RC: Despite their acronymic similarity, LEDs and LCDs...

[NL]

Ron, how do you read this RC passage to answer the questions?


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.

1/ The passage indicates that each of the following may be considered an advantage of LCDs relative to other display technologies EXCEPT:

A. they consume less power
B. they generally have more pixels per square inch of the display
C. they are able to replicate computer images with little to no loss of detail
D. they reflect a widely adopted standard
E. they represent the latest, most advanced technology


2/ Which of the following can be inferred about uncoiled liquid crystals in an LCD pixel?
A. Electric currents cause them to release photons.
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.


3/ The process through which an LCD monitor displays different colors is most closely analogous to

A. the partial blocking of an hourglass so that a limited stream of grains of sand fall into the lower portion
B. the use of rigid sizing boxes at an airport security checkpoint in order to allow the passage of certain sizes of luggage while excluding other sizes of luggage
C. the soundproofing of a recording studio so that any performances within are muted to those outside
D. the cutting out of characters from a sheet of paper so that a lamp in front of the paper casts shadows in the shapes of the characters
E. the emission of warmer air by an air vent on the outside of a building while an air conditioning system cools the interior of the building


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My difficulties are:

- The main idea that I get from reading is very general, so it helps little to answer the questions or even to predict the answers. (Main idea: the difference between LEDs and LCDs is something about technology. LEDs is in the 1st paragraph, LCDs is in the 2nd one. That is.)

- When answering the questions, although I know where the detail information located, I can't figure out the answers because right after my eyes turn out (from the passage) to check the choices, nothing about details sticks in my mind. It likes in a thick-dark forest, although standing right on the area that has gold, I can't find any because I don't have a flash-light.

(MGMAT CAT. Answers: 1.E; 2.D; 3.B)
Thank you.

[RonPurewal]

Well, there are basically just 2 reasons to "read" the passage in the first place (quotes around "read" because you shouldn't actually be reading too much of it, especially when it's full of scientific detail):
1/ To get the main idea;
2/ To create a rough mental "index" that will help you locate specifics more quickly.
Reading for any other reason would be pointless, as you won't have any idea what the questions will be like.
So, your relatively vague summary of the passage is, in my opinion, pretty much exactly what you should be doing.

[RonPurewal]

In terms of your "inability to see the forest": Yeah, I'm with you on that. There's no way I could retain this kind of detail in my memory"”especially not while trying to think through a test question!

Now, time for the good news: There's a technique that can help here. You ready for it?
Here it is:
Write stuff down.
So you can look at it with your eyes.

Basically, here's the best practice for answering detail questions:

1/
Recognize that it's a detail question.
I.e., it has some specific keyword(s) that narrow the search to at most a few lines of the passage.

2/
Decide EXACTLY what you need to go find.
Imagine you're looking for something in a long encyclopedia article, or in a big reference book. If you don't have a very specific search in mind, you could be scanning that thing for days.

3/
Find it.
You should very seldom need to read more than a few lines of the passage (although you may need to scan a larger area of text to find that focus in the first place).

4/
WRITE DOWN an INVENTORY OF EXACTLY WHAT THE PASSAGE ACTUALLY SAYS about it.
You should probably abbreviate to save time"”but don't abbreviate so heavily that you can't understand your own notes.

5/
Look for a match.

This.

[RonPurewal]

Let's try this for question #2.

1/
Coiled and/or uncoiled thingys are not a main theme"”i.e., they are not the subject of the entire passage"”so they're a keyword, and this is a detail question. ("LCD" and "LED" are not keywords in this passage, since the entire passage is about those things.)

2/
The question doesn't ask about any specific feature/aspect/behavior of uncoiled thingys, so I just need to do a general "fact dump" about them.
Okay. I have a goal.
Search topic: What is EVERYTHING THE PASSAGE ACTUALLY SAYS about coiled and uncoiled thingys?
Note that coiled things are part of this, too. If the passage presents information about them side-by-side, then there's clearly going to be a contrast. (I.e., if you see "Coiled thingys do X" and then the passage immediately starts talking about uncoiled thingys, then they must not do X. Otherwise that whole paragraph structure would be nonsense/misleading.)

