Microscope Activities, 13: Chromatic Aberration

In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.

EXPERIMENT 13: Chromatic Aberration

Goal

To become familiar with the detection of chromatic aberration, and its correction.

Level

Basic

Materials Needed

Stage micrometer or Motic Calibration Slide

Procedure

Using your 40X objective, focus on the scale of a stage micrometer or the Motic Calibration Slide, move the slide so that the scale runs across the entire field of view, or at least, from center to edge. Now, look toward the extreme edge of the field of view, where you will see the scale or crossline of the Calibration Slide surrounded with a yellow-orange halo. If you were to focus on a microscope slide preparation of opaque metal dust, you would notice color fringes around the edges of the particles; not in the center of the field, but worse and worse the farther you look toward the edges of the field of view. In addition to yellow-orange fringes, you may see blue-to-blue-magenta on the other sides of the opaque particles. This color fringing will give a false impression with colorless transparent particles or structures, yielding the impression of color where there is none.

Discussion

The color fringing you noticed in this experiment is another one of the several aberrations inherent in lenses; this one is known as “chromatic aberration.” This aberration is common in single-lens magnifiers, and was a problem with early microscope objectives until a way was found to greatly reduce the problem through the use of combinations of lenses made from glasses with different refractive index and dispersion.

Such combinations of lenses composed of different glasses were introduced in the 1830s by Joseph Jackson Lister, the father of the famous surgeon. When two lenses of different composition and optical properties are cemented together to reduce chromatic aberration (and, incidentally, spherical aberration), the combination is referred to as an “achromat” (a- without; -chroma color), or “achromatic doublet,” etc.

The vast majority of microscope objectives manufactured today are made with achromats, but they are not engraved as such on the objective, because such correction is now assumed. Your Boreal/Motic objectives are classified as achromats; however, as you have seen in this experiment, there is still color fringing due to secondary color, i.e., not all of the different wavelengths that make up white light can be brought to a common focus with just a two lens combination.

Over the years, it has been found that if the mineral fluorite (calcium fluoride) is ground and polished into lenses, and used in combination with glass lenses, most of the secondary spectrum could be eliminated. Fluorite of optical quality is scarce, and, therefore, expensive. For color photomicrography, however, the additional expense is justified. Objectives made with fluorite lenses, in combination with glass, will be engraved as such on the objective; they will be labeled “Fluorite” or “Fl” or, if made with modern synthetic fluorite, something like “Neofluar” (new fluorite).

Even with incorporated fluorite, there will still be a bit of residual tertiary spectrum, i.e., a very small amount of relatively minor color fringing. More recently, highly specialized glasses have been made in small batches, that when combined with fluorite elements, virtually eliminates all color fringing across the entire field of view. Such objectives, now incorporating many lens elements, are designated apochromats, and such high correction is always indicated engraved on the objective with such labels as “Apo” or “Apochromat” or “Apochromatic.” These are very expensive objectives, but their cost is justified where no color fringing can be tolerated.

Finally, the most expensive objectives are those which are both flat field and apochromatically corrected. These are the “Planapo” objectives.

Thus, you have three major classes of correction—achromats, fluorites, and apos, any one of which may also be corrected for flatness of field, yielding a total of six choices: achromat, planachromat, fluorite, planfluorite, apochromat, and planapochromat.

A typical 100X achromat consists of six lenses—a front hemisphere, a negative meniscus and two doublets—and may cost $100-$200. Whereas, a 100X planapochromat could consist of up to 23 lenses, and cost $5000-$6000 or more. Both objectives yield 100X magnification, but one with color fringing and field curvature, and the other perfect, free of all aberrations.

Fluorites are intermediate in correction and cost, and extremely good value for the money; these would be the best general choice for most work.

Another thing you need to keep in mind is that the higher the correction, the higher, generally, is the numerical aperture, and therefore, the ultimate resolving power of the objective; i.e., the ability to discriminate the smallest spaces between adjacent microscopic structures. A typical 10X achromat, such as comes on your Boreal/Motic microscopes, has a numerical aperture of 0.25; a 10X fluorite objective will have a numerical aperture of 0.30; and a 10X apochromat will have a numerical aperture of 0.32.

With all three objectives on a nosepiece, all three will yield 100X magnification with a 10X eyepiece, but, as you rotate from achromat to fluorite to apochromat, the ability to discern the finest structures increases—assuming you adjust the aperture diaphragm to match the numerical aperture in use.

Task

Perform the directions in the Procedure section, and note the color fringing in the image of the scale at the edges of the field of view.