(a) Some of the glass objects recovered from ancient monuments look milky instead of being transparent. Why?

(b) Iron (II) oxide has a cubic structure and each side of the unit cell is 5$\stackrel{\circ }{A}$. If density of the oxide is 4 g cm-3, calculate the number of Fe2+ and O2- ions present in each unit cell.

[Atomic mass : Fe = 56 u, O = 16 u; Avogadro's number = 6.023$×$1023 mol-1]