Q. What do you understand by crystallographic notation of atomic planes/Explain with the help of examples.

Ans.Crystallographic Notation

1. Crystal Symmetry:                   2: Miller Indices

1. Crystal Symmetry: The symmetry of a crystal indicates that in a lattice if the parts of an ideal crystal are interchange, the various direction are geometrically equivalent. The unit atomic groups also have like properties just like the original crystal. Symmetry of a crystal form is determined by the position of similar faces, diagonals etc.

    (a)Symmetry plane: If we cut the crystal along the plane which divides it into two similar halves so that one half is the reflection of the other half, such a plane is called plane of symmetry.

     (b)Centre of symmetry: It is point in a crystal which is equidistant from all the faces of the crystal. If a line is a drawn from this, it shows a reflection through a point instead of a reflection in a plane.

A crystal may have one or more axis or plane of symmetry is only one.

Symmetry Axis: The axis of symmetry is the imaginary line-passing through the centre of the crystal, about which the crystal may be rotated so that, it presents an identical appearance more than once in the course of rotation.

2. Miller indicates                                                                                                                 

     Some characteristics of miller indicate are:

1. A plane which is parallel to any one of coordinate's axes has an intercept of infinity and thus, the miller index for that axis is zero.
2. Miller indices do not only define a particular plain but a set of parallel plane.
3. Only the ratio of indices is important.
4. if (h,k,l)are the miller indices of a plane, then the plane cuts the axes into a/h,b/h and c/l equal segment respectively.
5. All the parallel equidistant planes have the same miller indices, therefore the miller indices define a set of parallel planes.
6. A plane parallel to one of the coordinate axes has an intercept of infinity.