Control sections stained with Congo red show only yellow and no green birefringence. This could be corrected by ensuring the sections are cut at approximately what thickness?

• A. 8-10 nanometers
• B. 2-3 micrometers
• C. 50-60 nanometers
• D. 1-2 millimeters

Answer: (A)

Observing apple-green birefringence with Congo red relies on having the tissue section of the right thickness so polarized light can interact with the aligned amyloid fibers. If the section is too thick, light scattering and multiple interfaces smear the interference colors and the green birefringence can be lost. If the section is too thin, there isn’t enough material to produce a detectable birefringent signal. For light microscopy, this means cutting sections in the micrometer range—typically a few micrometers (roughly in the 5–8 µm area, depending on protocol)—so the optical path length is just right to reveal the characteristic birefringence.

Given the options, the thickness that aligns with standard light-m microscopy practice is in the micrometer range. Very thin (nanometer-scale) or very thick (millimeter-scale) sections won’t produce the desired effect, while micrometer-thick sections are appropriate for Congo red staining and polarized light examination.