Only show overlapping portion of shape (LibGDX)

So I’m stuck again Hopefully someone can help

I’m working on my first app, a 2d side-scroller. It’s a bat in a cave and he will shoot out a Circle shaped sonar. I want this circle to be generally invisible and only show the parts of it that are overlapping my triangular “Polygon” objects (cave spikes). I looked through the LibGDX documentation and had trouble finding a way of locating coordinates of the point where shapes are overlapping. Any quick tips would be awesome to get me in the right direction.