.back_img_1{background-size:cover;background-repeat:no-repeat;position:absolute;top:100%;left:0;width:100%;height:0;transition:all 1s;z-index:5;opacity:0}.back_img_1_end{background-size:cover;background-repeat:no-repeat;position:absolute;top:0;left:0;width:100%;height:100%;transition:all 1s;z-index:5;opacity:1