When sections are 8 nm thick, all nuclei will be in the same plane of focus.

• A. True
• B. False
• C. Only if stained
• D. Only with fluorescence

Answer: (B)

The idea being tested is how plane of focus and depth of field work in microscopy. At any one focusing position, only a thin zone of the specimen is sharply in focus—the plane of focus. The depth of field is the range above and below that plane that still looks acceptably sharp.

Even if a section is extremely thin, the material inside it is three-dimensional and the features within it are at slightly different depths along the z-axis. The nucleus itself has height and can be positioned at a small range of depths within the slice, and the section can be not perfectly flat or perfectly aligned. Because the depth of field is a finite thickness, not every nucleus will lie exactly in that single sharp plane at once; some will require a tiny adjustment of focus to be seen clearly.

So, you cannot guarantee that all nuclei will be in the same plane of focus just because the section is very thin. This is why the statement is false.