Sorry if the brief was / is confusing.
"portray penetrating light" to me suggests; that the viewer of the photograph should be able to determine / see the penetrating light recorded within the image. As would be seen with images with translucent objects in them, images displaying beams of light, or both etc.
It was really quite clear to me Andy; it's one of those occasions where you can get lost (I can anyway), if you're trying too hard! Or thinking too hard.
I knew exactly what I was after, and photographed sunlight falling into a wood, through a gap in the trees. Penetrating light.
Then I spotted a yellow leaf on an ivy covered fence; The leaf was lit by sunlight, and lit up the whole area that was otherwise in the shade. In my enthusiasm, it was some time before I realised what I was doing, and that I was off course. The sunlight had already penetrated the trees and had reached its destination. Penetrated light.
My gut instinct was right and I knew what you were after, and later got confused. And now that I'm thinking about it, I get confused again!
Language can be very confusing!