how photocopiers work by nafees

7/25/2019 How Photocopiers Work by nafees

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How Photocopiers Work

Walk into almost any business ofce, and you'll probably nd a photocopier

("copier") with a line o people waiting to use it !or most businesses, small or large,

the copier has become standard euipment, much like ha#ing a desk to work at and

a chair to sit in

What i you had to resort to making carbon copies o important documents, as many

people did beore copiers came along$ %r worse, imagine how tedious it would be i

you had to recopy e#erything by hand& ost o us don't think about what's going on

inside a copier while we wait or copies to shoot neatly out into the paper tray, but

it's pretty amaing to think that, in mere seconds, you can produce an eact replica

o what's on a sheet o paper& *n this article, we will eplore what happens ater you

press "+tart" on a photocopier

The Basics

he human-end o making a copy begins with a ew basic steps.


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%pen the copier lid

0lace the document to be photocopied ace-down on the glass

+elect the options you want (number o pages, enlargements, lighterdarker)

0ress the +tart button

What happens inside the copier at this point is amaing& 2t its heart, a copier works

because o one basic physical principle. opposite charges attract

2s a kid, you probably played with static electricityand balloons %n a dry winter

day, you can rub a balloon on your sweater and create enough static electricity in

the balloon to create a noticeable orce !or eample, a balloon charged with static

electricity will attract small bits o paper or particles o sugar #ery easily

A copier uses a similar process.

*nside a copier there is a special drum he drum acts a lot like a balloon --

you can charge it with a orm o static electricity

*nside the copier there is also a #ery ne black powder known as toner he

drum, charged with static electricity, can attract the toner particles

There are three things about the drum and the toner that let a copier

perform its magic:

he drum can be selectivelycharged, so that only parts o it attract toner *n

a copier, you make an "image" -- in static electricity -- on the surace o the

drum Where the original sheet o paper is black, you create static electricity

on the drum Where it is white you do not What you want is or the white

areas o the original sheet o paper to 3% attract toner he way this

selecti#ity is accomplished in a copier is with light -- this is why it's called

a photocopier&

+omehow the toner has to get onto the drum and then onto a sheet o paper

he drum selecti#ely attracts toner hen the sheet o paper gets charged

with static electricity and it pulls the toner o4 the drum

he toner is heat sensitive, so the loose toner particles are attached (used)

to the paper with heat as soon as they come o4 the drum
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he drum, or belt, is made out o photoconductivematerial 6ere are the actual

steps in#ol#ed in making a photocopy.

he surace o the drum is charged

2n intense beam o lightmo#es across the paper that you ha#e placed on the

copier's glass surace 7ight is re8ected rom white areas o the paper and

strikes the drum below

Where#er a photono light hits, electronsare emitted rom the

photoconducti#e atomsin the drum and neutralie the positi#e charges

abo#e 9ark areas on the original (such as pictures or tet) do not re8ect light

onto the drum, lea#ing regions o positi#e charges on the drum's surace

3egati#ely charged, dry, black pigment called toneris then spread o#er the

surace o the drum, and the pigment particles adhere to the positi#e charges

that remain

2 positi#ely charged sheet o paper then passes o#er the surace o the drum,

attracting the beads o toner away rom it

he paper is then heated and pressed to use the image ormed by the toner

to the paper's surace

his diagram helps see the process.

When the copier illuminates the sheet of paper on the glass surface of a

copier a pattern of the image is pro!ected onto the positively charged

photoreceptive drum below. "ight re#ected from blank areas on the pagehits the drum and causes the charged particles coating the drum$s surface

to be neutrali%ed. This leaves positive charges only where there are dark

areas on the paper that did not re#ect light. These positive charges

attract negatively charged toner. The toner is then transferred and fused

to a positively charged sheet of paper.

&nside a Photocopier

* you take a photocopier apart, you might be o#erwhelmed by how many di4erent

parts there are 6owe#er, the actual photocopying process relies on only a ew, key

pieces.

Photoreceptor drum(or belt)

'orona wires

"amp and lenses

Toner
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(user

&n the following sections you$ll learn about each of these parts.

)arious photocopier drums

Photoreceptor *rum

he photoreceptor drum(or, in some photocopiers, belt) is the heart o the

system 2 drum is basically a metal roller co#ered by a layer

o photoconductivematerial his layer is made out o a semiconductorsuch as

selenium germanium or silicon. What makes elements like selenium so cool is

that they can conduct electricity in some cases, but not in others *n the dark, the

photoconducti#e layer on the drum acts as an insulator, resisting the 8ow o

electrons rom one atomto another ;ut when the layer is hit by light, the energy o

the photons liberates electrons and allows current to pass through& hese newly

reed electrons are what neutralies the positi#e charge coating the drum to orm

the latent image

*t's easy to imagine how you might pro


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a small piece o the page he drum actually has to be cleaned, recharged with ions,

eposed to photons, and sprinkled with toner multiple times in order to duplicate

the entire original o the casual obser#er, the process appears continuous, because

it's all seamlessly coordinated inside the photocopier as the drum rotates

'orona Wires

!or a photocopier to work, a eld o positi#e charges must be generated on the

surace o both the drum and the copy paper hese tasks are accomplished by

the corona wires hese wires are sub


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toner particles nd the positi#ely charged ions on the uneposed areas on the

drum's surace much more attracti#e than the weakly charged bead he same

particles are subseuently e#en more drawn to the electro statically charged paper

he plastic in the toner lets you keep it rom


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his image is made #isible and transerred to paper using a special, charged toner

6ere's how it all comes together to make a copy.

