The early contributions of ancient China to optics predate those of Euclid, but are little known in the west. During the Warring States period, 2400 years ago in China, Mo Zi, a philosopher, thinker and scientist, stated explicitly the concepts of linear optics: the straight line propagation of light, reflection of light by planar, concave and convex mirrors, and the pinhole camera. Refraction of light was also discovered then, and the refractive index of water was measured to be 1.25, which is very close to the modern value of 1.33. These are recorded in the Book of Mo Zi. In the early Western Han Dynasty, Liu An, King of Huai-Nan, also compiled several works, where novel optical devices are mentioned, such as burning glasses made of ice, and the world’s first surveillance periscope.
The Warring States period of China, between 475 and 221 BCE, was a time of academic and scholarly prosperity when many schools of learning were set up. Some are well known in the world today, such as Confucianism and Taoism, but some are not so well known. One school which is little known in the west but is particularly important with regard to science and technology is Mohism, founded by Mo Zi, who paid great attention to natural science and engineering. Among his many contributions, it is noteworthy to recall his achievements in optics since we are celebrating the International Year of Light in 2015. His major contributions include: an outline of the basic concepts of linear optics, the straight-line propagation of light, images and shadows, the reflection of light by plane, concave and convex mirrors, the pinhole camera, and the refraction of light, with the refractive index of water measured to be 1.25 which is very close to the present-day value of 1.33. These are recorded in the Book of Mo Zi [1-5].
Fig. 1: Mo Zi, 468-376 BCE.
In another development, Liu An, (179 to 122 BCE), the King of Huai-Nan in the Western Han Dynasty (202 BCE to 9 CE) and a Taoist master and thinker, also made important contributions to optics. Taoism attaches great importance to natural science. The world famous Chinese bean curd food, tofu, was invented by Liu An as a by-product while making elixirs, or alchemical medicines. These are recorded in the Book of Huai-Nan and the Wan-Bi-Shu [6, 7]. In these writings the reflection of light by multiple mirrors, used to set up the world’s earliest surveillance periscope, was described . Also recounted are the focusing of sunlight to light a fire using a concave mirror or a lens made of ice.
Though these early contributions in ancient China have been noted and studied by certain renowned scholars such as Joseph Needham  and some famous popular science writers , they are not widely known. For instance, they are not even touched upon in various recent reviews of the history of optics [8, 9]. In this short blog article, we will introduce the reader to these ancient Chinese records on optics.
Mo Zi’S contributions
Mo Zi (also known as Mo Tzu, Mo Di, Mo Ti, or Micius) lived between 468 and 376 BCE in the Warring States period of China. He was a philosopher, thinker, scientist, engineer, and military strategist. He advocated the “universal love of mankind” as compared to the “extensive love of Confucius”, and also pushed for a simple lifestyle. His many contributions to the natural sciences in mechanics, acoustics, optics, and other fields are collected in the Book of Mo Zi (Mo Jing, Mo Ching). Unfortunately, over the course of history much of the original book was lost and the surviving transcripts are very fragmented. An informative account in English of optics in the Book of Mo Zi may be found in Vol. 4 of Needham’s monumental work Science and Civilization in China , as well as in Ref. . It seems evident that Mo Zi actually conducted experiments in optics, as his writings include the basic concepts and applications of linear optics. Below is a summary of his contributions, where the quotations are taken from Needham’s translated excerpts from the Book of Mo Zi [1, 4].
Straight-line propagation of light “An illuminated person shines as if he was shooting forth (rays).” This is in contrast to the ancient Greek conception of the eye shooting out rays onto the object of vision.
Shadow formation, umbra and penumbra “When there are two shadows (it is because there are two sources of light).” The reason is given under ‘doubling’. “Two (rays of) light grip (i.e. converge) to one light-point. And so you get one shadow from each light-point.” “If the source of light is smaller than the post, the shadow will be larger than the post. But if the source of light is larger than the post, the shadow will still be larger than the post. The further (from the source of light) the post, the shorter and darker will be its shadow; the nearer (to the source of light) the post, the longer and lighter will be its shadow.”
Fig. 2: Illustration of pinhole camera. Note the image is inverted.
