How to run Ajax after every 5 second

Question

I want to call my ajax after every 5 seconds. Below code is showing the html once but not calling the ajax after every five seconds. I am new in ajax, Can somebody explain this to me please.

$(document).ready(function(){
    function fresh_ajax(){
        $.ajax({ 
           type:"POST",
            url:"pic.php",
            success:function(html){
                $('.a').empty();
                $('.a').html(html).show();
            }   
        });
    }

    window.setInterval(function(){
        fresh_ajax();
    }, 500);
});