Reflection and Mirrors
Reading Preview
Key Concepts
• Whilt a,e tile kln<k of reflection?
• wtIlIt typft of 1""'9'" are
pro6l.o:ed by plane. <""""""",
and (onftX
mi"",,1
Key Terms
',.y • f~uWr reflKtiorl
•
•
•
•
dithM ...IIKtion
pl_ rrrirror • ~
virtual image
Con<i1"" mirror • optIul axil
• focal point ••ul ~
~ Target Reading Skill
Cont'a<ting
A. you .Nd. compaore and contrast
MId conw.
in a
Venn d~f"" ~t.. ttM. one t>o.low.
Write the .....ilariti<!s in the.pace
wM.e the circles ove.... p and
the diflerffOCg on the left and
«>nC_
mi"""
,Ight~.
(on. . .
DIScover
Activity
How Does Your Reflection Wink?
~
1. took at your face in a mirror. Wink your right
eye. VoIh;ch eye ~ your reflection wink?
2. Tape two mirrors together so that they
open and d~ like a book. Open
them so they form a 9().degree
angle with e.xh other. CAUTION:
8e careful of any $hiIrp
~9M.
3. Looking into both mirrors at 00(",
wink at your reflection il9"in. 'Nhich
eye does your reflection wink !>OW?
• con""" mi,,'Of
Com~fi",.rod
~
MI,ton
Think II Over
Observing How does your refle<11on wink
at you? How doe:; the ~ond reflection
compare with the first retlKtionl
You laugh as )'Qu and a friend move toward the eun'Cd mirror.
First your rdle<:tions look tall and skinny. Thl:"n they become
short and wide, At oue paint, your rcfle<:tions disappear e\'en
though you are still in front of the mirror. Imagine what it
would be like if this happened c\'Cry time you tried to comb
your hair in front of a mirror!
funhouse mirror ...
Chaptet" 4 0 • 113
Reflection of Light Rays
1be rdkclion you sec in a mirror depends on how the surfolCt'
Observing
In a daork room, hold a
flas.hlight nelet to a tablfo.
CAUTION: Do not look
directly Into the flashlight.
Point its beam ~raight up '10
no light mines on the
tabletop. Then hold a metal
un upright 5 em above the
fllls.hlight. Tilt the can '10 its
flat bottom reflect~ light
onto the tabl<!. Try thi:< <>gain
using a white paper cup.
How does the light reflected
by the un compare with the
light reflected by the cup1
retle<:ts light. To show how light rdleo$, )'Ou can represent light
wa\TS as straight lines called rays. You may rec.all lhal light rays
obey the law of refle<:tion----the angle of refleclion equals I~
angle: of incidence.
Figure 7 shows tIVO kinds of retle<:tion. In the choppy water,
)'Ou do not see a dear rdlcction of the person in the OOaL But in
the: smooth water, you s« a sharp retle<:tion. The two ways in
which a sucfa(:e can rdlert light are regular reOection and
diffuse reOection.
Regular Reflection When parallel ... ~ of light hit a smooth
surface. regular refle<:tion OCUln. All of the light ra~ are
rene<:ted at the same angle b«ausc: of the smooth surfa<:e. So,
)'Ou see a sharp reOect.ion.
Diffuse Reflection When parallel rays of light hit II bumpy
or uneven surface, diffuse reflection oc(:urs. Ea(:h light ray
obeys the law of rene<:tion but hits the surface al a different
angle becau5C the surfa(:e is unen:n. Therefore, each ray refllXts
at a different angle. and you don', see a clear rdle<tiOIl.
FtGIJIl[ 7
Diffu~1
and Regular Reflection
The type of reflection that oc:(U~
at II wrliKe depends on whether
the wrliKe i:< 'ough Q( smooth.
Diffuse Reflection
When paralletlight rll)'f f1rike
a rough surface. the TilY' are
reflected at different aogl~
~ ~t) What kind of fUrlac:l rewlts In ditfus.e rdleaion?
FIGI,I1It •
Im.ge In II'tIM Mirror
A pIjnot mirror lormi I
,,;nUll imaogot. The rellecled
light rlYllppNr 10 tome
from betund the mirror.
where the image forTM.
