检测固定元素是否在div之上

Question

如何检测固定元素何时超过div？在这种情况下，锚点超过.images：

var fixedElement = $('a'),
    images = $('.images'),
    imagesTop = images.offset().top,
    imagesLeft = images.offset().left,
    imagesWidth = images.width(),
    imagesHeight = images.height();            

if(`fixedElement` is over `images`) {
    fixedElement.addClass('active');
}

a {
  position: fixed;
  top: 0;
  left: 0;
}
a.active {
  font-weight: bold;
  color: #aaa;
}
.images {
  margin-top: 100px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<a>Hello world</a>
<div class="images">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
    <img src="http://placehold.it/200/200">
</div>

遇到Detect element if over another element via using CSS3 Transform，但看起来太复杂了。

更新：请参阅kthornbloom的评论