Consider a square lattice as shown in Fig 18.3(a).
Figure 18.3 (a) A few planes in a lattice. 18.3 (b) The coordinate system.
In this case, an atom occupies each lattice point. The coordinate system used here is also shown in Fig 18.3 (b). Several lines can be drawn in this lattice such that at least one (or more) atoms lie on the lines. A few examples are shown. The unit cell for the Lattice in Fig 18.3 (a) is a square. Lines 1 and 2 are similar in that atoms lie on these at the same separations. Line 3 is less "densely packed", i.e., the distance between adjacent atoms is larger than that in lines 1 and 2. We need a system to label or characterize each of these lines. In three dimensions our interest would be the different planes of atoms.
