The diameter of the parabola is and deep.

Set the vertex of the parabola is at and it opens up.

The general form of the parabola is  , where is distance from vertex to focus.

Since the parabola is across and deep the points on the parabola is  and .

Substiute in .

.

Where is the distance from the vertex to focus.

Therefore, the bulb located at the focus is  from vertex.

The bulb should be placed   from vertex.