lecture 11 a reflection

8/9/2019 Lecture 11 a Reflection

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LIGHT

REFRACTION , REFLECTION, GEOMETRIC OPTICS

and PHOTOELECTRIC EFFECT

LIGHT.

Light is a form of energy hi!h tra"e#s in a "a!$$m at a

s%eed of & ' ()* m+s( Light !an e'hi-it %ro%erties of -oth

a"es and %arti!#es . %hotons/+ This %ro%erty is referred to as

a"e0%arti!#e d$a#ity+ The st$dy of #ight, 1non as o%ti!s, is

an im%ortant area in %hysi!s+S$-stan!es hi!h a##o #ight to %ass thro$gh them are !a##ed

trans%arent materia#s and s$-stan!es hi!h %re"ent #ight

%assing thro$gh are !a##ed o%a2$e+

Light tra"e#s in straight #ines hi!h !an -e demonstrated -y

%#a!ing three !ard-oards A, 3 and C and ma1e a %inho#e at

their !entres+ P#a!e a -$rning !and#e on one side of the!ard-oard A and arrange the !ard-oards in s$!h a ay that the

three %inho#es and the !and#e f#ame are in a straight #ine+ The

!and#e f#ame i## -e "isi-#e thro$gh the %inho#e of the

!ard-oard C+ If any one of the !ard-oards is s#ight#y

dis%#a!ed the f#ame i## not -e "isi-#e+ From this it is !#earthat #ight tra"e#s in a straight #ine+

The #ight !an -e seen -y the o-ser"er at O if and on#y if A, 3

and C are in a straight #ine+

http://en.wikipedia.org/wiki/Wave

http://en.wikipedia.org/wiki/Particle_physics

http://en.wikipedia.org/wiki/Photon

http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

http://en.wikipedia.org/wiki/Optics

http://en.wikipedia.org/wiki/Physics

http://images.google.ie/imgres?imgurl=http://image.tutorvista.com/content/reflection-light/rectilinear-propagation-of-light.jpeg&imgrefurl=http://www.tutorvista.com/content/science/science-ii/reflection-light/rectilinear-propagation.php&usg=__ZxpEeQPMW6SF4CKe7Njspqb-bJ0=&h=257&w=469&sz=14&hl=en&start=6&itbs=1&tbnid=vNbcCrZsV3aAxM:&tbnh=70&tbnw=128&prev=/images%3Fq%3Dlight%2Bray%2Btravels%2Bin%2Ba%2Bstraight%2Bline%26gbv%3D2%26hl%3Den

http://en.wikipedia.org/wiki/Wave

http://en.wikipedia.org/wiki/Particle_physics

http://en.wikipedia.org/wiki/Photon

http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

http://en.wikipedia.org/wiki/Optics

http://en.wikipedia.org/wiki/Physics



A#so thro$gh one %inho#e the image is formed -y the #ight

tra"e##ing in straight #ines and is in"erted

A ray of #ight is the dire!tion of the %ath ta1en -y the #ight

A beam is a stream of #ight energy or a !o##e!tion of rays+

A -eam !an -e

(+ Para##e#

A !o##e!tion of %ara##e# rays

4+ 5i"erging

A !o##e!tion of rays di"erging from a %oint+



&+ Con"erging+

A !o##e!tion of rays !on"erging to a %oint+

REFLECTION OF LIGHT.

6hen #ight fa##s on a smooth high#y %o#ished s$rfa!e it is

ref#e!ted i+e t$rned -a!1+ A %ie!e of %o#ished meta# ma1es a

good ref#e!tor+

LAWS OF REFLECTION.

LAW 1. The in!ident ray, the ref#e!ted ray and the norma# a##

#ie in the same %#ane+

Norma# is the dire!tion at 7)o to the dire!tion of the s$rfa!e at

the %oint+

LAW 2. The ang#e of in!iden!e e2$a#s the ang#e of

ref#e!tion+ 3oth ang#es meas$red re#ati"e to the norma#+



Example : Plane Mrr!r. G#ass mirrors ha"e a thin #ayer of

si#"ering de%osited on the -a!1 of the g#ass hi!h is

%rote!ted+ An IMAGE is %rod$!ed in the mirror+ The term

image is $sed for any re%rod$!tion of an o-8e!t -y means ofrays of #ight+

The image in a %#ane mirror is a#ays

( Same si9e as the o-8e!t and the same ay $%

4+ As far -ehind the mirror as the o-8e!t is in front+

&+ T$rned sideays or #atera##y in"erted

:+ In the !ase of %#ane mirrors, the image is said to -e a

"r#$al ma%e+ ;irt$a# images are images hi!h are

formed in #o!ations here #ight does not a!t$a##y rea!h+

Light does not a!t$a##y %ass thro$gh the #o!ation on the

other side of the mirror< it on#y a%%ears to an o-ser"er as

tho$gh the #ight is !oming from this #o!ation+

Note an a#ternati"e des!ri%tion is that

A real ma%e !an -e fo!$sed on a s!reen, hereas a"irt$a# image !an not+



The ang#e of in!iden!e i is the ang#e -eteen the in!ident ray

and the norma#+

The ref#e!ted ray r is the ang#e -eteen the ref#e!ted ray and

the norma#+

The ang#e of in!iden!e i is e2$a# to the ang#e or ref#e!tion r

r i ∠−∠

Lo!ating an Image in a %#ane mirror

Ta1e a %oint o-8e!t O the image is a#so a %oint I

To identify one %arti!$#ar %oint yo$ need to #ines+

Therefore to #o!ate a %oint e i## $se the interse!tion of to

ref#e!ted rays+ From the %oint O %rod$!e to in!ident rays+Ea!h in!ident ray has a ref#e!ted ray

