i have some questions puzzling me about light. the answers are probably simple but i dont now the answers to them. so here i go Why is it that light slows down in dense transparent materials, such as perspex, and regains its speed once it leaves the material? N.B. is it an energy loss/gain or just illusion or something else. furthermore i read that light has infact beened slowed significantly and even stopped by passing it through a cold sodium gas. My question is that if light has been stopped then wouldnt time also be stopped? Which leads me to my next question if a transparent material slows time (not sure if its true or not) does this mean aquatic life living underwater experience a slightly slower time elapse than what we surface dwelling being experience? this may sound farout and i could very well be wrong. Its just something that popped into my head the other day whilst thinking about light. Unfortunately i dont have a deep understanding so i need some help. thanks.