### Explaining a steradian

We measure angles on paper, in two dimensions, in either degrees or radians but when we need to deal with three dimensions, for example when something spreads out like a cone, then the unit used is a steradian. The video below illustrates and explains exactly what a steradian is.

The total surface area of a sphere = 4*π**r*^{2}

so by this definition the total solid angle subtended at the centre of a sphere must be:

4*π**r*^{2 }*/r*^{2}

the *r*^{2} cancel out, which equals 4*π *steradians that is 4*π** *sr