Difference between basic units and derived unit

Basic units=those units of measurement which do not depend upon any other units are called basic units. Example length, mass, time, current etc.

Derived units=those units which are expressed in terms of fundamental units are called derived units. Example area, volume, density etc.

blue light is scattered in different directions by the tiny molecules of air. blue is scattered more than other colours because it has a shorter distance to travel which results in smaller waves. this is why we see a blue sky most of the time.