Let's try to design the interaction between two media, a (sunscreen) and B (the substrate) and light taking into accounut different physical effect. Lf the two madia have both smooth faces and low scattering properties,the tranmitted radiation path is straight into account different physical effects. lf the two media have both smooth faces and low scttering properties,the transmitted radiation path is straight forward while part of radiation is straight back reflected. lf medium A is diffussing,both the in-coming radiation and the back reflected at interface are then scattered in all direction,and part of the last one is aging re-directed in the substrate; if the substrate has a roughened suface, as in case of PMMA plates and roughened surface,as in case of PMMA plate and roughened