1 !or the photocopier to work its magic, the surace o the photoconducti#e

material must rst be coated with a layer o positi#ely charged ions by

the corona wire

1 When you hit the +tart button, a strong lamp mo#es across the inside o the

copier and casts light onto the paper you're copying, and the drum starts to

rotate 2s light re8ects o4 o blank areas o the paper, mirrors direct it

through onto the drum surace 7ike dark clothing on a hot sunny day, the

dark areas o the original absorb the light, and the corresponding areas on

the drum's surace are not illuminated

/ *n the places that light strikes the rotating drum, the energy o

the photonskicks electronsaway rom the photoconducti#e atoms

5 %pposites attract -- the positi#ely charged ions coating the photoconducti#e

layer attract the reed electrons he marriage o one ion and one electron

produces a neutral particle Bharged particles remain only in places where

light didn't hit the drum because it wasn't re8ected rom the original -- the

dark spaces taken up by tet and pictures on the page& his part o the

process loosely resembles how a camera takes a picture * you'#e read 6ow

0hotographic !ilm Works, you know that when lm is eposed to light, the

energy o the photons causes chemical changes in the silver halide grains

coating the lm

: his creates a negative imageo what you see through the #iewnder With

a photocopier, howe#er, you end up with a real imagecreated rom a

pattern o positi#e charges let ater eposure to light 2nd while you ha#e to

de#elop lm using special chemical processes and print it on light-sensiti#e

photographic paper, the photocopier produces a #isible image with only dry

ink, heat and regular paper

= )oltageis applied to the aluminum core o the drum +ince light renders

selenium conducti#e, current can 8ow through the photoconducti#e layer

while the drum is being illuminated, and the electrons released by the atoms

are uickly replaced by the electrons that orm the current 8owing through

the drum

> he eposed areas o the drum rotate past rollersencrusted with beads o

toner iny particles o toner are pressed against the drum's surace he

plastic-based toner particles ha#e a negati#e charge and are attracted to

areas o positi#e charges that remain on the drum's surace
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A The corona wirepasses o#er a sheet o paper so that the paper's surace

becomes electrically charged

C he area o the drum reshly coated with toner spins into contact with a

positi#ely charged sheet o paper he electric eld surrounding the paper

eerts a stronger pull than the ions coating the drum's surace, and the tonerparticles stick to the paper as the drum passes by

D %nce the entire original has been recreated on toner in the page, the paper

proceeds on through the copier to

1EThe fuser he weak attraction between the toner particles and the surace

o the sheet o paper can easily be disrupted o the toner image in place

on the paper's surace, the entire sheet is shunted through the fuser$s

heated rollers he heat melts the plastic material in the toner and uses

the pigment to the page

;y the time you reach or your copy in the collection tray, the photocopier has

already prepared or the net go-round by again cleaning o4 the drum's surace and

applying a resh coat o positi#ely charged ions to it

How "aser Printers Work

he term ink


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*n this article, we'll unra#el the mystery behind the laser printer, tracing a page's

path rom the characters on your computer screen to printed letters on paper 2s it

turns out, the laser printing process is based on some #ery basic scientic principles

applied in an eceptionally inno#ati#e way

The Basics: /tatic 0lectricity

he primary principle at work in a laser printer is static electricity, the same energy

that makes clothes in the dryerstick together or a lightning bolttra#el rom a

thundercloud to the ground +tatic electricity is simply an electrical charge built up

on an insulated ob!ect, such as a balloon or your body +ince oppositely

charged atomsare attracted to each other, ob


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*nitially, the drum is gi#en a total positive charge by the charge corona wire, a

wire with an electrical current running through it (+ome printers use a charged

rollerinstead o a corona wire, but the principle is the same) 2s the drum re#ol#es,

the printer shines a tiny laser beam across the surace to discharge certain points

*n this way, the laser "draws" the letters and images to be printed as a pattern o

electrical charges -- an electrostatic image he system can also work with thecharges re#ersed -- that is, a positive electrostatic image on a negati#e

background

2ter the pattern is set, the printer coats the drum with positi#ely charged toner-- a

ne, black powder +ince it has a positi#e charge, the toner clings to the negati#e

discharged areas o the drum, but not to the positi#ely charged "background" his

is something like writing on a soda can with glue and then rolling it o#er some 8our.

he 8our only sticks to the glue-coated part o the can, so you end up with a

message written in powder

With the powder pattern afed, the drum rolls o#er a sheet o paper, which ismo#ing along a belt below ;eore the paper rolls under the drum, it is gi#en a

negati#e charge by the transfer corona wire(charged roller) his charge is

stronger than the negati#e charge o the electrostatic image, so the paper can pull

the toner powder away +ince it is mo#ing at the same speed as the drum, the paper

picks up the image pattern eactly o keep the paper rom clinging to the drum, it is

discharged by the detac corona wireimmediately ater picking up the toner


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The Basics: (user

!inally, the printer passes the paper through the fuser, a pair o heated rollers 2s

the paper passes through these rollers, the loose toner powder melts, using with


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the bers in the paper he user rolls the paper to the output tray, and you ha#e

your nished page he user also heats up the paper itsel, o course, which is why

pages are always hot when they come out o a laser printer or photocopier

+o what keeps the paper rom burning up$ ainly, speed-- the paper passes

through the rollers so uickly that it doesn't get #ery hot

2ter depositing toner on the paper, the drum surace passes the discharge lamp

his bright light eposes the entire photoreceptor surace, erasing the electrical

image he drum surace then passes the charge corona wire, which reapplies the

positi#e charge

Bonceptually, this is all there is to it % course, actually bringing e#erything

together is a lot more comple *n the ollowing sections, we'll eamine the di4erent

