English Assignment

Mirror

EnglishMid-sembreakassignment
Dayangnurshafiqa
2Zamrud
Mr.Iqmal

Background
Thesilvered-glassmirrorsfoundthroughouttheworldtodayfirstgottheirstartinGermanyalmost200
yearsago.In1835,GermanchemistJustusvonLiebigdevelopedaprocessforapplyingathinlayerof
metallicsilvertoonesideofapaneofclearglass.

History
Thesilvered-glassmirrorsfoundthroughouttheworldtodayfirstgottheirstartinGermanyalmost200
yearsago.In1835,GermanchemistJustusvonLiebigdevelopedaprocessforapplyingathinlayerof
metallicsilvertoonesideofapaneofclearglass.

RawMaterial

Glassisconsideredagoodmaterialformirrorsbecauseitcanbemoldedintovariousshapesfor
specialtymirrors.

Glasssheetsaremadefromsilica,ofsilicaisknownasfusedquartz.

TheManufacturingProcess

Cuttingandshapingtheglass
⚫ 1Thefirststepinmanufacturinganymirroriscuttingtheoutlineoftheglass"blank"tosuitthe
application.Ifthemirrorisforanautomobile,forexample,theglasswillbecutouttofitinthemirror
mountonthecar.Althoughsomemirrormanufacturerscuttheirownglass,othersreceiveglassthathas

alreadybeencutintoblanks.Regardlessofwhocutstheglass,veryhard,finelypointedbladesareusedto
dothecutting.Diamondscribesorsaws—sharpmetalpointsorsawswithdiamonddustembeddedin
them—areoftenusedbecausethediamondwillweardowntheglassbeforetheglasswearsdownthe
diamond.Thecuttingmethoduseddependsentirelyonthefinalshapethemirrorwilltake.Inonemethod,
thebladesorscribesmaybeusedtocutpartwaythroughtheglass;pressurecanthenbeusedtobreakthe
glassalongthescoreline.Inanothermethod,amachineusesadiamondsawtocutallthewaythroughthe
glassbydrawingthebladebackandforthorupanddownmultipletimes,likeanautomatedbandsaw.
Cuttingisusuallydonebeforethemetalcoatingisapplied,becausethecoatingmayflakeofftheglassasa
resultofthecut.Analternativetocuttingtheglasstoformblanksistomoldtheglassinitsmoltenstate.
⚫ 2Blanksarethenplacedinopticalgrindingmachines.Thesemachinesconsistoflargebaseplatesfull
ofdepressionsthatholdtheblanks.Theblank-filledbaseisplacedagainstanothermetalplatewiththe
desiredsurfaceshape:flat,convex,orconcave.Agrindingcompound—agrittyliquid—isspreadoverthe
glassblanksastheyarerubbedorrolledagainstthecurvedsurface.Theactionissimilartogrindingspices
withamortarandpestle.Thegritinthecompoundgraduallywearsawaytheglasssurfaceuntilitassumes
thesameshapeasthegrindingplate.Finerandfinergritsareuseduntilthesurfaceisverysmoothandeven.
Handgrindingtechniquesexistaswell,buttheyareextremelytime-consuminganddifficulttocontrol.
Theyareonlyusedincaseswheremechanicalgrindingwouldbeimpossible,asisthecasewithverylarge
orunusuallyshapedsurfaces.Acommercialopticalgrindercanaccommodate50to200blanks,whichare
allpolishedsimultaneously.Thisismuchmoreefficientthanhandgrinding.Evenspecialtyopticscanbe
mademechanicallyinadjustableequipment.

Applyingthereflectivematerial
⚫ 3Whentheglasssurfacesareshapedappropriatelyandpolishedtoasmoothfinish,theyarecoatedwith
whateverreflectivematerialthedesignerhaschosen.Regardlessofthecoatingmaterial,itisappliedinan
apparatuscalledanevaporator.Theevaporatorisalargevacuum chamberwithanupperplatefor
supportingtheblankmirrors,andalowercrucibleformeltingthecoatingmetal.Itissocalledbecause
metalisheatedinthecrucibletothepointthatitevaporatesintothevacuum,depositingacoatingonthe
surfaceoftheglassmuchlikehotbreathwillsteam acoldwindow.Blanksarecenteredoverholesinthe
upperplatethatallow themetalvaportoreachthesurfaceoftheglass.Metalscanbeheatedtoseveral
hundredsorthousandsofdegrees(dependingontheboilingpointofthemetal),beforetheyvaporize.The
temperatureandtimingforthisprocedurearecontrolledverypreciselytoachieveexactlytheright
thicknessofmetal.Thismethodofcoatingcreatesveryuniformandhighlyreflectivesurfaces.
⚫ 4Theshapeoftheholesintheupperplatewillbetransferredtotheglassinmetal,likepaintthrougha
stencil.Thiseffectisoftenusedtointentionallypatternthemirror.Metalstencils,ormasks,canbeapplied
tothesurfaceoftheglasstocreateoneormorepatterns.
⚫ 5Dielectriccoatings—eitherasreflectivelayersorasprotectivelayersovermetalones—areappliedin
muchthesameway,exceptthatgasesareusedinsteadofmetalchunks.Siliconoxidesandsiliconnitrides
aretypicallyusedasdielectriccoatings.Whenthesegasescombineinextremeheat,theyreacttoform a
solidsubstance.Thisreactionproductformsa
coatingjustlikemetaldoes.
⚫ 6Severalevaporationstepsmaybecombinedtomakeamultiple-layercoating.Cleardielectric
materialsmaybeevaporatedontopofmetalorotherdielectricstochangethereflectiveormechanical
propertiesofasurface.Mirrorswithsilveringonthebackoftheglass,forinstance,oftenhaveanopaque
dielectriclayerappliedtoimprovethereflectivityandkeepthemetalfromscratching.One-waymirrorsare
theexceptiontothisprocedure,inwhichcasegreatcaremustbetakennottodamagethethinmetalcoating.
⚫ 7Finally,whenthepropercoatingshavebeenapplied,thefinishedmirrorismountedinabaseor
packedcarefullyinashockresistantpackageforshipping.

Thefuture
A non-reversingmirror,calleda‘truemirror’,wasdevelopedspecificallytoundothedistortion
causedbytraditionalmirrors.Itallowstheusertoseetheirreflectionasothersseethem.Atruemirror
givesyoua3D reflection,whichmovesasyoudo,ratherthangivingyoutheflatimagefrom an
ordinarymirror.

Thankyou!