What is Steradian? Waht is ratio of plane angle to solid angle?

Steradian is the solid angle (3 dimensional) subtended at the center of a sphere by an area of its surface equal to the square of a radius of a sphere

• 3D
• Solid Angle
• Ratio of plane angle to solid angle= 1 : 2    :-  plane angle / solid angle = 2π / 4π = 1 : 2