2022-01-25 ศัพท์ น่าสับสน - Set – C - concave & convex

Revision C

2022-01-25

ศัพท์ น่าสับสน - Set – C - concave & convex

แนะนำการใช้ ตามที่ส่วนใหญ่ใช้ แต่ละท้องถิ่น 

ความหมาย อาจผันแปร ตาม ตำแหน่ง/หน้าที่ ในประโยค

 

Dictionary.com:

ออกเสียง concave – adj. & verb = “kon-KEYV, KON-keyv” 

         – noun = “KON-keyv

ออกเสียง convex – adj. = kon-VEKS or Kuhn”

 – noun = “KON-veks”

 

Dictionary of Problem Words and Expressions:

concave & convex

Concave means “curved inward,” and 

Convex means “curved outward.”

That is, concave means having a surface 

that is curved or rounded inward, 

like a segment of the interior of a hollow sphere or circle.

In keeping the meaning of concave straight, 

it may help to think of a cave 

as something that actually turns or curves inward from the earth.

Convex means precisely the opposite.

 

Dictionary.com:

“Concave” vs. “Convex”: What’s The Difference?

Published July 13, 2021

Concave and convex are literal opposites

—one involves shapes that curve inward 

and the other involves shapes that curve outward. 

The terms can be used generally, 

but they’re often used in technical, scientific, and geometric contexts.

 

Lenses, such as those 

used in eyeglasses, magnifying glasses, binoculars, and cameras

are often described as concave or convex,

depending on which way they curve. 

 

In geometry, 

both words can be used to describe 

the shape of POLYGONS, 

depending on whether they have any inward-facing angles.

 

Not only will this article help you remember which is which, 

you’ll also learn a little bit of the science behind 

why concave and convex shapes

create different OPTICAL effects in lenses and mirrors.

⚡️Quick summary

Concave means curving inward

—like the shape of the inside of a bowl. 

Convex means curving outward

—like the shape of the outside of a contact lens. 

 

To put it another way,

a concave shape can be filled, 

while a convex shape creates a dome.

 

What does concave mean?

CONCAVE means “curving inward” or “hollow and curved,” 

like the inside of a bowl, 

or, more technically, 

the inside of a partial sphere. 

A crater is roughly concave.

 

What does convex mean?

CONVEX means “having a surface that is curved or rounded outward,”

like the outside of a contact lens 

(the part that a person touches when putting it on their eye). 

A mound is roughly convex.

 

Concave vs. convex lenses

Concave lenses

  - those that are thicker around the edges 

    than they are in the center

-  light rays, which can have the effect of making things 

look smaller or farther away. 

Convex lenses

-  those that dome outward 

    and are thicker in the center 

    than they are around the edges

—make light rays converge. 

 

Convex lenses have the opposite effect of concave ones: 

they can make things look closer or bigger.

 

A lens is a transparent medium bound by two surfaces. 

There are mainly two types of lens. 

1) Convex lens 

2) Concave lens

 

Magnifying glasses and some binoculars use convex lenses. 

Concave lenses are used in movie projectors 

to spread out the image onto the screen 

(if you look back through the lens, inside the projector, the image will look tiny). 

Some optical devices, like telephoto lenses for cameras, 

use a combination of concave and convex lenses.

Concave lenses are used in eyeglasses 

for people who are nearsighted 

(those who have trouble seeing things that are far away). 

Convex lenses are used in glasses 

for people who are farsighted 

(those who can’t see things very well up close). 

 

This may seem counterintuitive,

but it all has to do with

how the lenses help the image get processed by the eye.

 

Convex lenses sometimes have 

the effect of inverting far-away images. 

 

Many binoculars that use convex lenses solve this issue 

through the use of prisms that turn the image right-side up. 

A similar thing happens with your eyes. 

Because the eyeball is curved, 

the images that pass through it are inverted, 

and the brain has to flip them so you can see them the right way.

 

Concave vs. convex mirrors

Convex mirrors have the effect of making things look farther away 

and allowing for a field of view that’s wider 

than the one you would normally be able to see. 

 

Most cars have convex passenger side mirrors 

that allow the driver to see objects 

that would normally be in the vehicle’s blind spot. 

This is why the mirror says 

Objects in mirror are closer than they appear 

(they look farther away than they actually are).

