## Detailed description

x squared = 4ay, parabola, turning point at (0,0). Tangent at a point P(2ap,ap squared) drawn. The tangent intersect the y-axis at (0,−ap squared). Light ray parallel to the y-axis reflected to the focus S(0,a)

x squared = 4ay, parabola, turning point at (0,0). Tangent at a point P(2ap,ap squared) drawn. The tangent intersect the y-axis at (0,−ap squared). Light ray parallel to the y-axis reflected to the focus S(0,a)