Kishore,

To find the focal point of a parabola, take any two rays, not on the axis of symmetry, but parallel to the axis of symmetry, and calculate thier angles of incidence as they strike the parabola. The angle of incidence is measured between the incident ray and the normal drawn to the parabola at the point of intersection. The reflected angle will always equal the incident angle. Project the reflected angle of the two rays across the axis of symmetry. Where they intersect is the focal point.

Chris