asp.net ajax 弹出层问题

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style>        html, body {
         height: 100%;
         width: 100%;
        }
        .white_content {
         display: none;
         position: absolute;
         top: 25%;
         left: 25%;
         width: 50%;         border: 12px solid #D6E9F1;
         z-index:1002;
        }
        .black_overlay {
         display: none;
         position: absolute;
         top: 0%;
         left: 0%;
         width: 100%;
         height: 100%;
         background-color:#f5f5f5;
         z-index:1001;
         -moz-opacity: 0.8;
         opacity:.80;
         filter: alpha(opacity=80);
        }
        .close {
         float:right;
         clear:both;
         width:100%;
         text-align:right;
         margin:0 0 6px 0
        }
        .close a {
         color:#333;
         text-decoration:none;
         font-size:14px;
         font-weight:700
        }
        .con {
         text-indent:1.5pc;
         line-height:21px
        }
</style>
<script>
    function show(tag) {
        var light = document.getElementById(tag);
        var fade = document.getElementById('fade');
        light.style.display = 'block';
        fade.style.display = 'block';
    }
    function hide(tag) {
        var light = document.getElementById(tag);
        var fade = document.getElementById('fade');
        light.style.display = 'none';
        fade.style.display = 'none';
    }
</script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
        <a href="javascript:void(0)" onclick="show('light')">打开</a>
<div id="light" class="white_content">
      <div class="close"><a href="javascript:void(0)" onclick="hide('light')"> 关闭</a></div>
      <div class="con"> 
          <asp:Button ID="Button1" runat="server" Text="Button" />
      </div>
</div>
        </ContentTemplate>
        </asp:UpdatePanel>
    
    </div>
    </form>
</body>
</html>
我一点击Button按钮弹出层就被刷没了，求指教啊