When talking about normal refractions, it can be argued that thoose are still two peices of straight lines just like reflections.

But there was some experiment which light bends in curved path in a solution of varying concentration from top to bottom. So there was a gradient of optical density(higher at bottom) and caused smooth curving of light

Edit: Oh it was just sugar solution http://physicsed.buffalostate.edu/pubs/StudentIndepStudy/EURP09/Sugar/sugar.html

Edit 2: Found this too which is much better demo https://m.youtube.com/watch?v=rnNjV3fh-4M