Earlier on there was a press report showing a photograph of a 'glory' taken from an aeroplane. I took a similar photo a few years back while flying over Yunnan. It shows the shadow of the plane on the clouds, with concentric rings of blue on the inside and red on the outside. There have been observations elsewhere of these primary rings being accompanied by as many as three to four similar rings of larger diameter.
Figure 1 'Glory'
These rings are produced when light is refracted and reflected after entering water droplets (a cloud is made up of water droplets). The ring diameter is related to the size of the water droplets. The smaller the glory diameter, the larger the droplets that produce it.
Figure 2 Light refracted and reflected inside a water droplet
Here a light ray hitting the edge of a water droplet is refracted, reflected and then refracted again, before clinging to the surface of the droplet and conveyed the rest of the way around the droplet to the backward direction (180 degrees).
A glory may also be seen from a high point, for example on a mountain, when there are clouds down below and the sun is behind you, projecting your shadow onto the cloud. You may be rewarded by the sight of a shadow of your head surrounded by coloured haloes. What is unique about the shadow, moreover, is that if someone else is with you, his/her shadow does not have such haloes. This is probably how the name 'glory' came about in the first place.
The observation of light travelling in a straight and forming an image after entering the eyes dates back a long time. During China's Warring states period, the philosopher Mozi had this to say about light forming an image: "Light falling on a person comes from a source radiating in all directions. Light from beneath forms a tall shadow; light from above gives a short shadow. Blocking the light with the foot produces an image at the top; blocking with the head gives an image down below."
We do not know why this concept of light was not further developed in China. A few centuries later, the great scientist Alhazen (Abū 'Alī al-Hasan ibn al-Hasan ibn al-Haytham, 965 - 1039), born in Basra of present-day Iraq, was the first to prove by experiments the formation of images. He pointed out that an image of an object is formed by light reflected from the object and subsequently entering the eyes, and argued that vision occurs in the brain, rather than the eyes.
Figure 3 Alhazen (965 - 1039)
Apart from optics, Alhazen was involved in the study of many science subjects and published a large number of works. His insights had far-reaching effects on the development of science for more than five centuries. His Book of Optics has been ranked alongside Isaac Newton's (1643 - 1727) Philosophiae Naturalis Principia Mathematica as one of the most influential books in physics for introducing an early scientific method, and for initiating a revolution in optics and visual perception.
Next time you go outdoor, be on the lookout for what is around you. God knows you may be able to discover an interesting phenomenon or two.
(a) Atmospheric phenomena - Readings from 'Scientific American', W.H. Freeman and Co., 1980.
(b) A Field Guide to the Atmosphere, V.J. Schaefer and J.A. Day, Houghton Mifflin Co. Boston (1981).
(c) 'Scientific American', September 2001.
(d) 「科學，從好奇開始」，郭中一著，文經社 (中國台灣) 2005 年出版。 (in Chinese only).