how to make browser back close magnific-popup

I have the popup working but sometimes a user clicks the back button on their browser to close the popup.

How can I make the browser back button close a 'magnific-popup' that is already open?

Thanks

After some digging found history.js and then the following

var magnificPopup = null;
jQuery(document).ready(function ($) {
    var $img = $(".img-link");
    if ($img.length) {
        $img.magnificPopup({
            type: 'image',
            preloader: true,
            closeOnContentClick: true,
            enableEscapeKey: false,
            showCloseBtn: true,
            removalDelay: 100,
            mainClass: 'mfp-fade',
            tClose: '',
            callbacks: {
                open: function () {
                    History.Adapter.bind(window, 'statechange', closePopup);
                    History.pushState({ url: document.location.href }, document.title, "?large");
                    $(window).on('resize', closePopup);
                    magnificPopup = this;
                },
                close: function () {
                    $(window).unbind('statechange', closePopup)
                        .off('resize', closePopup);
                    var State = History.getState();
                    History.replaceState(null, document.title, State.data["url"]);
                    magnificPopup = null;
                }
            }
        });
    }
});

function closePopup () {
    if (magnificPopup != null)
        magnificPopup.close();
}