Steradian

Steradian is a unit of solid angle in a three dimensional space. The steradian is defined as the solid angle subtended at center of a sphere by an area of its surface equal to square of the radius of that sphere. OR .

The steradian is defined as the solid angle that,  having its vertex in the center of a sphere, cuts off an area of the surface of sphere equal to that of a square with side of length equal to the radius of the sphere.

                                          The surface are of a closed sphere of radius "r" is 4πr².According to the definition                                                      Sr= 4πr²/r²

                                             Sr =4π

Any closed surface subtends a solid  angle of  4π at any of its interior point.

Prefixes of the Power of 10:

Sometimes, we have to deal with quantities that are either very large or very small compared with the basic standard units. For this purpose we use certain prefixes such as centi, mili, micro, kilo etc which are given in the table below.

Prefix	Symbol	Power	Prefix	Symbol	Power
Deca	d	10¹	Deci	d	10¯¹
Hector	h	10²	Centi	C	10¯²
Kilo	K	10³	Mili	m	10¯³
Mega	M	10⁶	Micro	µ	10¯⁶
Giga	G	10⁹	Nano	n	10¯⁹
Terra	T	10¹²	Pico	p	10¯¹²
Peta	P	10¹⁵	Femto	f	10¯¹⁵
Exa	E	10¹⁸	Alto	a	10¯¹⁸