Logout in Browser problem

I  am having 2 pages (Page1.aspx,Pag2.aspx).In Page1.aspx I am having a Login Button which is navigating to Page2.aspx .In page2.aspx I am having a Logout button. When I click on Logout I will be navigated to Page1.aspx.But if I click back button of the IE, Mozilla or any other browser I can go back to Page2.aspx.So I want to restrict this to the user i.e. if the user want to navigate to Page2.aspx they need to insert their Username, Password and the only they can navigate to Page2.aspx. Pls respond me ASAP ...