The solids are classified into two types:

a) Crystalline /true solids

b) Amorphous solids

To distinguish between crystalline & amorphous solids, isotropy & anisotropy plays a vital role.

Isotropy – The substances which have the same physical properties such as electrical conductivity,thermal conductivity,mechanical strength, refractive index etc in all the directions in the space are called isotropic substances & this property is called as isotropy.

Anisotropy – The substances which have physical properties such as electrical conductivity,thermal conductivity,mechanical strength, refractive index etc different  in different  directions in the space are called anisotropic substances & the property is called as anisotropy.

    So a isotropic solid will have same refractive index in all the directions,Hence, AMORPHOUS SOLIDS  are isotropic in nature.The reason for this is that in amorphous solids the arrangement of particles is random & disordered, therefore all directions are equivalent & properties are independent of direction .On the other hand,the particles in a crystalline solids are arranged & well ordered.Thus the arrangement of particles may be different in different directions.

 CLEAVAGE-A crystalline solid when cut with a sharped edged tool such as knife gives a clean cleavage but an amorphous solid undergoes an irregular cleavage i.e  When an amorphous solid is cut with a sharp edged tool, it cuts into two pieces with irregular surfaces.