Hi, I was reading my astrophysics text book and came across solid angles. I'm not sure I fully understand, for example there was a problem in the book that went as follows. The attached "math.jpg" shows a light source (yellow) in the centre of an arc. The problem is 2D, but the arc is rotated about the x axis to form a 3D sphere. I have the flux (F photons/sec) crossing the red line (hight h cm, zero thickness). But how do I translate that into the flux crossing the entire area after it's been rotated to be 3D. I guess it would form a cone shape and I want the area of the face of that cone or something. So do I need to multiply the flux by the solid angle? In this case would that be 4*pi*r^2? "math2.jpg" would represent the same problem, but just showing the cone bit. So again I have the flux crossing line of length h and zero thickness. And I want the flux that would come out of the entire cone. I hope that makes sense. I would really appreciate any help please. Thank you.