Hi,
this function worked on my computer,i don't know your html structure.
try this
<style>
.item{
height:300px;
width:300px;
background-color:yellow
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
$(function(){
$(".item").hover(function(){
$(this).css('opacity','0.9');
},function(){
$(this).css('opacity','0');
});
})
</script>
<div id="div1" class="item">
</div>