Pinhole camera, focal point and inversion “The bottom part of the man becomes the top part (of the image) and the top part of the man becomes the bottom part (of the image). The foot of the man (sends out, as it were) light (rays, some of which are) hidden below (i.e. strike below the pinhole) (but others of which) form its image at the top. The head of the man (sends out, as it were) light (rays, some of which are) hidden above (i.e. strike above the pinhole) (but others of which) form its image at the bottom. At a position farther or nearer (from the source of light, reflecting body, or image) there is a point (the pinhole) which collects the (rays of) light, so that the image is formed (only from what is permitted to come through the collecting-place).” (See Fig.2 for illustration.)
Fig. 3: Illustration of the reflection of light by multiple mirrors (the world’s first surveillance periscope).
Reflection by plane mirrors “A shadow can be formed by the reflected (rays of) the sun.” “The reason is given under ‘turning’.” “If the light (-rays) from the sun are reflected (from a plane mirror perpendicular to the ground) on to a person, the shadow (of that person) is formed (on the ground) between that person and the sun.” “Standing on a plane mirror and looking downward, one finds that one’s image is inverted. (If two mirrors are used) the larger (the angle formed by the mirrors within the limit of 180 degrees) the fewer (the images).” “A plane mirror has only one image. If now two plane mirrors are placed at an angle, there will be two images. If the two mirrors are closed or opened (as if on a hinge), the two images will reflect each other. The image-targets are numerous (i.e. there are many images) but (the angle between the two mirrors) must be less than when they were originally in the same line (i.e. 180 degree). The reflected images are formed from the two mirrors separately.”
Fig. 4: Focusing sunlight by a lens to start a fire.
Reflection and focusing of light by concave mirrors “With a concave mirror, the image may be smaller and inverted, or larger and upright”. “(Take first) (an object in) the region between the mirror and the focal point. The nearer the object is to the focal point (and therefore the farther away from the mirror), the larger the image will be. The farther away the object is from the focal point (and therefore the nearer to the mirror), the smaller the image will be. In both cases the image will be upright. From the very edge of the centre region (i.e. almost at the focal point), and going towards the mirror, all the images will be larger than the object, and upright.” “(Take next) (an object in) the region outside the centre of curvature and away from the mirror. The nearer the object is to the centre of curvature, the larger the image will be. The farther away the object is from the center of curvature, the smaller the image will be. In both cases the image will be inverted.” “(Take lastly) (an object in) the region at the centre (i.e. the region between the focal point and the centre of curvature). Here the image is larger than the object (and inverted).”
Reflection of light by convex mirrors “With a convex mirror there is only one kind of image.” “The nearer the object is to the mirror the larger the image will be. The farther away the object is the smaller the image will be. But in both cases the image will be upright. An image given by an object too far away becomes indistinct.”
Later, Shen Kuo in the 11th century wrote: “The ancients made mirrors according to the following methods. If the mirror was large, the surface was made flat (or concave); if the mirror was small the surface was made convex. If the mirror is concave it reflects a person’s face larger; if the mirror is convex it reflects the face smaller. The whole of a person’s face could not be seen in a small mirror, so that was why they made the surface convex. They increased or reduced the degree of convexity or concavity according to the size of the mirror, and could thus always make the mirror correspond to the face. The ingenious workmanship of the ancients has not been equaled by subsequent generations. Nowadays, when people get hold of ancient mirrors, they actually have the surfaces ground flat. So perishes not only ancient skill, but even the appreciation of ancient skill.” .
Refraction of light and the refractive index of water “The (apparent) size of a thorn (in water) is such that the sunken part seems shallow…. If you compare it (the difference between the real and apparent depth is) one part in five. This gives a refractive index of 1.25 for water, which is not bad compared with the actual value 1.33.”
