A parabolic reflector with diameter PR = 20 cm and OQ = 5 cm.

Vertex of the parabola is (0, 0).
Let focus of the parabola be (a, 0)
Now, PR = 20 cm ⇒ PQ = 10 cm
∴ Coordinate of point P are (5, 10)
Since the point lies on the parabola y2 = 4ax
∴ (10)2 = 4 × 5 ⇒ a =

100

20

⇒ a = 5
Thus required focus of the parabola is (5, 0) which is the mid-point of the given diameter.