Almost in all diagrams in mechanics, I noticed that a surface is represented by a line with a lot of slashes ("/") on one side, like the one shown below:
I've seen this in optics where such slashed lines are used to denote perfectly reflecting surfaces (mirrors). But what is its significance in mechanics? Before reading the chapter friction, I thought these denote frictionless surfaces, but these slashed lines are even used to show rough surfaces. Is there any difference between a plain line and a line with slashes on one side? Or is that because it looks nice?
I searched various books to find the valid reason for this, but I could not find any. If possible, kindly support your answer with a reference.