components in greater detail to see how they produce tet and images so uickly

and precisely

The 'ontroller: The 'onversation

;eore a laser printer can do anything else, it needs to recei#e the page data and

gure out how it's going to put e#erything on the paper his is the


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hese days, you might ha#e hundreds o di4erent onts to choose rom, and you

wouldn't think twice about printing a comple graphic o handle all o this di#erse

inormation, the printer needs to speak a more ad#anced language

he primary printer languages these days are 6ewlett 0ackard's Printer 'ommand

"anguage(0B7) and 2dobe's Postscript ;oth o these languages describe thepage in vectororm -- that is, as mathematical #alues o geometric shapes, rather

than as a series o dots (a bitmapimage) he printer itsel takes the #ector images

and con#erts them into a bitmap page With this system, the printer can recei#e

elaborate, comple pages, eaturing any sort o ont or image 2lso, since the

printer creates the bitmap image itsel, it can use its maimum printer resolution

+ome printers use a graphical device interface(H9*) ormat instead o a

standard 0B7 *n this system, the host computer creates the dot array itsel, so the

controller doesn't ha#e to process anything -- it


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The "aser Assembly

+ince it actually draws the page, the printer's laser system -- orlaser scanning

assembly-- must be incredibly precise he traditional laser scanning assembly

includes.

2 laser

2 movable mirror

2 lens

he laser recei#es the page data -- the tiny dots that make up the tet and images

-- one horiontal line at a time 2s the beam mo#es across the drum, the laser emitsa pulse o light or e#ery dot to be printed, and no pulse or e#ery dot o empty

space

he laser doesn't actually mo#e the beam itsel *t bounces the beam o4 a

mo#able mirrorinstead 2s the mirror mo#es, it shines the beam through a series

o lenses his system compensates or the image distortion caused by the #arying

distance between the mirror and points along the drum

Writing the Page

he laser assembly mo#es in only one plane, horiontally 2ter each horiontal

scan, the printer mo#es the photoreceptor drum up a notch so the laser assembly

can draw the net line 2 small printengine computersynchronies all o this

perectly, e#en at diying speeds

+ome laser printers use a strip o light emitting diodes(7J9s) to write the page

image, instead o a single laser Jach dot position has its own dedicated light, which

means the printer has one set print resolution hese systems cost less to
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manuacture than true laser assemblies, but they produce inerior results ypically,

you'll only nd them in less epensi#e printers

7aser printers work the same basic way as photocopiers, with a ew signicant

di4erences he most ob#ious di4erence is the source o the image. 2 photocopier

scans an image by re8ecting a bright light o4 o it, while a laser printer recei#es theimage in digital orm

2nother ma


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assembly he electrostatic image has a stronger negati#e charge than the

de#eloper beads, so the drum pulls the toner particles away

he drum then mo#es o#er the paper, which has an e#en stronger charge and so

grabs the toner 2ter collecting the toner, the paper is immediately discharged by

the detac corona wire 2t this point, the only thing keeping the toner on the pageis gra#ity-- i you were to blow on the page, you would completely lose the image

he page must pass through the user to af the toner he user rollers are heated

by internal-uart% tube lamps, so the plastic in the toner melts as it passes

through

;ut what keeps the toner rom collecting on the user rolls, rather than sticking to

the page$ o keep this rom happening, the user rolls must be coated with Te#on,

the same non-stick material that keeps your breakast rom sticking to the bottom

o the rying pan

'olor Printers

*nitially, most commercial laser printers were limited to monochrome printing (black

writing on white paper) ;ut now, there are lots o color laser printers on the market

Jssentially, color printers work the same way as monochrome printers, ecept they

go through the entire printing process our times -- one pass each or cyan (blue),

magenta (red), yellow and black ;y combining these our colors o toner in #arying

proportions, you can generate the ull spectrum o color

here are se#eral di4erent ways o doing this +ome models ha#e our toner and

de#eloper units on a rotating wheel he printer lays down the electrostatic image

or one color and puts that toner unit into position *t then applies this color to the

paper and goes through the process again or the net color +ome printers add all

our colors to a plate beore placing the image on paper

+ome more epensi#e printers actually ha#e a complete printer unit -- a laser

assembly, a drum and a toner system -- or each color he paper simply mo#es

past the di4erent drum heads, collecting all the colors in a sort o assembly line

Advantages of a "aser Printer

+o why get a laser printer rather than a cheaper ink


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use a laser printer as their "work horse," their machine or printing long tet

documents *n most models, this mechanical efciency is complemented by

ad#anced processing efciency 2 typical laser-printer controller can ser#e

e#erybody in a small ofce

When they were rst introduced, laser printers were too epensi#e to use as apersonal printer +ince that time, howe#er, laser printers ha#e gotten much more

a4ordable 3ow you can pick up a basic model or


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he dots can ha#e di4erent colors combined together to create photo-uality

images

*n this article, you will learn about the #arious parts o an ink


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Thermal wa2printers are something o a hybrid o dye-sublimation and solid

ink technologies hey use a ribbon with alternating BKL color bands he

ribbon passes in ront o a print head that has a series o tiny heated pins

he pins cause the wa to melt and adhere to the paper, where it hardens in

place

Thermal auto chromeprinters ha#e the color in the paper instead o in the

printer here are three layers (cyan, magenta and yellow) in the paper, and

each layer is acti#ated by the application o a specic amount o heat he

print head has a heating element that can #ary in temperature he print

head passes o#er the paper three times, pro#iding the appropriate

temperature or each color layer as needed

%ut o all o these incredible technologies, ink


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Belt- 2 belt is used to attach the print head assembly to the stepper motor

