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Despite their acronymic similarity, LEDs and LCDs represent

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Despite their acronymic similarity, LEDs and LCDs represent [#permalink] New post 07 Dec 2012, 11:12
Despite their acronymic similarity, LEDs and LCDs represent distinct display technologies. In LEDs, or light-emitting diodes, two different semiconductor materials are layered together: n-type, in which mobile electrons carry negative charge, and p-type, in which “holes” in an otherwise bound sea of electrons carry positive charge. When electric current flows through the p-n junction between layers, an n-type electron falling into a p-type hole releases a photon, a specifically colored particle of light.
The dominant technology currently used in most consumer product displays is the active matrix liquid crystal diode display (LCD). LCDs apply thin-film transistors (TFTs) of amorphous silicon sandwiched between two glass plates. The TFTs supply voltage to liquid-crystal-filled cells, or pixels, between the sheets of glass. Liquid crystals can twist the polarization, or wave orientation, of light. Just as a guitar string can vibrate sideways or up and down, so a light wave can be polarized horizontally or vertically. Polarizing filters act as selective gates, transmitting light polarized one way but not the other. Within a pixel, liquid crystals in their relaxed, coiled state rotate the polarization of ambient light enough to make surrounding filters transparent. Alternatively, applied electrical signals uncoil the crystals, causing the filters to block light and the pixel to become opaque. LCDs that are capable of producing color images, such as in televisions and computers, reproduce colors by blocking out particular color wavelengths from the spectrum of white light until only the desired color remains. The variation of the intensity of light permitted to pass through the matrix of liquid crystals enables LCD displays to present images full of gradations of different colors.
The amount of power required to untwist the crystals to display images is much lower than that required for analogous processes using other technologies, such as plasma. The dense array of crystals displays images from computer sources extremely well, with full color detail, no flicker, and no screen burn-in. Moreover, the number of pixels per square inch on an LCD is typically higher than that for other display technologies; LCD monitors are excellent at displaying large amounts of data with exceptional clarity and precision.

Which of the following can be inferred about uncoiled liquid crystals in an LCD pixel?

A They are opaque to ambient light.

B They are in a relaxed state, in comparison to their high-energy coiled state.

C They are found in one of two wave orientations, horizontal or vertical.

D They fail to rotate the polarization of surrounding photons enough to allow them to pass through nearby filters.

E They cause the pixel to become transparent.
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Re: LEDs and LCDs [#permalink] New post 08 Dec 2012, 11:30
Hi,

Let me see if I can help, here's my thought process...

Which of the following can be inferred about uncoiled liquid crystals in an LCD pixel?
OK. So need to scan through passage to find section talking about uncoiled liquid crystals and read that bit thoroughly...


Polarizing filters act as selective gates, transmitting light polarized one way but not the other. Within a pixel, liquid crystals in their relaxed, coiled state rotate the polarization of ambient light enough to make surrounding filters transparent. Alternatively, applied electrical signals uncoil the crystals, causing the filters to block light and the pixel to become opaque.


Found it. Now for the answer choices...

A They are opaque to ambient light. No, this doesn't work.. We're told as a FACT that it blocks light, we have no reason to assume ambient light is affected any differently to other light, so nothing inferred here.

B They are in a relaxed state, in comparison to their high-energy coiled state. No this is actually incorrect. We're told that crystals are relaxed in their coiled state

C They are found in one of two wave orientations, horizontal or vertical.No, these 2 orientations are mentioned, but not in relation to the liquid crystals

D They fail to rotate the polarization of surrounding photons enough to allow them to pass through nearby filters. Looks good. Correct. We are told that coiled crystals rotate to allow light photons through. We know uncoilded crystals do not allow light photons through, it is inferred that this is the reason why

E They cause the pixel to become transparent. No this doesn't work. We see something about opaque, nothing about any transparency

Hope it helps. James
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Re: LEDs and LCDs [#permalink] New post 09 Dec 2012, 06:31
Which of the following can be inferred about uncoiled liquid crystals in an LCD pixel?

A They are opaque to ambient light.

a very nice distractor, It is surrounding filter which becomes opaque or transparent by the action of crystal not crystal itself

B They are in a relaxed state, in comparison to their high-energy coiled state.


This is opposite answer.

C They are found in one of two wave orientations, horizontal or vertical.
They make light photons polarized not get polarized themselves

D They fail to rotate the polarization of surrounding photons enough to allow them to pass through nearby filters.
A difficult but correct inference. Passage says the coiled ones in relaxed state allow light to pass through filters amd they act as transparent. And the uncoiled ones make filters opaque by doing something to polarization of light. what they do is mentioned in D .

E They cause the pixel to become transparent.

not transparent but opaque
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Re: LEDs and LCDs   [#permalink] 09 Dec 2012, 06:31
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Despite their acronymic similarity, LEDs and LCDs represent

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