Awesome! So we can get the index of anything with respect to its property group, by applying the expression to its child parameter. Thank you so much, it helps a lot.
Thank you for you quick reply. But I think I need to explain the scene more. I am trying to do a animation like this. https://www.eamaemirkin.com/site/echo/. I chose a point light(w/trapcode lux) to bounce on a surface, some how I still couldn’t get it right. I very much appreciate your help.