Light difussion

I am struggling to work out how to create a realistic back light through a transparent material such as liquid. Im attaching a project where we are trying to create a simple, diffused, backlight through the liquid. However whatever we do the light source always appears to have hard edges top bottom and sides. Nothing we do seems to diffuse the light to create the natural ‘glow’ with soft edges that would happen in reality.  We have tried adjusting the liquid settings also but nothing we do softens the light. Please help!?

Im attaching our working file for ref. The light source in question is the layer called ‘BACK LIGHT'

THanks

Greg