Question:
why does the grey piece of plastic that covers my calculator screen make the numbers visible?
wildmanski
2008-04-07 12:09:21 UTC
when its removed the screen appears blank. the plastic appears to be tinted but nothing more.
Four answers:
2008-04-07 12:17:43 UTC
because its polarised

try this experiment

look at your calc through the lens of a pair of polarised sunglasses then rotate the calc 90 degrees the screen will black out - why

because polarised glass only lets light pass in 1 orientation

-put 2 pieces together - if they are both the same way up they will permit - for example horizontal light but blocks vertical

if you turn the 1 lens through 90 degrees, the 1 piece blocks the horizontal light - the other blocks the vertical - the result? no light gets through



liquid crystal in your calc does the same thing
Katja D
2008-04-07 19:46:16 UTC
That grey piece of plastic is a polarizing filter. A polarizing filter passes only (more or less) light which oscillates at the same angle. As an example it oscillates verticaly, i.e top to bottom of the display. In a calculator you have usualy two of this filters with the same orientation. The liquid cristals are between them, like a sandwich. Normal light from a bulb or the sun oscillates in every possible direction. If light gets on the display only verticaly oscillating light can pass the first filter, the liquid cristals and the second filter. There it will be reflected and does the same way back. You will see a bright display.



Liquid cristals can change the direction of the light oscillation and that effect can be electricaly controled.



So with that effect turned on, light passes again the first filter and becomes verticaly polarized. Than the liquid cristals turns the polarization of the light by 90 degree and the light can not pass the second filter. You get a dark display.



Without the first filter the light doesn't get poliarized and it does not matter if that unpolarized light will be turned or not. A part of it will allways pass the second filter and find the way back. You get allways a bright display.



P.S. The text above describes not the only possible arrangement of filters and cristals, there are many more.

If you are interested have a look at LCD on the wikipedia
Jos v
2008-04-07 19:42:33 UTC
LCD displays work with polarised light, that means that the waves go either Up and Down, or Left to Right. Daylight has both (and everything in between) in roughly equal quantities. Let's call the light UD and LR. UD and LR light enters the display through the plastic filter which removes the LR. The remaining UD light then reflects off the silvery gray screen at the back. If the display, or that part of the number is switched off then the UD light, unaffected by the display shines back out through the filter because the plastic filter allows UD to pass. When part of a number needs to be black the liquid crystals are electrically activated and turn the reflected UD light through 90 degrees into LR light. But this LR light can't get back through the plastic filter so you see no reflected light, part of a number becomes black.



Removing the filter means that both LR and UD light gets through, without filtering out the unwanted LR turning the UD through 90 degrees makes no difference to the unaided human eye.
sospeter m makanya
2008-04-07 19:41:21 UTC
Because the grey piece of plastic allow black ink to transparent easy than other colour when is inside.Normally calculator feeded with blank catilage.


This content was originally posted on Y! Answers, a Q&A website that shut down in 2021.
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