/tabili%er bar- he print head assembly uses a stabilier bar to ensure that

mo#ement is precise and controlled

Paper feed assembly:

Paper tray3feeder- ost ink


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Pie%oelectric- 0atented by Jpson, this technology uses pie%o crystals 2

crystal is located at the back o the ink reser#oir o each nole he crystal

recei#es a tiny electric charge that causes it to #ibrate When the crystal

#ibrates inward, it orces a tiny amount o ink out o the nole When it

#ibrates out, it pulls some more ink into the reser#oir to replace the ink

sprayed out

'lick on the button to see how a pie%oelectric ink!et printer works.

7et's walk through the printing process to see


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C ultiple dots are made at each stop *t sprays the BKL colors in precise

amounts to make any other color imaginable

D 2t the end o each complete pass, the paper eed stepper motor ad#ances

the paper a raction o an inch 9epending on the ink pages per minute(00) o black tet but

take a couple o minutes to print one, ull-color, page-sied image

11%nce the printing is complete, the print head is parked he paper eed

stepper motor spins the rollers to nish pushing the completed page into the

output tray ost printers today use inks that are #ery ast-drying, so that you

can immediately pick up the sheet without smudging it

A typical color ink cartridge: This cartridge has cyan magenta and yellow

inks in separate reservoirs.

Paper and &nk

*nk


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you can't buy a printer rom anuacturer 2 and ink cartridges rom anuacturer ;

hey will not work together

2nother way that they ha#e reduced costs is by incorporating much o the actual

print head into the cartridge itsel he manuacturers belie#e that since the print

head is the part o the printer that is most likely to wear out, replacing it e#ery timeyou replace the cartridge increases the lie o the printer

he paper you use on an ink


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2s stated, eathering is caused by the paper absorbing the ink o combat this, high-

uality ink


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"+tar Wars," "+tar rek," ";attlestar Halactica" -- laser

technology plays a pi#otal role in science ctionmo#ies

and books *t's no doubt thanks to these sorts o stories

that we now associate lasers with uturistic warare and

sleek spaceships

;ut lasers play a pi#otal role in our e#eryday li#es, too

he act is, they show up in an amaing range o

products and technologies Kou'll nd them in

e#erything rom B9 playersto dental drills to high-

speed metal cutting machines to measuring

systemsattoo remo#al, hair replacement, eyesurgery

-- they all use lasers ;ut what is a laser$ What makes a

laser beam di4erent rom the beam o a 8ashlight$

+pecically, what makes a laser light di4erent rom

other kinds o light$ 6ow are lasers classied$

*n this article, you'll learn all about the di4erent types o lasers, their di4erent

wa#elengths and the uses to which we put them ;ut rst, let's start with the

undamentals o laser technology. go to the net page to nd out the basics o an

atom

The Basics of an Atom

The 8ptical *amage

Threshold test station

at 1A/A "angley

4esearch 'enter.
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An atom in the simplest model consists of a nucleus and orbiting

electrons.

here are only about 1EE di4erent kinds o atomsin the entire uni#erse J#erything

we see is made up o these 1EE atoms in an unlimited number o combinations

6ow these atoms are arranged and bonded together determines whether the atoms

make up a cup o water, a piece o metal, or the that comes out o your soda

can&

2toms are constantly in motion hey continuously #ibrate, mo#e and rotate J#en

the atoms that make up the chairs that we sit in are mo#ing around +olids areactually in motion& 2toms can be in di4erent states of e2citation *n other words,

they can ha#e di4erent energies * we apply a lot o energy to an atom, it can lea#e

what is called the groundstate energy leveland go to an e2cited level.he

le#el o ecitation depends on the amount o energy that is applied to the atom #ia

heat, light, or electricity

his simple atom consists o a nucleus(containing the protons and neutrons) and

an electron cloud.*t's helpul to think o the electrons in this cloud circling

the nucleusin many di4erent orbits

Absorbing 0nergy
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Absorption of energy: An atom absorbs energy in the form of heat light

or electricity. 0lectrons may move from a lowerenergy orbit to a higher

energy orbit.

Bonsider the illustration rom the pre#ious page 2lthough more modern #iews o

the atom do not depict discrete orbitsor the electrons, it can be useul to think o

these orbits as the di4erent energy le#els o the atom *n other words, i we applysome heat to an atom, we might epect that some o the electrons in the lower-

energy orbitalOs would transition to higher-energy orbitalOs arther away rom the

nucleus

his is a highly simplied #iew o things, but it actually re8ects the core idea o

how atomswork in terms o lasers

%nce an electron mo#es to a higher-energy orbit, it e#entually wants to return to the

ground state When it does, it releases its energy as a photon-- a particle o light