The contributions of Liu An, King of Huai-Nan
Huai-Nan is a place in today’s Anhui province of China. Liu An (179-122 BCE), King of Huai-Nan during the Western Han dynasty, was the grandson of Liu Bang, the first emperor of the Han dynasty and the leader of a peasants uprising. Liu An was a Taoist master, and a thinker. Taoist scholars and practitioners were pragmatic, and attached great importance to natural science and engineering. One goal of Taoism was to achieve longevity through exercise and taking elixirs, and so in their quest for longevity they made many important contributions to the sciences. Liu An’s great contribution was in the compiling of two books: Book of the Master of Huai-Nan (or The Huainan Philosophers), and Huai-Nan Wan-Bi-Shu [6, 7].
The reflection of light by multiple mirrors, as well as from the surface of water, was well known in those days, and it is recorded that “A large mirror being hung up (above a large trough filled with water), one can see, even though seated, four ‘neighbors’” . Using this setup, a soldier behind a wall could see outside the wall and a landlord could oversee his laboring serfs. (See Fig. 3 for illustration.) This was probably the world’s first surveillance periscope. The use of concave bronze mirrors to light fires was also very common and is clearly described in the Book of the Master of Huai-Nan (although the phenomenon was already mentioned in even earlier writings): “It is like collecting fire with a burning-mirror. If (the tinder is placed) ‘too far away’, (the fire) cannot be obtained. If (the tinder is placed) too near, the centre point will not (be hit either). It should be just exactly between ‘too faraway’ and ‘too near’.” [4, 6].
Making fire from ice was also mentioned in the Huai-Nan Wan-Bi-Shu: “A piece of ice is cut into the shape of a round ball and held facing the sun. Mugwort tinder is held to receive the bright beam from the ice, and thus fire is produced.” [4, 7] (See Fig. 4 for illustration.) This is well known in northeast China, where to this day children playing outdoors in the winter like to make ice lenses; even a very crudely fashioned round lump of ice can start a fire. On the other hand, the ancient burning-mirrors were bright and highly polished, and indeed are evidence of the high level of metalworking in those days.
The earliest bronze mirrors that have been found were excavated from the tomb of Queen Liang of the Guo state (central China), dating back to 760 BCE, more than 200 years earlier than Mo Zi. It is a pity that so much knowledge has been lost over the course of China’s history, as the ancients certainly knew much more about optics than what is just found in currently existing written records.
As we are celebrating the International Year of Light in 2015, it is meaningful to look back on the role of light in the very early days of human society. This special year will aim to let everyone, young and old, in every part of the globe, understandand appreciate the basics of light, and to promote optical technology for the benefit of all countries. Optics and light are indispensable to our everyday life. The earliest discoveries and inventions in optics of ancient China and the world stemmed from the need and desire to emerge from the darkness of backwardness to the light of progress. The mirrors described above were luxury commodities in ancient times that were affordable only to a minority of the privileged. Today, light-based technology is a major economic driver with the potential to revolutionize the 21st century and to bring prosperity to all mankind. Let us strive towards that goal.
 Mo Zi, Book of Mo Zi, originally written around 385 BCE. See “Book of Mo Zi”, (in Chinese), proofed and noted version, by Yu-Jiang Wu and Qizhi Sun, Zhonghua Book Press, 2008.
 Lin-Zhao Qian and Qingshi Zhu, The Collected Works of Lin-Zhao Qian, (in Chinese), pp. 458-467, Anhui Education Press, 2001.
 Nian-zu Dai, Ancient Chinese Physics, (in Chinese), Shandong Education Press, 1991.
 J. Needham, Science and Civilization in China, v.4, Part 1,Physics, Cambridge Univ. Press, 1962.
 C. Cooper, Our Sun: Biography of a Star. Race Point Publishing, 2013.
 Liu An, Book of Huai-Nan, written between 179 – 122 BCE. See “Book of Huai-Nan”, (in Chinese), Guang-Zhong Chen, Zhonghua Book Press, 2012.
 Liu An,Wan-Bi-Shu, written between 179 -122 BCE. See “Huai- Nan Wan-Bi-Shu and Four Other Ancient Books”, (in Chinese), The Commercial Book Press, 1939.
 M. Herzberger, Optics from Euclid to Huygens, Applied Optics, Vol. 5, Issue 9, pp. 1383-1393 (1966).
 J. N. Mait, A History of Imaging, Optics and Photonics News, Feb, pp. 22-27 (2006) .