Most LCDs, such as your smartphone and tablet, use a polarizing filter to help you see the screen in bright sunlight. But so do polarized sunglasses, meaning the two essentially cancel each other out, causing your LCD screen to appear dark or completely black when you look at it. Polarized sunglasses are designed to block glare — overly bright light reflected off shiny surfaces such as water and snow. Natural light consists of protons bouncing in many directions; polarized lenses filter that light, causing those protons to travel in a single, uniform direction usually horizontal.
Polarized sunglass lenses are coated with a chemical compound composed of molecules that are parallel to one another. LCD screens and sunglasses typically contain a polarizing filter for the same reason: to make it easier for you to see clearly, especially in bright sunlight.
Normally, LCDs are designed with not one but two polarizing filters. The liquid crystals used in the LCD are sandwiched between these two polarizing filters. Like polarizing sunglasses, these filters work by only allowing light in a certain pattern or orientation to pass through. The two polarizing filters of LCDs are arranged so that light becomes polarized as it passes through the first filter.
Recently I've dug out my polarized sunglasses for driving and noticed a number of interesting phenomenon. First, I assume that the sunglasses are vertically polarized because most glare experienced when driving is from horizontal objects - car roofs, wet roads, lakes, etc - making the reflected light horizontally polarized, so vertical polarization removes this glare.
Based on this assumption, my digital watch must also be vertically polarized so that it is its brightest when my arm is horizontal and I'm wearing sunglasses; when I rotate my arm 90 degrees so that it is vertical the watch display goes completely black, as expected. However, the LCD displays in my car Citroen C4 are polarized at 45 degrees; those in the central console speedometer and stereo are polarized 45 degrees clockwise from vertical and the display above the steering wheel the rev counter is polarized 45 degrees anti-clockwise from vertical.
This means the displays on the left central console, right-hand drive car go black when I tilt my head to the left ie, towards the displays and conversely the display above the steering wheel goes black when I tilt my head to the right when wearing the sunglasses.
So why are they rotated 45 degrees? I suspect it is because the displays can be read equally when wearing vertically polarized glasses whether rotated clockwise or anti-clockwise, but I do not know why they are not vertically polarized like my digital watch. As suggested above by David Carey, it may be because the displays can be mounted horizontally or vertically without the risk of appearing totally black when wearing polarized sunglasses, which is vitally important when the display is the speedometer of a car!
Slight correction to the posts above. When light encounters non-absorbing surfaces, a fraction of the incident light is transmitted through the surface and the remaining fraction is reflected.
For dielectric surfaces water, glass etc light that polarised in the plane of the surface s-wave is generally preferentially reflected, whereas light that polarised perpendicular to this p-wave is preferentially transmitted. Light from the sun is mostly unpolarised containing equal amounts of s and p wave light. Sunlight reflected from a surface becomes partially polarised in the plane of the reflecting surface.
If this surface is horizontal the light will be partially polarised horizontally. The polaroid in sunglasses should be orientated to admit only light with a vertically polarised component - that is, the polaroid is orientated to filter out the polarised reflection from horizontal dielectric surfaces.
This property eliminates the glare from water, for example, so that fishermen can see the fish below, or so that a driver is not blinded by reflections from a wet road.
Sign up to join this community. However, in an LCD, the brightness of each pixel needs to be independently controlled. Since a typical display has millions of pixels, it would be impractical to mechanically rotate polarizers within each one of the pixels. In the next section, we learn about a new type of material called a liquid crystal and how it helps overcome this problem.
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