https://zh.frojeostern.com/post/as-shown-in-the-figure-if-a-pencil-is-erected-at-a-distance-of-15-cm-in-front-of-the-plane-mirror-the-image-of-the-pencil-can-be-seen-under-the-plane-mirror-at-this-time-the-pencil-does-not-move-and-the-plane-mirror-is-moved-to-the-original-image-of-the-pencil-then-the-distance-between-the-image-of-the-pencil-in-the-plane-mirror-and-the-pencil