The equation for the cross-section of the paraboloid is given as `y^2=20x` . Since the light rays are to diverge, we need the light source to be placed between the focus and the vertex of the parabola.

The standard form for a parabola is `(y-k)^2=4p(x-h)` ; here h=k=0 so the parabola has vertex (0,0). p is the distance from the vertex to the focus (and to the directrix.)

For `y^2=20x` we have h=k=0 and p=5. The parabola opens to the right with vertex (0,0) and focus (5,0).

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The light source can be placed at a distance d, 0<d<5, from the vertex in order for the light rays to diverge.

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