To the right of line PQ is a uniform magnetic field B. B1A is the line of incidence of a charged particle, which comes out of the field along DE ; CA and DF are the normals at A and D. B1AC = theta . Angle measured from CA in clockwise direction is taken as positive . what will be the value of theta so that angle subtented by the part of the circle ( along which charged particles moves in the field ) at its centre and facing the circle is less than pie?