From: dpatkinson 5/02/00 23:31:00 Subject: polaroid sunglasses post id: 35593 I know how polaroid sunglasses work with the "fence pailing" plastic molecules and stuff. I also know that if you put 2 lenses together, you can see through them, but when you turn one 90 degrees, they go dark, but why does this also happen when you look at the sky? and also, why does it only happen in the east? From: Michael Gunter 6/02/00 0:36:55 Subject: re: polaroid sunglasses post id: 35620 Scattering of light in the sky does lead to polarisation of the scattered light, and not just in the east. Try this: lie on your back, stare at the blue sky, with the sun about half way between the horizon and the zenith ("top of the sky"). Remove your sunglasses and holding them by the arms rotate the spectacles, as if rapidly tilting your head from side to side. I haven't done this for a while and can't remember which areas of the sky show greatest polarisation, or what orientation the polarised light has with respect to the line between sun and observer. (one to try tomorrow) The polarisation effect does seem quite weak when the sun is very high OR very low in the sky (from memory) From: Mark H 6/02/00 11:24:33 Subject: re: polaroid sunglasses post id: 35680 Hi All. Polaroid sunglasses will have a greater effect when the light source is directly above. Using the horizion as your image, with the sun being at 90° , tilt your head through an arc from 45° to 135°. You will notice that the image that you are viewing will appear darker when your head is Perpendicular to the light source ( 90° ), Being the sun in this example. Polarising filters are setup in this fashion: EG 1 With the sun directly overhead: o ------------------ ------------------ ------------------ ------------------ ------------------ EG 2 With the sun closer to the horizion: o ------------------ ------------------ ------------------ ------------------ ------------------ There is a greater poliseration effect in EG 1. With EG 2, if you were to tilt your head toward the sun ( sorry...can't draw that !! ), you would have the same/similar effect as in EG 1. I hope this makes sense. Mark From: dpatkinson 6/02/00 23:05:42 Subject: re: polaroid sunglasses post id: 35765 yeah. that makes sense actually... but why does it only change in the east and not the west? I have tried it at all times of the day. From: Michael Gunter 7/02/00 12:20:50 Subject: re: polaroid sunglasses post id: 35925 I went outdoors at sunset last night. The sky was blue. Putting on the shades, looking straight upwards, and the sky was deeply polarised at the zenith, extending in a band from north to south. For comfort try this: Put on the sunglasses and lie on the ground at sunset, feet to the west, head to the east. Look straight up: you will see a dark band of sky stretching from left (south) to right (north) across the whole sky. The dark band lightens if you tilt your head away from this orientation. I don't see why exactly the same effect would not occur at dawn. Don't bees navigate by detecting polarisation of the sky? Note: This description applies to the equinox, but the sun is setting slightly south of west just now. From: Dr Paul (Avatar) 7/02/00 15:48:46 Subject: re: polaroid sunglasses post id: 35977 Hi Michael Gunter, this is my third try at getting a satisfactory answer to the page. Twice I have gone too deep and would lose the non BSc Physicist person. When the light comes from the horizon, the direction of propagation is almost directly across the sky. The light you see from directly over head is due to scatter within the atmosphere. The scatter is at an angle of 90 degrees to the original plane of propagation. When this occurs (scatter from particles in the atmosphere whose size of about the same as the wavelengths of light hitting them) the scattered light is polarised. This is due to being able to observe only one transverse component of the light. Light has transverse components (related to the amplitude of the light) which are like the surf wave amplitudes, except the amplitude "vectors" (directions and amplitudes) are always perpendicular to the direction of propagation and can be anywhere about the perpendicular plane. (Did I say that this was an easy explanation?). (Like a circle plane being pushed along the propagation direction, the front of your car carrying a circular face) (This is the hard bit to get right) The light comes from the horizon, you are laying parallel to the direction of the light. The light scatters from the atmosphere and the light that reaches you has a plane of polarisation which will be perpendicular to your body, all across the 90 degree segment of the sky. So with polarising sunglasses, you can cut off the observation of the light where the plane of polarization of the polaroid is perpendicular to that of the scattered light. Hence the dark band. Some of the light scattered to you at 90 degrees will rescatter through the section of atmosphere, so the sky will not be completely black (as would be the case for completely crossed polarisers) It also works somewhat for light bouncing off the road and light scattered off the surface of swimming pools. This is one of my favourite sections of physics in nature, totally explanable and totally enjoyable. (If you were off world, and looking down the face of the earth from one of the poles at sunrise/set the plane of polarisation would be perpendicular to the surface of the earth. Try imaging that one in the mind!!) (Sorry, I am a physicist at heart and the view inside the head is too much sometimes {:~)} ) Hope this helps Paul From: Martin B 8/02/00 9:20:29 Subject: re: polaroid sunglasses post id: 36223 Also light from the horizon contains a much greater component of light scattered from large particles in the atmosphere (which is why the horizon is whiter) and this light, I believe, is not polarised (or at least not as strongly as light scattered by air molecules.) This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.