AJAX :: Showing Modalpopup In Button Click?

I am tring to get modal popup when enter value in one text box then click the button modal popup will show , without entering the value in textbox, button validation to enter the value. I written javascript validation to the button ,it showing alert message, but after showing alert message clicking ok button in alert window modalpopup is showing . How can i resolve it .

<asp:TextBox ID="TextBox1" runat="server" />
<asp:Button ID="Button1" runat="server" Text="Show popup" />

Here java script validation :

function Validate() {
if (document.getElementById("<%=TextBox1.ClientID%>").value == "") {
alert("Please Enter any Value ");
document.getElementById("<%=TextBox1.ClientID%>").focus();
return false;
}
}