Opacity settings

Does anyone know how to set opacity of a black background colored div to be 50% lighter, but so that the text and images that rest inside the same div are not also changed to 50% opacity? I tried the following found on w3schools, but it's still showing the text as 50% opacity along with the div...

div.transbox
{
width:400px;
height:180px;
margin:30px 50px;
background-color:#ffffff;
border:1px solid black;
/* for IE */
filter:alpha(opacity=60);
/* CSS3 standard */
opacity:0.6;
}
div.transbox p
{
margin:30px 40px;
font-weight:bold;
color:#000000;
}

Thanks!