Figure 11 shows Figures 9 and 1 from parts 1 and 2 with the 2-mm marker bars sized more or less equally. Figure 12 shows Figures 3 and 13 with both marker bars sized equally. Again, note the highly superior depth of field and surface detail visible in the SEM image compared to the light microscope. (These two images are of different screws.) At even 100x original magnification settings, the SEM depth of field and clarity leaves the optical microscope in the dust.
If you did not get your rulers out last time to measure the marker bars on some of the images and calculate your viewed magnification, I suggest you do so now on Figures 11 and 12. If you do it in millimeters, it’s easy to calculate. You simply take the actual length of the marker bar in millimeters as it appears on your screen and divide that by the number of millimeters the marker represents, either 2 mm for Figure 11 or 0.5 mm for Figure 12. Do it now!
Hopefully, you got a higher magnification number for Figure 12 than for Figure 11.
Now let’s look at something different. Figure 13 shows four pieces of iron-pyrite mineral that I have collected at various gift shops over the years. A small gold-colored paper clip and large silver paper clip are included for scale. As can be seen in Crystal A, the natural crystal shape is “cuboid,” or “almost cube-shaped.” If you go to the Wikipedia article on pyrite, you will see some crystals similar to my Crystal A, which are said to be from a mine in Spain. The cubic crystal shape is less obvious in Crystal B, and the individual crystals that make up the chunk of mineral are smaller than the larger of the two individual crystals of Crystal B. Crystals C and D have smaller crystals, and it is harder to find crystals that have recognizable shapes. The crystals in Crystal B may have eventually become more obviously “cuboid” had their growth not been interrupted. The triangle marked “4" is visible because of a “missing corner.” There is a similar larger triangle at the upper right of Crystal B. The series of fine ridges are letting us know that the layers of atoms that were adding themselves to the crystal surface were deeper in some places than others.
Note also in Figure 13 that the large facet of Crystal A is very dark compared to the small one protruding from the top. The “real color” of these two crystals is identical. Crystal B has a much greater variation in color as shown due to a greater variation in the angle of incident light. This image was obtained by laying the four pyrites on my scanner glass so that the incident light was coming from the same absolute direction for each “pixel” scanned. The relative angles of the facets sometimes reveal and sometimes hide details. See Feature E in Figure 13. There are two adjacent facets (orange arrows) that show very high contrast. We can guess that both are quite flat, as if there were protruding features the light angle would show it. But we can’t be sure that there are none of the small “v” features shown at F. There is nothing that beats having the object in your hand so that you can rotate it and look for appearance changes. A small feature that is invisible in one orientation can literally jump out in a slightly different orientation. That’s part of why it takes so long to get decent light-microscope images.
Crystals C and D will get us into a discussion in part 4 about interpreting images of metallographic cross sections, such as are used to measure grain size in steels or other metals and alloys. If you don’t know the magnification at which you are viewing, you really don’t know what you are looking at. If I could increase the magnification of Crystal D, I could probably make it look more like Crystal C. Likewise, I could possibly make Crystal C look more like some areas of Crystal B.
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