How to make light shine through a prism? I'm trying to make a scene like this:

But no matter how I mess with the bounce settings in cycles; the light just isn't coming out of the other side of the prism:

BlendFile
What am i doing wrong?
It was the &quot;indirect light&quot; in the &quot;clamping&quot; section of the &quot;scene&quot; properties tab.  It needs to be set to 0 for light to go through the prism.  I do not understand why it is so dark that that happens though.