The setup is
like in this picture, with the light coming from the left.
You can see the prism edge-on, and it casts a shadow on the box straight
off to the right. The incoming light is bent down through an angle of
about 30°. If you look closely, you can see that the top of the light
stripe is a bit redded than the middle, and the bottom is bluer than the
middle - but that is another story.