3/
I find coiled and uncoiled thingys.

4/
What is EVERYTHING THE PASSAGE ACTUALLY SAYS about them?
Not much:
* Coiled=relaxed "”> rotate light "”> surrounding filters transparent
* Uncoiled "”> nearby filters opaque
(Note that I have absolutely no idea what "surrounding filters" are, nor do I care. Also, I didn't write "block light" because that's the definition of "opaque".)
... aaaand that's absolutely EVERYTHING THE PASSAGE ACTUALLY SAYS about these things.

5/
Answer choices.
* A and C have nothing to do with this inventory at all.
* B and E contradict facts that are in the inventory.
* D wins.

[RonPurewal]

If you think about this process for a while, you'll realize that it's similar to a dispute over a contract. With contracts, after all, "main ideas" and "themes" are, essentially, non-things; in the end, all that matters is what the contract actually says about the topic in dispute.

In fact, this is the motivation behind the GMAT's way of testing details.
The GMAT is for future managers; managers regularly have to resolve disputes based on contracts or other policy instruments / systems of rules. So, that's the skill set they're testing.

Main idea questions test a completely different skill set"”the ability to scan through a highly technical document, quickly, without necessarily understanding all the details, and still grasp its relevance to the company's larger goals and overall situation.
It's not a manager's job to understand the details; (s)he delegates that task to the appropriate department(s).
The manager's job is to figure out overall relationships. "Yeah, ok, you just dumped this 109-page tech spec on my desk. What does this mean for the client? For our manufacturers/subcontractors/licensees? For our profit margin? For the competition? For our public image/marketing?" Etc.

If you understand why the problems work the way they do, you'll find it much easier to attack them in a way that makes sense.

[NL]

Very well-organized explanation, Ron! It cracked my stubborn brain, making it give up "what if" and start digging.

I’ll try the remaining questions:

3/ The process through which an LCD monitor displays different colors is most closely analogous to

Basically, it requires us to understand details.
- What details? How LCD displays colors
- Where is that? Near the end of 2nd paragraph

"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."

- Write down: block "whole colors" to get desired colors.
- Search choices:

A. the partial blocking of an hourglass so that a limited stream of grains of sand fall into the lower portion
B. the use of rigid sizing boxes at an airport security checkpoint in order to allow the passage of certain sizes of luggage while excluding other sizes of luggage
C. the soundproofing of a recording studio so that any performances within are muted to those outside
D. the cutting out of characters from a sheet of paper so that a lamp in front of the paper casts shadows in the shapes of the characters
E. the emission of warmer air by an air vent on the outside of a building while an air conditioning system cools the interior of the building

B is closest.
(Honestly, I would eliminate to A and B, then chose B because my brain thinks about "technological beam" here. I don’t reason too much in this case)


Q1.

1/ The passage indicates that each of the following may be considered an advantage of LCDs relative to other display technologies EXCEPT:

This is also a detail question.
- What details? NOT an advantage of LCDs.

- Where is that? Spreading out through 2nd to 3rd paragraph.
Hum, it’s challenging. I didn’t see "advantage" or similar words anywhere.

- Write down: nothing.
- Search choices:

"A. they consume less power
B. they generally have more pixels per square inch of the display
C. they are able to replicate computer images with little to no loss of detail
D. they reflect a widely adopted standard
E. they represent the latest, most advanced technology"

In this case, I'd read each choice, then check it back to the passage. Time consuming!
(I think the answer for this kind of question is often a too-far assumption or a distorted detail, but I better guess and move on soon if I fall back in time frame)

[NL]

If you understand why the problems work the way they do, you'll find it much easier to attack them in a way that makes sense.

This is a lesson for life.

[RonPurewal]

This exam can actually teach you a lot about real-life problem solving, if you let it.
Not enough people let it.