Kou see atoms releasing energy as photons all the time !or eample, when the

heating element in a toasterturns bright red, the red color is caused by atoms,ecited by heat, releasing red photons When you see a picture on aF screen,

what you are seeing is phosphor atoms, ecited by high-speed electrons, emitting

di4erent colors o light 2nything that produces light -- 8uorescent lights, gas

lanterns, incandescent bulbs-- does it through the action o electrons changing

orbits and releasing photons

The "aser3Atom 'onnection

2 laseris a de#ice that controls the way that energied atoms release photons

"7aser" is an acronym or light ampli,cation by stimulated emission of

radiation, which describes #ery succinctly how a laser works

2lthough there are many types o lasers, all ha#e certain essential eatures *n a

laser, the lasing medium is PpumpedQ to get the atomsinto an ecited state

ypically, #ery intense 8ashes o light or electrical discharges pump the lasing

medium and create a large collection o ecited-state atoms (atoms with higher-

energy electrons) *t is necessary to ha#e a large collection o atoms in the ecited

state or the laser to work efciently *n general, the atoms are ecited to a le#el
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that is two or three le#els abo#e the ground state his increases the degree o

population inversion he population in#ersion is the number o atoms in the

ecited state #ersus the number in ground state

%nce the lasing medium is pumped, it contains a collection o atoms with some

electrons sitting in ecited le#els he ecited electrons ha#e energies greater than

the more relaed electrons @ust as the electron absorbed some amount o energy to

reach this ecited le#el, it can also release this energy 2s the gure below

illustrates, the electron can simply rela, and in turn rid itsel o some energy

his emitted energycomes in the orm o photons(light energy) he photon

emitted has a #ery specic wa#elength(color) that depends on the state o the

electron's energy when the photon is released wo identical atoms with electrons in

identical states will release photons with identical wa#elengths

"aser "ight

7aser light is #ery di4erent rom normal light 7aser light has the ollowing

properties.

he light released is monochromatic.*t contains one specic wa#elength o

light (one specic color) he wa#elength o light is determined by the

amount o energy released when the electron drops to a lower orbit

he light released is coherent *t is PorganiedQ -- each photon mo#es in step

with the others his means that all o the photons ha#e wa#e ronts that

launch in unison

he light is #ery directional 2 laser light has a #ery tight beam and is #erystrong and concentrated 2 8ashlight, on the other hand, releases light in

many directions, and the light is #ery weak and di4use

o make these three properties occur takes something called stimulated

emission his does not occur in your ordinary 8ashlight-- in a 8ashlight, all o the

atoms release their photons randomly *n stimulated emission, photon emission is

organied
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he photon that any atom releases has a certain wa#elength that is dependent on

the energy di4erence between the ecited state and the ground state * this photon

(possessing a certain energy and phase) should encounter another atom that has

an electron in the same ecited state, stimulated emission can occur he rst

photon can stimulate or induce atomic emission such that the subseuent emitted

photon (rom the second atom) #ibrates with the same reuency and direction asthe incoming photon

he other key to a laser is a pair o mirrors, one at each end o the lasing medium

0hotons, with a #ery specic wa#elength and phase, re8ect o4 the mirrors to tra#el

back and orth through the lasing medium *n the process, they stimulate other

electrons to make the downward energy


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/ he 8ash tube res and in


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51

: +ome o these photons run in a direction parallel to

the ruby's ais, so they bounce back and orth o4 the

mirrors 2s they pass through the crystal, they

stimulate emission in other atoms

How Atoms Work


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*t has been said that during the /Eth century, man harnessed the power o the

atom We made atomic bombsand generated electricityby nuclear power We e#en

split the atom into smaller pieces called subatomic particles

;ut what eactly is an atom$ What is it made o$ What does it look like$ he pursuit

o the structure o the atom has married many areas o chemistry and physics in

perhaps one o the greatest contributions o modern science *n this article, we will

ollow this ascinating story o how disco#eries in #arious elds o science resulted

in our modern #iew o the atom We will look at the conseuences o knowing theatom's structure and how this structure will lead to new technologies
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&n a silicon lattice all silicon atoms bond perfectly to four neighbors

leaving no free electrons to conduct electric current. This makes a silicon

crystal an insulator rather than a conductor.

etals tend to be good conductors o electricity because they usually ha#e "ree

electrons" that can mo#e easily between atoms, and electricity in#ol#es the 8ow o

electrons While silicon crystals look metallic, they are not, in act, metals 2ll o the

outer electrons in a silicon crystal are in#ol#ed inperfect covalent bonds, so they

can't mo#e around 2 pure silicon crystal is nearly an insulator-- #ery little

electricity will 8ow through it

;ut you can change all this

What is an Atom; The "egacy of Ancient Times through the


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entitled "he Jtraordinary Bhemistry o %rdinary hings, 5rd Jdition"by Barl 6

+nyder.

1 ake a pile o paper clips (all o the same sie and color)

/ 9i#ide the pile into two eual piles

5 9i#ide each o the smaller piles into two eual piles

: Iepeat step 5 until you are down to a pile containing only one paper clip

hat one paper clip still does the


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or hydrogen peroide) +o, he could not say anything about the numbers o each

atom in the molecules o specic substances 9id water ha#e one oygen with one

hydrogen or one oygen with two hydrogens$ his point was resol#ed when

chemists gured out how to weigh atoms

/implest model of an atom

How @uch *o Atoms Weigh;

he ability to weigh atoms came about by an obser#ation rom an *talian chemist

named Amadeo Avogadro 2#ogadro was working with gases (nitrogen, hydrogen,

oygen, chlorine) and noticed that when temperature and pressure was the same,

these gases combined in denite #olume ratios !or eample.

