There is a subtle indicator that tells you whether or not you’re seeing polarized light, and which direction the light is polarized on. Find out about Haidinger’s Brush.
Polarization of light has come in handy over the last few years. Light travels in waves, like the kind you would see traveling along a suspended piece of string if you pulled one end rapidly up and down. Most naturally-emitted light waves are oriented every which way – the crests and valleys can be oriented up and down, side to side, or at any diagonal angle.
Polarized light, on the other hand, has all its waves oriented only one way. Light can be polarized when it reflects off something, refracts through something, or it can be emitted already polarized. One of the easiest ways to polarize light is just to make it go through slits oriented perpendicular to the direction that you’d like light to be polarized. All the light that’s oriented in the wrong direction will be blocked as it tries to get through the slit. Many pairs of glasses are covered with a coating with tiny, imperceptible slits which do just that.
Windshield and sunglass makers use polarization a lot, to make lenses that reduce glare. When light reflects off water or snow, or the horizontal surfaces of cars, much of it is polarized horizontally. The reflection of a lot of sun makes the surface look like a giant sheet of light which drowns out all the light around it. By making glass with a coating that polarizes light vertically, most of the horizontal glare is choked off, allowing the person behind the glass to see more clearly.
Movies that use 3D use polarization as well. They film the movie from two different angles, present one angle only in horizontally polarized light, and one in only vertically polarized light. By making the audience wear glasses, one lens of which only allows vertical light and the other only allows horizontal light, the movie presents two different images, which the viewer’s brain interprets as one three dimensional image.
One of the advantages of polarization is human’s lack of perception of it. Some animals can differentiate between light polarized in different directions, but for humans it’s just a handy way of filtering out a certain percentage of the incoming light without changing the perceived image.
OR IS IT?
After last week’s focus on Battlestar Galactica‘s series finale, we turn to some items from other shows, that fell through the cracks. First up is a recent episode of Dollhouse, in which Echo, (played by Eliza Dushku) is imprinted so that she can infiltrate a cult’s compound that the Bureau of Alcohol, Tobacco, Firearms and Explosives happens to be very interested in. So that the ATF can keep tabs on what is going on, Echo has a device surgically implanted in her brain that allows the ATF to tap into what her eyes are seeing (for dramatic purposes, the implant’s diversion of her optic signal renders Echo blind.)
This is a technology that has already seen a proof-of-concept demonstration. In 1999, researchers from Berkley and Harvard inserted electrodes into the brains of anesthetized cats that monitored the activity of 177 neurons located in the lateral geniculate nucleus, a key visual processing center. Using a computer to process the signals from the brain, the researchers were able to reconstruct different test images places in front of the cat’s eyes, albeit at a low resolution. While some people see this work as a possible pathway to give sight to the blind, by feeding images into the lateral geniculate nucleus instead of extracting them, it would require (as demonstrated on Dollhouse invasive brain surgery that would carry commensurate risk.