<html>
<body>
<div style="height:0px;400px;background:rgba(88,116,178,0.6);overflow:hidden;" class="div3"></div>
</body>
</html>
<script>
window.onscroll=function(){
var scrollTop=document.body.scrollTop;
if(scrollTop>=100){
$('.div3').animate({
height:'200px'
},2000)
}
}
</script>