%ne liter o nitrogen combined with three liters o hydrogen to orm ammonia

(365)

%ne liter o hydrogen combined with one liter o chlorine to make hydrogen

chloride (6Bl)

2#ogadro said that at the same temperature and pressure, eual #olumes o the

gases had the same number o molecules +o, by weighing the #olumes o gases, he

could determine the ratios o atomic masses !or eample, a liter o oygen

weighed 1> times more than a liter o hydrogen, so an atom o oygen must be 1>

times the mass o an atom o hydrogen Work o this type resulted in a relati#e mass

scale or elements in which all o the elements related to carbon (chosen as the

standard -1/) %nce the relati#e mass scale was made, later eperiments were able

to relate the mass in grams o a substance to the number o atoms and an atomic

mass unit (amu) was ound? < amuor *altonis eual to 1>> 1E-/:grams

2t this time, chemists knew the atomic masses o elements and their chemicalproperties, and an astonishing phenomenon


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newly disco#ered atomic masses in cards 6e arranged the elements by increasing

atomic mass and noticed that elements with similar properties appeared at regular

inter#als or periods endelee#'s table had two problems.

here were some gaps in his "periodic table"

When grouped by properties, most elements had increasing atomic masses,

but some were out o order

o eplain the gaps, endelee# said that the gaps were due to undisco#ered

elements *n act, his table successully predicted the eistence o gallium and

germanium, which were disco#ered later 6owe#er, endelee# was ne#er able to

eplain why some o the elements were out o order or why the elements should

show this periodic beha#ior his would ha#e to wait until we knew about the

structure o the atom

The /tructure of the Atom: 0arly th 'entury /cience

o know the structure o the atom, we must know the ollowing.

What are the parts o the atom$

6ow are these parts arranged$

3ear the end o the 1Dth century, the atom was thought to be nothing more than a

tiny indi#isible sphere (9alton's #iew) 6owe#er, a series o disco#eries in the elds

o chemistry, electricity and magnetism, radioacti#ity, and uantum mechanics in

the late 1Dth and early /Eth centuries changed all o that 6ere is what these elds

contributed.

he parts o the atom. chemistryand electromagnetism---

S electron(rst subatomic particle)radioactivity---

S nucleusprotonneutron

6ow the atom is arranged - -uantum mechanics puts it all

together. atomic spectra---S Bohr modelo the atom waveparticle

duality---S Cuantum modelo the atom

'hemistry and 0lectromagnetism: *iscovering the 0lectron

*n the late 1Dth century, chemists and physicists were studying the relationshipbetween electricity and matter hey were placing high #oltage electric currents

through glass tubes lled with low-pressure gas (mercury, neon, enon) much

like neon lights Jlectric current was carried rom one electrode (cathode) through

the gas to the other electrode (anode) by a beam called cathode rays *n 1CDA, a

;ritish physicist,>. >. Thomsondid a series o eperiments with the ollowing

results.
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6e ound that i the tube was placed within an electric or magnetic eld, then

the cathode rays could be de#ected or moved(this is how the the

cathode ray tube (BI) on your tele#isionworks)

;y applying an electric eld alone, a magnetic eld alone, or both in

combination, Thomson could measure the ratio of the electric chargeto the mass of the cathode rays

6e ound the same charge to mass ratio of cathode rays was seen

regardless of what material was inside the tubeor what the cathode

was made o

homson concluded the ollowing.

'athode rays were made of tiny negatively charged particles, which

he called electrons

he electrons had to come from inside the atomso the gas or metalelectrode

;ecause the charge to mass ratio was the same or any substance,

the electrons were a basic part of all atoms

;ecause the charge to mass ratio o the electron was #ery high, the electron

must be very small

7ater, an 2merican 0hysicist named Iobert ilikan measured the electrical charge

o an electron With these two numbers (charge, charge to mass ratio), physicists

calculated the mass o the electron as D1E 1E-/C

grams !or comparison, a G+penny has a mass o /= grams? so, /A 1E/Aor /A billion billion billion electrons

would weigh as much as a penny&

wo other conclusions came rom the disco#ery o the electron.

;ecause the electron was negati#ely charged and atoms are electrically

neutral, there must be a positive charge somewhere in the atom

;ecause electrons are so much smaller than atoms, there must be other

more massive particles in the atom

!rom these results, homson proposed a model o the atom that was like awatermelon he red part was the positi#e charge and the seeds were the electrons
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4utherford$s view of the atom

4adioactivity: *iscovering the 1ucleus the Proton and the 1eutron

2bout the same time as homson's eperiments with cathode rays, physicists such

as by 6enri ;ecuerel, arie Burie, 0ierre Burie, and Jrnest Iutherord were

studying radioacti#ity Iadioacti#ity was characteried by three types o emitted

rays

Alpha particles- positi#ely charged and massi#e Jrnest Iutherord showed

that these particles were the nucleus o a helium atom

Beta particles- negati#ely charged and light (later shown to be electrons)

amma rays- neutrally charged and no mass (ie, energy)

he eperiment rom radioacti#ity that contributed most to our knowledge o the

structure o the atom was done by Iutherord and his colleagues Iutherord

bombarded a thin oil o gold with a beam o alpha particles and looked at the

beams on a 8uorescent screen, he noticed the ollowing.

