如何判断一个Div是否在可视区域，判断div是否可见

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
    <title>js</title>
    <script src="http://code.jquery.com/jquery-latest.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(document).ready(function () {
            $(window).scroll(function () {
                var a = document.getElementById("eq").offsetTop;
                if (a >= $(window).scrollTop() && a < ($(window).scrollTop()+$(window).height())) {
                    alert("div在可视范围");
                }
            });
        });
    </script>
</head>
<body>
    <div style="width:1px;height:2000px;"></div>
    <div id="eq" style=" width:100px; height:100px; background-color:Red;">1</div>
    <div style="width:1px;height:2000px;"></div>
</body>
</html>