May be because of Postback your code is not working.
Look at code below....
<script type="text/javascript">
function show() {
document.getElementById('light').style.display = 'block';
document.getElementById('fade').style.display = 'block';
return false;
}
</script>
Here I included "return false". Which avoids postback for a button click and your code will work fine...
Hope it works...