ost o the particles went straight through the oil and struck the screen

+ome (E1 percent) were de8ected or scattered in ront (at #arious angles) o

the oil, while others were scattered behind the oil

Iutherord concluded that the gold atoms were mostly empty space, which

allowed most o the alpha particles through 6owe#er, some small region of the

atom must have been denseenough to de8ect or scatter the alpha particle 6e

called this dense region the nucleus(seehe Iutherord Jperimentor an

ecellent @a#a simulation o this important eperiment&)? the nucleus comprised

most o the mass o the atom 7ater, when Iutherord bombarded nitrogen withalpha particles, a positi#ely charged particle that was lighter than the alpha particle

was emitted 6e called these particles protonsand realied that they were a

undamental particle in the nucleus 0rotons ha#e a mass o 1>A5 1E -/:grams,

about 1,C5= times larger than an electron&

6owe#er, protons could not be the only particle in the nucleus because the number

o protons in any gi#en element (determined by the electrical charge) was less than
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5D

the weight o the nucleus hereore, a third, neutrally charged particle must eist&

*t was>ames 'hadwick, a ;ritish physicist and co-worker o Iutherord, who

disco#ered the third subatomic particle, the neutron Bhadwick bombarded

beryllium oil with alpha particles and noticed a neutral radiation coming out his

neutral radiation could in turn knock protons out o the nuclei o other substances

Bhadwick concluded that this radiation was a stream o neutrally charged particleswith about the same mass as a proton? the neutron has a mass o 1>A= 1E -

/:grams

3ow that the parts o the atom were known, how were they arranged to make an

atom$ Iutherord's gold oil eperiment indicated that the nucleus was in the center

o the atom and that the atom was mostly empty space +o, he en#isioned the atom

as the positi#ely charged nucleus in the center with the negati#ely charged

electrons circling around it much like a planet with moons 2lthough he had no

e#idence that the electrons circled the nucleus, his model seemed reasonable?

howe#er, it presented a problem 2s the electrons mo#ed in a circle, they would lose

energy and gi#e o4 light he loss o energy would slow the electrons down 7ike

any satellite, the slowing electrons would all into the nucleus *n act, it was

calculated that a Iutherord atom would last only billionths o a second beore

collapsing& +omething was missing&

White light passing through a prism.

Cuantum @echanics: Putting &t All Together

2t the same time that disco#eries were being made with radioacti#ity, physicists

and chemists were studying how lightinteracted with matter hese studies began

the eld o-uantum mechanicsand helped sol#e the structure o the atom

Cuantum @echanics /heds "ight on the Atom: The Bohr @odel

0hysicists and chemists studied the nature o the lightthat was gi#en o4 when

electric currents were passed through tubes containing gaseous elements

(hydrogen, helium, neon) and when elements were heated (eg, sodium, potassium,

calcium, etc) in a 8ame hey passed the light rom these sources through a

spectrometer (a de#ice containing a narrow slit and a glass prism)
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'ontinuous spectrum of white light.

0hoto courtesy 32+2

3ow, when you pass sunlight through a prism, you get a continuous spectrum o

colors like a rainbow 6owe#er, when light rom these #arious sources was passed

through a prism, they ound a dark background with discrete lines

Hydrogen spectrum

0hoto courtesy 32+2

Helium spectrum

0hoto courtesy 32+2

Jach element had a uniue spectrum and the wa#elengtho each line within a

spectrum had a specic energy (see 6ow 7ight Worksor details on the relationship

between wa#elength and energy)

*n 1D15, a 9anish physicist named 1iels Bohrput Iutherord's ndings together

with the obser#ed spectra to come up with a new model o the atom in a real leap ointuition ;ohr suggested that the electrons orbiting an atom could only eist at

certain energy le#els (ie, distances) rom the nucleus, not at continuous le#els as

might be epected rom Iutherord's model When atoms in the gas tubes absorbed

the energy rom the electric current, the electrons became ecited and


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:1

Bohr models of various atoms.

he ma


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:/

;ohr's model was the predominant model until new disco#eries in uantum

mechanics were made

C+A1T+@ @0'HA1&'/

;ranch o physics that deals with the motion o particles by their wa#e properties at

the atomic and subatomic le#el

0lectrons 'an Behave as Waves: The Cuantum @odel of the Atom

2lthough the ;ohr model adeuately eplained how atomic spectra worked, there

were se#eral problems that bothered physicists and chemists.

Why should electrons be conned to only specied energy le#els$

Why don't electrons gi#e o4 light all o the time$ 2s electrons change

direction in their circular orbits (ie, accelerate), they should gi#e o4 light

he ;ohr model could eplain the spectra o atoms with one electron in the

outer shell #ery well, but was not #ery good or those with more than one

electron in the outer shell

Why could only two electrons t in the rst shell and why eight electrons in

each shell ater that$ What was so special about two and eight$

%b#iously, the ;ohr model was missing something&

*n 1D/:, a !rench physicist named "ouis de Brogliesuggested that, like light,

electrons could act as both particles and wa#es (see 9e ;roglie 0hase Wa#e

2nimationor details) 9e ;roglie's hypothesis was soon conrmed in eperimentsthat showed electron beams could be di4racted or bent as they passed through a

slit much like lightcould +o, the wa#es produced by an electron conned in its orbit

about the nucleus sets up a standing wa#eo specic wa#elength, energy and

reuency (ie, ;ohr's energy le#els) much like a guitar string sets up a standing

wa#e when plucked

2nother uestion uickly ollowed de ;roglie's idea * an electron tra#eled as a

wa#e, could you locate the precise position o the electron within the wa#e$ 2

Herman physicist, Werner Heisenberg, answered no in what he called

the uncertainty principle.

o #iew an electron in its orbit, you must shine a wa#elength o light on it that

is smaller than the electron's wa#elength

his small wa#elength o light has a high energy

he electron will absorb that energy
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he absorbed energy will change the electron's position

We can ne#er know both the momentum and positiono an electron in an atom

hereore, 6eisenberg said that we shouldn't #iew electrons as mo#ing in well-

dened orbits about the nucleus&

With de ;roglie's hypothesis and 6eisenberg's uncertainty principle in mind, an

2ustrian physicist named0rwin /chrodingerderi#ed a set o euations or wave

functionsin 1D/> or electrons 2ccording to +chrodinger, electrons conned in

their orbits would set up standing wa#es and you could describe only the probability

o where an electron could be he distributions o these probabilities ormed

regions o space about the nucleus were called orbitalDs %rbitalOs could be

described as electron density clouds he densest area o the cloud is where you

ha#e the greatest probability o nding the electron and the least dense area is

where you ha#e the lowest probability o nding the electron

Cuantum model of a sodium atom.

