Close ModalPopup From frame(another page)

hello, m showing another page (i.e Default2.aspx) for modalpoup . Default2.aspx has gridview.

By clicking on 'Select ' linkbutton in Gridview,i want to close the popup window

How to close it....Please help

FirstPage.aspx-->

<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panl1" TargetControlID="Button1"
CancelControlID="Button2" BackgroundCssClass="Background" BehaviorID="mpeBehavior">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panl1" runat="server" CssClass="Popup" align="center" style = "display:none">
<iframe style=" width: 350px; height: 300px;" id="irm1" src="Default2.aspx" runat="server"></iframe>
<br/>
<asp:Button ID="Button2" runat="server" Text="Close" />

-------------------------------------------------------------------------------------------------------------------

Default2.aspx-->

<asp:GridView ID="grvDemo" runat="server" onrowcommand="grvDemo_RowCommand">
<Columns>
<asp:TemplateField>
<ItemTemplate>

<asp:LinkButton ID="lnkSelect" runat="server" Text="Select" CommandName = "Select" OnClientClick = "return GetSelectedRow(this)" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>