of differences in the refractive index between the particle and the mounting medium. John Delly provides a more detailed definition in his book, ESSENTIALS OF POLARIZED LIGHT MICROSCOPY AND ANCILLARY TECHNIQUES. His definition is "Contrast between a specimen and its surroundings (typically mounting media) due to the difference between their refractive indices. The greater the numerical difference in refractive indices, the stronger the relief. Expressed as positive or negative; high, medium, or low."

The first image shows a glass fiber from an office ceiling acoustic tile. The refractive index of the glass fiber is about 1.520. The relief is moderate. The sign cannot be determined, positive or negative, from this image. If it were negative, refractive index lower than the mounting medium, then the fiber would become dark when the stage is lowered (bright Becke line out). If it were positive, refractive index higher than the mounting medium, then the fiber would become bright when the stage is lowered (bright Becke line in).

This image shows a glass fiber (lower left) and a silica phytolith (upper right) with oblique illumination. The light is coming in from the right. The silica phytolith has a lower refractive index, relief is negative, and so is bright on the right. The glass fiber has a higher refractive index, relief is positive, and is dark on the right side. With oblique illumination the relative refractive index can be determined without having to defocus the particle.

Calcite has an omega refractive index of 1.658 or higher (in this case 1.669) and an epsilon prime refractive index in this orientation of about 1.550. The first image shows the relief created when a linear polarizer is oriented to show epsilon prime. The refractive index difference is 0.114. The result is high relief. The second image is with the crystal rotated 90 degrees, showing the omega refractive index. The difference is 0.005 refractive index units. The result is low relief. In this position, lowering the stage results in a orange Becke' line moving into the particle and an blue Becke' line moving out, as shown in the third image. If Oblique illumination is used the dispersion staining colors are evident on oposite sides of the crystal.

The optical glasses have a refractive index of 1.64, 1.66, and 1.67. The first image shows the calcite crystal in high relief and the 1.67 refractive index glass in low relief. The 1.66 refractive index glass is invisible. With oblique illumination, the second image, the optical glasses show dispersion staining colors.

The first image shows the olivine (upper left) and quartz (lower center) in slightly off crossed polarized light.
The second image is taken with oblique illumination and a single linear polarizing filter oriented parallel to the high refractive index of the olivine particle. The quartz is showing high negative relief. It is bright on the side of the light source and dark on the other side. That indicates that the refractive indices of the quartz (1.544 and 1.553) are well below 1.664. The high refractive index of the olivine particle is about 1.668, based on the dispersion colors shown.
The third image shows the particles with the linear polarizer rotated 90 degrees. The relief of the quartz particle has not changed. The olivine particle is now showing its low refractive index of about 1.656. It is now showing low negative relief because its refractive index in this position is slightly lower than the 1.664 of the mounting medium.

Dispersion Staining Color Charts are approximations of the colors shown by a particles that matches the refractive index of the mounting medium at a specific wavelegnth in the visible part of the electromagnetic spectrum. The colors vary slightly in the real world as a result of the size and shape of the particle, the type of dispertion staining used, and the physical configuration and design of the microscope.

This section shows Cargille Standard Optical Glasses in Standard Cargille High Dispersion Refractive Index Liquids. The intent is to demonstrate the effects of size and of different microscope configurations on the colors.

If the refractive index of a transparent particle is much different than the medium in contact with it, then the polarized beam can be changed at the interface as a result of reflection. If the interface is aligned with the polarizer or analyzer then the beam is not affected. In other orientations reflection at the interface results in rotation of the polarized beam and the interface appears to show a first order white interference color.

Polarized light is depolarized at the interface between a conductive particle and a non-conductive mounting medium. This light halo effect with transmitted crossed polarized light indicates an opaque particle is a wear metal particle or at least is conductive. Graphite is sufficiently conductive to produce this effect. Pencil debris can be distinguished from combustion residue by this effect.

Fine structure can act as a diffraction grating.

Reflection at an interface is a function of the difference in the real part of the refractive index between the particle and the medium.

There are literally hundreds of ways to enhance contrast at the edge of a particle. Simply using crossed polarized light for birefringent partices is one example. Phase contrast and any of the other hundreds of interference methods are examples. Rhineberg filters and darkfield illumination are two more examples. Selecting a contrasting refractive index mounting medium is an important consideration in desiding whether the surface features or interial structuctures of the particles are of interest. A few of those methods are shown here.

If the mounting medium is near the refractive index of the particle then any internal materials with a different refractive index or bondaries become more easily visible. Surface features become less apparent. if a mounting medium with a very different refractive index is selected, then surface features become easy to visuallize.

Reducing the angle of the cone of light impinging on the particle increases contrast but decreases resolution.

Oblique illumination increases resolution normal the the axis of the illuminating beam but decreases the resolution for structures parallel to the projection of the axis on the stage. It also distorts the image to some extent, exagerating the shape of the particle perpedicular to the axis of the illuminating beam.

Darkfield illumination makes all particles, even opaque particles, bright on a dark field of view. This is an advantage for automated particle counts and particle obscuration measurements. Resolution is increased.

Rheinberg filters can create color contrast and can be used to balance brightness for the objects in the field of view relative to the background.

Modifying the shape or the angle of the illuminating beam relative to the optic axis of the microscope can create a number of different effects.

Anisotropic particles become easily visible when crossed linear polarized light is used, dependent on the orientation of the particle. If circular polarized light is used then the contrast is no longer dependent on orientation.

Phase contrast is best used with particles that have low contrast when viewed with brightfield illumination. Phase contrast can be positive or negative and have a variety of other characteristics that may better suit a specific application.

Nomarski, Jamin-Lebedeff, Mach-Zehnder, Hoffman, and many other systems that alter the diffraction beam path to bring it into interference with the image beam path create contrast. Many of these systems can be used quanitatively to measure a few nanometers difference in pathlengths.