Wave (unctions

he wa#e unction o each electron can be described as a set o three uantum

numbers.

Principal number 5n6- describes the energy le#el

A%imuthal number 5l6- how ast the electron mo#es in its orbit (angular

momentum)? like how ast a B9spins (rpm) his is related to the shape o

the orbital

@agnetic 5m6- its orientation in space
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*t was later suggested that no two electrons could be in the eact same state, so a

ourth uantum number was added his number was related to the direction that

the electron spins while it is mo#ing in its orbit (ie, clockwise, and

counterclockwise) %nly two electrons could share the same orbital, one spinning

clockwise and the other spinning counterclockwise

he orbitals had di4erent shapes and maimum numbers at any le#el.

s(sharp) - spherical (ma T 1)

p(principal) - dumb-bell shaped (ma T 5)

d(di4use) - our-lobe-shaped (ma T =)

f(undamental) - si-lobe shaped (ma T A)

he names o the orbitalOs came rom names o atomic spectral eatures beore

uantum mechanics was ormally in#ented Jach orbital can hold only twoelectrons 2lso, the orbitalOs ha#e a specic order o lling, generally.

6owe#er, there is some o#erlap (any chemistry tetbook has the details)

he resulting model o the atom is called the -uantum modelo the atom

+odium has 11 electrons distributed in the ollowing energy le#els.

1 one s orbital- two electrons

/ one s orbital- two electrons and three p orbitals(two electrons each)

5 one s orbital- one electron

Iight now, the uantum model is the most realistic #ision o the o#erall structure o

the atom *t eplains much o what we know about chemistry and physics 6ere are

some eamples.


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:=

The modern periodic table of the elements 5elements are ordered based

on atomic number rather than mass6.

Bhemistry.he 0eriodic able- the able's pattern and arrangement re8ects

the arrangement o electrons in the atom Jlements ha#e di4erent atomic

numbers - the number o protons or electrons increases up the table as

electrons ll the shells Jlements ha#e di4erent atomic masses - the number

o protons plus neutrons increases up the table Iows - elements o each row

ha#e the same number o energy le#els (shells) Bolumns - elements ha#ethe same number o electrons in the outermost energy le#el or shell (one to

eight) 'hemical reactions- echange o electrons between #arious atoms

(gi#ing, taking, or sharing) Jchange in#ol#es electrons in the outermost

energy le#el in attempts to ll the outermost shell (ie, most stable orm o

the atom)

0hysics 4adioactivity- changes in the nucleus (ie, decay) emit radioacti#e

particles 3uclear reactors- splitting the nucleus (ssion) 3uclear bombs-

splitting the nucleus (ssion) or orming a nucleus (usion)Atomic spectra-

caused by ecited electrons changing energy le#els (absorption or emission

o energy in the orm o light photons)
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/T@ image 5E nm 2 E nm6 of a single %ig%ag chain of cesium atoms 5red6 on

a galliumarsenside surface 5blue6

'an We /ee Atoms;

2toms are so small that we cannot see them with our eyes (ie, microscopic) ogi#e you a eel or some sies, these are approimate diameters o #arious atoms

and particles.

atom T 1 1E-1Emeters

nucleus T 1 1E-1=to 1 1E -1:meters

neutron or proton T 1 1E-1=meters

electron - not known eactly, but thought to be on the order o 1 1E-

1Cmeters

Kou cannot see an atom with a light microscope 6owe#er, in 1DC1, a type o

microscope called a scanning tunneling microscope 5/T@6was de#eloped he

+ consists o the ollowing.

2 #ery small, sharp tip that conducts electricity (probe)

2 rapid pieoelectricscanning de#ice to which the tip is mounted

Jlectronic components to supply current to the tip, control the scanner and

accept the signals rom the motion sensor

Bomputer to control the system and do data analysis (data collection,

processing, display)

he + works like this.

2 current is supplied to the tip (probe) while the scanner rapidly mo#es the

tip across the surace o a conducting sample
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When the tip encounters an atom, the 8ow o electrons between the atom

and the tip changes

he computer registers the change in current with the ,y-position o the

atom

he scanner continues to position the tip o#er each ,y-point on the sample

surace, registering a current or each point

he computer collects the data and plots a map o current o#er the surace

that corresponds to a map o the atomic positions

he process is much like an old phonograph where the needle is the tip and the

groo#es in the #inyl record are the atoms he + tip mo#es o#er the atomic

contour o the surace, using tunnelingcurrent as a sensiti#e detector o atomic

position

he + and new #ariations o this microscope allow us to see atoms *n addition,the + can be used to manipulate atoms as shown here.

Atoms can be positioned on a surface using the /T@ tip creating a custom

pattern on the surface.

*n summary, science in the /Eth century has re#ealed the structure o the atom

+cientists are now conducting eperiments to re#eal details o the structure o the

nucleus and the orces that hold it together

how photocopiers work by nafees

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