ObMnI"lII Is fhe raMd
hMId in thf! ..... lleh
hMId or I right "-d?
Plane Mirrors
Did you look into a minur this morning to comb fum hair or
brush your t«IM If yuu did,}'OO pr'ONbly used a plane mirror.
A plane mirror is a Oal sheet of glass that h:as .. smooth, silver·
,oloroo coating on one side. Often tht roating is on the ba.:k
of the mirror 10 protect it from damage. Whtn lighl SIriUs a
mirror, the coating (('"eelS the light. Because the GOating is
smOOlh, roesu1ar rdk'<lion occurs and a d~ar image forms. An
image is a copy of an ubje<:1 formed by relleeted or refracted
raysoflighl.
What Kind of Image Fonns The image }'Ou s« in a plane
mirror is I virlual imago---an uprighl image Ihal forms where
lighl $Cnns 10 come from. ·Virtual~ JCKribei something lhat
dots IlOI rall)' aisl. Your image ap~ars to be behind Ihe
mirror, bul)'UU can', reach behind the mirror aootou,h il.
A plane mirror produces a virtual image that is upright
and the same size as the object. But the image is not quitt the
same as the object. lbe l.eft and right of the image are ~.
For exampk-. wbm you Iool: in .. mirror, )'OOr right hand
appears to be a left!wxl in the i~.
How Im.ges Fonn Figure 8 dlow5 how a plane mirror
forms an inlagc. Some lighl rays from the brate "OOent "riu
Ihe mirror and refk.ct low,ud his~. ~ though the' 1'1)'$ a~
rdkcted, Ihe stud'nr's brain treats them as if they haJ comc
from behind the mirror. The dashed lines show where the light
rays appear to rome from. Bccall50C the light "ppc'al'$ 10 rome
from hchind the mirror, this is whl're the studml's image
appears to be localed.
flGUll( 9
focal PalM of;ll Concave Mirror
A concave mirror reflects rays of
light parallel to the optical alh
baock through the focal point.
Concave Mirrors
A mirror wilh a surface Ihal curws inward tik~ the inside of a
bowl is a concave mirror. Figure 9 shows how a conGwe mir­
ror can r~flcci parallel raY!' of light SO Ihatther mcrl at a point.
NOlic~ lhal Ih~ rays o( IiKht shown are parallel to the oplical
axis. The optical axis is an imaginary line thilt divides a mirror
in half, much like the Equator that divides brlh inlo northern
and southern hahu. The poinl al which ra)'$ paralkl to the
optical axis meel is called the (ocal poinl. The IlXation of the
focal point depends on lhe shape of th~ mirror. The mo[~
curved tht:' mirror is, the closeT the focal p·oint is to the mirror.
Representing How Images Form Ray diagrams arc used
Concave mirror
show where a fOCUsN image forms in a concave mirror.
A ray diagr,lm shows rays oflighl coming (rom points on the
objlxl. Two rays coming (rom one point on lhe objCC1 meet or
appear 10 ,neet at th~ corresponding point on Ihe imag~.
Figure 10 shows how a ray diagram is drawn.
10
.......
"
Drawing a Ray Diagram
Ray diagrams sIlowwllere an Image forms and the size 01 the
Image. The steps below 1how how to draw a rit)' diagram,
o
o
Draw a red ray from a
point 00 the object (point A)
10 the mirror. Make thh
'll'I parallel to the optical
;oxis. Then draw the
,eflected ray, whi<h pas"",
through the f~ point.
Draw the green ray from
the s.ame point on the
object to the mirror. Draw
thIs ray as If It COfTIe< hom
the focal point. Then draw
lhe reflected ray, whic:h is
parallel 10 the opti<al axis,
o the
Or_ dashed lines behind
mirror to showwhere
the reflected rays apptar
to come from. The
corresponding point on
the Image Is Iouoted wher~
the d.Mhed lines <J"OSI.
116 • 0
....
'
FtwRt 11
Images In Conava Mirrors
The type of image formed depends
on the location of the ob;ect.
InWr~tl"9 Diagrams When light
rays actually fflel>t, wh.1t tind of
image is formed?
Object
~~)
When the objt<t is
/
~I beyond the fOCiI
polnt, a relll, redoced
image
fOfll~.
the object n blIyond the
focal polnt but near it, I <Nl,
enlarged image fOfIM.
