Light Travels in a Straight Line Demonstration - Boulder, CO
Posted by: Groundspeak Premium Member Outspoken1
N 40° 01.092 W 105° 15.363
13T E 478149 N 4429808
The question of why the sun (or any light) can make a circular image when it shines through a square hole has been pondered since 330 BCE.
Waymark Code: WMCH5N
Location: Colorado, United States
Date Posted: 09/07/2011
Published By:Groundspeak Regular Member Math Teacher
Views: 9

This moveable apparatus explores the question 'why does a square hole make round light?" The principle explored here became the basis for the pinhole camera. Other apparatus helped Dutch artists as well as

"The first mention of the principles behind the pinhole camera, a precursor to the camera obscura, belongs to Mo-Ti (470 BCE to 390 BCE), a Chinese philosopher and the founder of Mohism. Mo-Ti referred to this camera as a "collecting plate" or "locked treasure room". The Mohist tradition is unusual in Chinese thought because it is concerned with developing principles of logic. The Greek philosopher Aristotle (384 to 322 BCE) understood the optical principle of the pinhole camera. He viewed the crescent shape of a partially eclipsed sun projected on the ground through the holes in a sieve, and the gaps between leaves of a plane tree.

The camera obscura was known to earlier scholars since the time of Mozi and Aristotle. Euclid's Optics (ca 300 BC), presupposed the camera obscura as a demonstration that light travels in straight lines.

In the 4th century BC, Aristotle noted that "sunlight travelling through small openings between the leaves of a tree, the holes of a sieve, the openings wickerwork, and even interlaced fingers will create circular patches of light on the ground." In the 4th century AD, Theon of Alexandria observed how "candlelight passing through a pinhole will create an illuminated spot on a screen that is directly in line with the aperture and the center of the candle." In the 9th century, Al-Kindi (Alkindus) demonstrated that "light from the right side of the flame will pass through the aperture and end up on the left side of the screen, while light from the left side of the flame will pass through the aperture and end up on the right side of the screen."

In the 6th century, Byzantine mathematician and architect Anthemius of Tralles (most famous for designing the Hagia Sophia), used a type of camera obscura in his experiments.

The Song Dynasty Chinese scientist Shen Kuo (1031–1095) experimented with a camera obscura, and was the first to apply geometrical and quantitative attributes to it in his book of 1088 AD, the Dream Pool Essays.[7][verification needed] However, Shen Kuo alluded to the fact that the Miscellaneous Morsels from Youyang written in about 840 AD by Duan Chengshi (d. 863) during the Tang Dynasty (618–907) mentioned inverting the image of a Chinese pagoda tower beside a seashore. In fact, Shen makes no assertion that he was the first to experiment with such a device. Shen wrote of Cheng's book: "[Miscellaneous Morsels from Youyang] said that the image of the pagoda is inverted because it is beside the sea, and that the sea has that effect. This is nonsense. It is a normal principle that the image is inverted after passing through the small hole."

In 13th-century England Roger Bacon described the use of a camera obscura for the safe observation of solar eclipses. Its potential as a drawing aid may have been familiar to artists by as early as the 15th century; Leonardo da Vinci (1452–1519 AD) described camera obscura in Codex Atlanticus. Johann Zahn's Oculus Artificialis Teledioptricus Sive Telescopium was published in 1685. This work contains many descriptions and diagrams, illustrations and sketches of both the camera obscura and of the magic lantern.
Camera obscura, from a manuscript of military designs. Seventeenth century, possibly Italian.

The Dutch Masters, such as Johannes Vermeer, who were hired as painters in the 17th century, were known for their magnificent attention to detail. It has been widely speculated that they made use of such a camera, but the extent of their use by artists at this period remains a matter of considerable controversy, recently revived by the Hockney-Falco thesis. The term "camera obscura" was first used by the German astronomer Johannes Kepler in 1604." (excerpted from (visit link)

Another good resource is (visit link) .
Scientific Principle(s) being demonstrated:
This apparatus explores the scientific principle that light travels in a straight line. This principle, called 'Camera Obscura', has given us a critical tool for Waymarking - the camera!


Briefly explain if the experiment was effective for you. Could it have been better?:
This is a well designed apparatus, but the explanation depends on the nearby kiosk that provides a video explaining the scientific principle. If the kiosk is not working, then the visitor has no way to get the 'answer' to the question 'square hole = round circle of light.' I feel this is a case of too much technology. Signs could help make this a better learning experience.


When is the apparatus/experiment available to the public:
Dawn to dusk since it is dependent upon sunlight.


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