Why Line defects are 1 D and grain boundary defects are considered 2-D defects? In line defects, a plane is missing even then it is considered as 1-D? Why?

See that’s not the perspective we see in real life, I mean practically and we interpret microscopically.

Yeah it’s true that in line defects also like dislocations the plane of atoms are missing but when we see them in 3-D with respect to the large layers of atoms considering them as full spheres the breadth and height seems negligible w.r.t to line. In simple words as the atoms have some size and arrangements and when a plane of atoms is missing such as the length is very large w.r.t. to the height or breadth we can visualise this as a line instead in our 2-D visualisation of working on a paper.

Similar condition is with grain boundary , it’s totally true that the grains when form whole over the material and the grains get eventually separated by them. But when we look over macroscopically we see it as large sheet of atomic defect with negligible height and so with appreciable length and breadth  we can consider this as 2-D defect.

It’s all depends on relative observation as we see in every field of science. The atoms are not as we draw or say simply , they are arranged in a more complicated pattern. But to visualise and interpret it properly to do our work we used to make this as simple as we can. Hence, the Dislocations which rather have a large length is said as the line defect or grain boundary appearing as large sheet is said as 2-D.

I hope I cleared some of your doubts, if not, you can ask me further.