why shows index.php/new_controller when loading any controller in codeigniter

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PHP

when i click a button in view, control goes to controller then controller
redirect me to another view.
Its working perfectly but without .css, controller is loaded like localhost/templateSample/index.php/new_controller and if i directly access localhost/tempalteSample/new_controller it gives me proper output with .css

So, my question is when i access new_controller from view using form_open why its
not giving me perfect output?