You don't anything wrong with the light. It only needs something more to 
interact with. It can be a media or some surface.

You may do as Jim said and use some media, but there are other way to 
make the light visible:

You can place a white plane just behind the prism, placing so that the 
back side of the prism intersect it. That way, you'll be able to see the 
path of the light inside the prism.

I added this:
plane{-z,-7.9 pigment{rgb 1}finish{ambient 0 diffuse 1}}

This plane acts like a projector's screen.


I changed your light to a light and a slit like this:
union{
	light_source {
	    <-50, 0, -3>
	    color rgb 100 // Much brighter
	    translate <-5, 5, -5>
	    parallel // instead of a spotlight
	    point_at <0,0,0>
	    photons { // Optional
	       refraction on
	       reflection on
	    } // OK even if not needed.
	
	}
// those 2 boxes make a nice slit
	box{<-40,  0.1, -1000><-41,  1000, 10>}
	box{<-40, -0.1, -1000><-41, -1000, 10>}
	rotate 49*z
	translate -6.3*x
	}

I reduced the photon's spacing down to 0.0045.
I increased the ior to get closer to the image you refered.
It needs something like 1.88.
In the original image, the dispersion seems exagerated. Increased 
dispersion to 1.06. You can try some larger values.
I bumped up the dispersion_samples to 50 and got very smooth results.
You will see more reflections in the prism. Those are correct.
You need a strong light as the incident angle to the plane is shallow 
and the dispersion spreads it's brightness over a much larger area.

Advantage compared to the use of media: Much faster.



Alain

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