Name the electromagnetic waves used for the following and arrange them in increasing order of their penetrating power.
(a)    Water purification
(b)    Rewrite sensing
(c)    Treatment of cancer.

(a) Ultra violet waves are required for water purification.
(b) Micro waves are used for rewrite sensing.
(c) Gamma rays are used for the treatment of cancer.

In increasing order of penetration powers:
microwaves < ultra violet waves < gamma rays