 

Concave mirrors

—like that one they stick in your mouth at the dentist’s office

—are used for close-up work 

because they make the things they reflect 

easier to see by making them look bigger.

 

Reflective objects that are concave 

have the ability to invert the images they reflect. 

 

Grab a spoon from the silverware drawer and you can see this in action. When you look at the concave part of the spoon

—the bowl part that you scoop up soup with

—you’ll see that your reflection is upside down. 

The concave side shows an upside-down image 

because the top part reflects light downward 

and the bottom part reflects light upward. 

Flip the spoon over to the convex side and your reflection will be upright.

 

Concave vs. convex polygons

In geometry, 

concave has a more specific meaning that’s used to describe 

the shape of a polygon (multi-sided figure) 

that has at least one interior angle greater than 180 degrees 

(translation: it’s a shape with at least one inward angle). 

 

Convex polygons are those in which all the angles are no greater than 180 degrees—meaning that none of them point inward.

For example, 

a classic star shape is a concave polygon 

because each of the arms of the star comes inward 

to produce an inward-facing angle. 

An octagon—like a stop sign—is a convex polygon, 

because all of its angles point outward.

 

Where do the words concave and convex come from?

Concave comes from the Latin concavus, 

meaning “arched” or “hollow,” from the root cavus, 

which also means “hollow”

and is also the basis of the word cave. 

Convex comes from the Latin convexus, 

meaning “vaulted” or “rounded.”

 

How to use concave vs. convex

Here’s the best way to remember the difference: 

concave things curve or turn inward, 

          like the inside of a cave. 

Convex things curve or turn outward

          —regular hexagons are excellent examples of convex polygons.

 

Of course, generally speaking, 

some shapes could be considered both concave and convex 

depending on how you’re looking at them. 

In a semicircle, for example, the part that curves inward is concave, 

and the part that curves outward is convex.

 

Examples of concave and convex used in a sentence

Concave and convex are mostly used in technical, scientific, and geometric contexts. 

You might use them in everyday descriptions, 

but you’re more likely to use less formal words like rounded or curved.

 

Still, they may be the best way to be clear about the shape of something. 

• Our company manufactures convex lenses for a variety of applications, including telescopes and binoculars.
• The convex surface of the magnifying glass lens is what produces the magnification.
• Most of the basic polygons that you’re familiar with are convex polygons, including triangles, rectangles, pentagons, hexagons, and octagons.
• Many cameras use both concave lenses and convex lenses to provide the ability to zoom in and out.
• The description said the car has “several concave surface features”—which was a misleading way of saying “dents.”

 

Merriam-Webster Dictionary:

Usage Notes

'Concave' vs. 'Convex'

A simple mnemonic device should help

What to Know

Concave means "hollowed out or rounded inward" 

and is easily remembered because these surfaces "cave" in. 

The opposite is convex meaning "curved or rounded outward." 

Both words have been around for centuries but are often mixed up.

 

Merriam-Webster is a very serious company 

(although we prefer to be thought of as a lexicographic concern, 

employing sense 4 of concern as a noun), 

and so when we are asked to help people distinguish 

between two easily confused words 

we do not wallow in silly little mnemonics.

 

No! Not for us are the vapid memory aids, 

the admonitions to “remember that X begins with a Y.” 

We instead provide you with serious tools, 

such as the etymology of each word, 

and thus allow you to distinguish between them as a scholar would.

 

Remembering 'Concave' vs. 'Convex'

So if you need to tell the difference 

between concave and convex 

simply remember that convex comes from 

the Latin word convexus,

which may mean either “convex” or “concave” or … wait a minute. 

Please disregard our earlier advice.

 

If you need to distinguish 

between these two words simply remember 

that concave has the word cave in it 

(because it is like a cave) 

and means “hollowed or rounded inward like the inside of a bowl”. 

 

Convex has the word vex in it 

(because it is vexing that this word is hard to remember), 

and means “curved or rounded outward.”

 

Neither word is particularly recent; 

concave has been in English since the 15th century, 

and convex since the 16th.

 

Each of these words has multiple meanings, 

but the reason most people look them up, 

we are fairly certain, is simply because 

they have trouble remembering which one bows out 

and which one bows in. 

Sometimes silly little mnemonics are the best tool for the job.