We
can imagine two equal and small elements of area placed at points R and
S normal to the field lines there The number of field lines in our picture
cutting the area elements is proportional to the magnitude of field at
these points The picture shows that the field at R is stronger than at S To understand the dependence of the field lines on the area, or rather
the solid angle subtended by an area element, let us try to relate the
area with the solid angle, a generalisation of angle to three dimensions