Sharpness – Let the light shine
Sharpness can be defined simply as two surfaces coming together to form a perfect angle. An arris with no interruption.
So the next time you want to know if something is reasonably sharp, take a good hard look at it. Take a good look at it with a light source on the opposite side of the edge in question. When the edge is “sharp”, there is no facet that will reflect light. In other words, the edge will appear as a straight black line.
Now, take a look at an edge that I “rolled” over with a burnisher. It’s a bit exaggerated, but it will give you a very idea of what I’m talking about.
You might see a chisel or plane iron that has been polished to the enth degree, that just won’t cut worth squat. Hold it up to the light. You’ll very likely find that there isn’t a true arris. Consequently, the blade is not sharp.