~
.",,",
~
Image
,
'''''''
Wher1 the object is (Iosef than
the !<Xill point a "'n~l. eN;tfged
Image forms.
Determining the Type of Image The tw of image Ihal
is formed by a conan"" mirror depends on the location of the
object. Concave mirrors can form either Yirtual images or
,ta) images. If the object is farther .\\oa)' frOIli the mirror than
the fOC<lI point, the reflected ra)'$ form a rC'a1 image as shown in
foigull' II. A real image forllls when rays actually m«1. Real
images are upside dOWlI, A real image may be larger or smaller
than the object.
If the object is between the mirror and the focal point, the
r(flccte<! rays form a virtual image. The image appears to be
behind the mirror and is upright. Virlual images formed by a
umc;m:: mirror an:: al",ays larger than the object. Concave mir­
rol"l; produce the magnified images }oou see in a makeup mirror.
If an objecl is placed at the focal point, no image furms. But
if a lighl source is placed at the focal poinl, the mirror can
project paralkl rays of light. A car headlight, for ClC.lmple, has a
light bulb althe focal point of a concave mirror. Light hits the
mirror, forming a beam oflighl that shines on Ihe road ahead.
.P~t)
Wh"" ....limage7
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----'''''--'''"/_ "
--. -
-, ~..
OptKal
I
f«al Point The focal
point of a (onvex mirror
is behind the mirror.
..
'-a~--- ---::?;t~~1
•
•
7'
~'....
....
.......nl
,.,...
Virtu" Reduced Image No ~"er ~
where the object is, the image is
virtual, upright. and redlKed.
7.
Convex mirror
FIGtM( 12
ConveK Mirrors
light r~ par;allel to the "Plkal
axis reflect ,"il they (ame from
the focal poionl behind the mirror.
The image formed by a (l,)fl...ex
miHor is always virtual.
Making G.nerali~ations Describe
fix> difKT;ons of lhe ~,.'IeII'iI)"<
reflected by ..
mirror.
COlI"""
"'"dlll"
Convex Mirrors
1\ mirror with a surface that curva outward is called a
convex mirror, Figure 12 shows how convex mirrors reflel;:1
parallel rays of light. The rays spread oul but appear to come
from a foc.11 point behind lhe mirror. The focal point of a cun­
''eX mirror is the lI'Oim from whkh the rays appear 10 come.
Bec1Iuse the rays nCYer meet, images formed by convex
mirrors are always virtual and smaller than the obj«t.
"'erhaps}'OU ha\"e seen lhis warning on a car mirror:KObjects
in mirror are closer than they appear." Com'~ mirrors are used
in cal'lii as ~nger-sKk mirrors. The ad"anlage of a convcx
mirror is that it allows you to see a larger area than you can with
a planc mirror. The disadvantage is lhat the image is reduced in
si~e, so it appears to be farlher away Ihan it actually is.
2 AsseSSll1C'nt
.f@Target Reading Skill
c. Comparing and Contrasting How are
Comparing and Contrasting U5e)'Ou,Venn
diagT';lnl about mirru." 10 hell> )'Otl al1s~r
QuO'Stion 2 bdow.
Reviewing Key Concepts
1. a. Reviewing Whal an! ''''0 kinds of
relle<:lion?
b. bplalning Explain ho",' both kioos u(
rdkctioll olxoy the law of n:lIe<:lion.
t. InfHring Why is an image dear in a shin)'
spoon but (UlZY in a tarnished sptK>n?
2.
~llnlf!9 What is all ima!l"!
b. CI/luifying Which mlrn'll'5C1n form n:aI
iI.
im3{;cs! Which OlIn form
118 • 0
,·;rt~l
iINgc$!
ima~ in COIK;;l,'e mirrors like imagl'S in
con\on mirTOr'$? How are they dirrermt!
Writing in Scienet'
01..' .... At. funhou5e "*"or, your,ounger
btothM- .iiAkeS tilt can ~ Ns linage
d!t'I',l • • he-.lks~'"minor. He
as rou to ....... but your.mwet!Ncb to
more~Write thedl.aogue~might
~ pa.ee bethU.. ~.-.cI row brott-.