Mohist school made an in-depth observation and research on concave mirror, and made a clear and detailed record in the book Mohist Classic. "Looking down, the scenery is small and easy, and the big one is right, saying outside and inside." "Low" is a deep depression; If you put it in the middle, the image you get is bigger and more upright than the object. Shen Kuo measured the focal length of concave mirror in the Northern Song Dynasty. He put his finger in front of concave mirror, observed the imaging situation, and found that the image changed with the distance between his finger and the mirror. It is recorded in Meng Qian's pen talk: "Yang Xiang's face is sunken, and it is right to take photos with one finger. If he is far away, he can't see anything." After that, you will fall down. " Explain that when the finger approaches concave mirror, it seems to be upright and gradually moves away from somewhere (near the focus), then "nothing can be seen", that is, there is no image (like at infinity); After moving this distance, it's like a handstand. This experiment not only expresses the imaging principle of concave mirror, but also is a rough method to measure the focal length of concave mirror.

Mohism also studied convex lenses. The Mohist Classic wrote: "Jiantuan, Jing Yi. It is said that the punishment is very strong. " "Sword Regiment" jiyan Sleeping Sutra, also known as the Regiment Sutra. "Scene 1" shows that there is only one kind of convex mirror imaging. The homonym of "punishment" refers to an object, which is always bigger than an image. Our ancestors made use of the characteristic that plane mirrors can reflect light, and combined many plane mirrors, and achieved interesting results. For example, Zhuangzi Tianxiapian's related annotation "Zhuangzi's Correction" records: "Take the shadow as a mirror, then the shadow as a mirror, and take the surface as a mirror, then the ghost is infinite." Such a device has received the effect of "taking photos in front of the mirror and taking photos in the back, and the flowers set each other off". It is recorded in the series "Jian" and "Huainan Bay Wall Book": "You can see your neighbors with a big mirror and a basin under it." It shows that someone made the earliest open-tube "periscope" a long time ago, and you can watch the outdoor scenery through the partition wall.