reperiendi

Passive polarization clock

Posted in Astronomy, Math, Time by Mike Stay on 2008 February 22

Here’s a design for a passive polarization clock.

The sky is polarized in concentric circles around the sun. The polarization of the southern sky moves through around 180 degrees during daylight hours. It is polarized vertically in the morning, horizontally at noon, and vertically again in the evening.

Align slices of polarized film such that they are parallel to the contours. Any given ray from the center of the sundial outward always hits the contours at the same angle; the angle changes by 360 degrees as the ray passes through 180 degrees. In other words, the clock goes from 6am to 6pm as the sun moves through the sky.

sundial

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4 Responses

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  1. Gert said, on 2013 June 13 at 12:40 pm

    hello Mike,
    I found this while looking for polarization sundials. I wonder if you would like to explain a little more how it is supposed to work.

    best,
    Gert

    • Mike Stay said, on 2013 June 13 at 2:21 pm

      Polarized filters are made from long polymers that allow electrons to move freely along the molecule, but not to get too far away from the molecule. A light wave that’s polarized parallel to the molecule will act on the electron to slide it along the molecule. The electron will be pushed around by the light wave and move along the molecule, vibrating the atoms; the energy from the light wave will get turned into heat. A light wave that’s polarized perpendicular will pull the atom away from the molecule; the energy of the wave gets turned into potential energy, like in a spring. The electron will be attracted back to the molecule and will re-emit the light wave. So only light that’s polarized perpendicular to the molecules will make it through a polarizing filter.

      The light bouncing off the atmosphere and into our eyes is polarized just like light bouncing off of a glossy magazine or the surface of a pool. The angle of polarization depends on the position of the sun. Facing north, as the sun rises and then sets, the angle of polarization goes through a 180 degree turn. I chose the contours of the template in this post such that when the sun is at an angle A, the ray from the center of the sundial at angle B filters cos(A-B) of the light. The result is that the darkest part of the polarized film will behave like an hour hand and will go through a 360-degree turn from sunrise to sunset, roughly 6AM to 6PM.

  2. David Ring said, on 2016 February 5 at 7:52 am

    Can this concept be made from bent wire? Recommended guage. Is there only one sweet spot viewing angle or is the image approximatedly the same from several viewing angles?

    • Mike Stay said, on 2016 February 5 at 9:12 am

      Bent wire doesn’t polarize light, but it would work as a reflective clock (i.e. if you’re in the northern hemisphere, make the clock face south); the smaller gauge the better. For the polarization clock, the dark part depends on the polarization of the sky behind it, so changing your viewing angle too much won’t work.


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