Download
1 / 28
300 likes | 493 Views
Polarization Sundials. Montgomery College Planetarium by Dr. Harold Williams http://montgomerycollege.edu/Departments/planet/ http://montgomerycollege.edu/Departments/planet/planet/polarsun.html. All Sundials.
E N D
Polarization Sundials Montgomery College Planetarium by Dr. Harold Williams http://montgomerycollege.edu/Departments/planet/ http://montgomerycollege.edu/Departments/planet/planet/polarsun.html
All Sundials • All sundials tell time by the position of the sun in the sky. Mostly they tell local apparent sun time. • At sunrise the sun is just coming up on the horizon. • At midday, noon, the sun is crossing the meridian, the line that runs from north to south through the zeinth, (the point over head). • At sunset the sun is just going down on the horizon.
Most Sundials • Most sundials work by casting shadows with a “gnome,” shadow caster. • The shadow is projected on a surface that is a dial face telling you the time.
Polarization Sundials • Determine the position of the sun by measuring the polarization of the sky. • The polarization of scattered sunlight in our atmosphere tells us where the sun is even when the sun has no shadows because it is covered by a cloud or you are shadowed by a building. • But people can not see polarization so how do you measure it in the sky!
What is Polarization and how can you see it to measure it? • Most light sources produce unpolarized light, or light with both polarization states mixed equally. • So you must have an analyzer to see polarization, unless you are an bee or some other insects or octopus, squid, cuttlefish, or mantis shrimp, but evidently not pigeons then you have an analyzer in your eye already evolved. • Haidinger's brush, some people can be trained to see polarization weakly!
Visible Light • Visible light is electromagnetic radiation of a limited frequency and wavelength. • All electromagnetic radiation: radio waves, micro waves, infrared, visible light, ultraviolet, x-ray, through gamma rays, has two polarization states!
http://en.wikipedia.org/wiki/Polarization Elliptical Linear Circular
Polarization is perpendicular to the plane containing the sky point, the sun, and the observer.
Maximum polarization in the sky occur at a point 90 degrees from the sun in the plane containing the sun observer and zenith.
The plane containing the sun, observer, and the celestial pole rotate counter clockwise uniformly about the celestial pole making one turn in a local apparent solar day. The major polarization vector is always 90 degrees to the plane.
So it should be possible to make a gnomeless, no shadow caster, sundial using the polarization pattern in the sky. This should be easiest if you use the polarization pattern around the celestial pole since it rotates uniformly. Sort of like an equatorial gnomned sundial has uniform dial with hours 15 degrees apart since 15 degrees/hour x 24 hours/revolution=360 degrees/revolution.
History of Polarization • Etienne-Louis Malus (1775-1812) Discovered polarization in 1808 • François Arago (1786-1853) discovered polarization of the sky in 1809 • Babinet, Brewster, and others extended sky polarization • Charles Wheatstone (1802-1875) actual built a polarization sundial in 1848 • EventuallyJohn William Strutt alias Lord Rayleigh (1842-1919) in 1870s Rayleigh scattering in the sky
Wheatstone’s polarizing sundial at the Science Museum, London
Folding-pattern polarization sundial designed for demo by Charles Wheatstone
So how come polarization sundials are not common • Improvement in technology has made polarization sundials much easier and cheaper to make now than in Wheatstone's time when Nicol prism (calcite prism where one polarization encountered black paint) were used. • Edwin Land discovers how to embed iodosulphate of quinine (herapathite) microscopic crystals, which are dichroic crystals which are both birefringent and colored, aligning them by stretching them in polyvinyl alcohol films between sheets of cellulose acetate-butyrate (CAB)HN32 linear polarizer, Polaroid. • All references in the Literature are in obscure English Journals.
Of course why build sundial at all when we have ephemeral watches and clocks • To teach physical principles about the movement of the sun and the earth, the day and the seasons, in the case of shadow sundials. • Polarization sundials besides teaching about daily motion can be used to explain spooky facts about polarization.
Spooky Polarization • Regeneration of polarization by putting the third Polaroid between the two crossed Polaroids. • Quantum mechanics in action 2. The polarization of photons pages 4-7 in the 4th edition of Quantum Mechanics by P.A.M. Dirac. This is in the first chapter on The Principle of Superpositon after 1. The need for a quantum theory section. Polarization is a true quantum mechanical effect! In the limit of h goes zero there is no polarization.
Novelty factor of making a simple instrument to do science • This sundial works when it is in the shadows since it does not use shadows. So the sun behind a cloud is no problem. Works in courtyards, too, with a patch of blue sky available. • The sun being down is also no problem for an hour before sunrise and an hour after sunset. • When won’t it work? White clouds depolarized sunlight. So you do need a patch of blue in the north for the simplest type of polarization sundial to work.
Reference List • David Colchester, “A Polarized Skylight Sundial,” Bulletin of the British Sundial Society, 96.3, pp. 1315 (1996) • Allan A. Mills, “The Sellotape Sundial,” Bulletin of the British Sundial Society, 98.1, pp. 3-9 (1998) • Allan A. Mills, “Polarization of Light from the Sky, and its application to timetelling and Navigation,” Bulletin of the Scientific Instrument Society, No. 33, pp. 8-14 (1992) • A.H. Pfund, “A Polaroid Half-Shade Analyzer,” Journal of the Optical Society of America, 26, p. 453 (1936)
