when page scrolls 50px down code example

Example 1: jquery on scroll x pixels

$(document).scroll(function() {

        if ( $(document).scrollTop() >= 1000 ) {
        } else if ( $(document).scrollTop() >= 500 ) {
        } else {
        }
    });

Example 2: jquery on scroll x pixels

$(document).scroll(function() {
           var value=$(document).scrollTop();/* <== here*/

            if ( value >= 1000 ) {
            } else if ( value >= 500 ) {
            } else {
            }
        });