The interse!tion of the ref#e!ted rays indi!ates the #o!ation ofthe image+



In the !ase of the %#ane mirror e noti!e that the ref#e!ted

-eam %rod$!ed is a di"erging -eam and that the rays

APPEAR to -e di"erging from a %oint -ehind the mirror+

The distan!e from M to O = the distan!e from M to I

The ref#e!ted rays form a di"erging -eam hi!h APPEAR to

!ome from I

In a %#ane mirror e !an see that

(+ Image is same si9e as the o-8e!t the same ay $% and

the same distan!e -ehind the mirror as the image is infront



4+ Latera##y in"erted

&+ ;irt$a# Image



C$r"e& Re'le(#!r).

C$r"ed Mirrors are %arts of s%heri!a# s$rfa!es+

There are to ty%es

(+ Con!a"e or Con"erging Mirrors

4+ Con"e' or 5i"erging Mirrors

The mirror is %art of a s%here of !entre ! 1non as the !entreof !$r"at$re of the mirror+

The midd#e %oint % of the mirror is 1non as the %o#e of themirror+

The mirror is symmetri!a# a-o$t the #ine %! hi!h is !a##ed

the PRINCIPAL A>IS+ The radi$s of the s%here of hi!h the

mirror is %art i+e+ PC = r and is 1non as the radi$s of

!$r"at$re+

The idth of the mirror ST is 1non as the a%erat$re+



Con!a"e Mirror

Con"e' Mirror



It !an -e shon that a %ara##e# -eam of #ight in!ident on a

!on!a"e mirror is ref#e!ted to a !on"erging -eam that

%asses thro$gh a %oint F on the %rin!i%a# a'is 1non asthe %rin!i%a# fo!$s+

In a !on!a"e mirror the rays ref#e!ted -y the mirror

a!t$a##y %ass thro$gh F and it is !a##ed a real fo!$s+

The distan!e from F to the %o#e of the mirror P is !a##ed

the fo!a# #ength of the mirror, f

F P = f

Any in!ident ray %ara##e# to the %rin!i%a# a'is is ref#e!ted

so as to %ass thro$gh the %rin!i%a# fo!$s F+



In a !on"e' mirror hen a %ara##e# -eam is in!ident on

the mirror the ref#e!ted rays form a di"ergent -eam+

Hoe"er hen %rod$!ed -a!1 a## the rays seem to !ome

from one %oint F -ehind the mirror+

The rays on#y a%%ear to !ome from F and it is said to -e a

"irt$a# fo!$s+

Again

F P = f fo!a# #ength

For -oth mirrors

F P = f fo!a# #ength

C P = r radi$s of !$r"at$re

It !an -e shon that4

r f =



If the o-8e!t in a !$r"ed ref#e!tor is a %oint O then e find

the %oint of the image -y the %oint of interse!tion of to

ref#e!ted rays+

There is a set of r$#es to fo##o hen finding rays

(+ Rays in!ident at the %o#e are ref#e!ted ma1ing the same

ang#e ith the %rin!i%a# a'is+

4+ Rays %assing thro$gh the !entre of !$r"at$re are ref#e!ted

-a!1 a#ong their on %ath+

&+ Rays %ara##e# to %rin!i%a# a'is are ref#e!ted thro$gh the

%rin!i%a# fo!$s+

:+ Rays thro$gh the %rin!i%a# fo!$s are ref#e!ted %ara##e# to

the %rin!i%a# a'is+



NOTES

(+ A REAL image is formed -y the a!t$a# interse!tion of

ref#e!ted rays+

4+ A ;IRT?AL image is one formed -y the a%%arent

interse!tion of ref#e!ted rays hen their dire!tions ha"e

-een %rod$!ed -a!1ards+

&+ The %rin!i%#e of re"ersi-i#ity states that #ight i## fo##o

e'a!t#y the same %ath if its dire!tion of tra"e# is re"ersed+

If a #ight ray tra"e#s from A to 3 a#ong a %ath it i## tra"e#from 3 to A -a!1 a#ong the same %ath in the re"ersedire!tion+

:+ The dire!tion 8oining any %oint on the mirror to the %oint

! .the !enter of !$r"at$re / is the Norma# to the mirror at

that %oint+

@+ A## ang#es are meas$red re#ati"e to the Norma#+

?sing the a-o"e r$#es e may no easi#y find the images

formed -y a !on!a"e mirror+



For ea!h image formed e need to determine

.i/ Its #o!ation re#ati"e to the mirror+

.ii/ 6hether it is rea# or "irt$a#+

.iii/ 6hether it is $%right or in"erted+

.i"/ Its si9e re#ati"e to the o-8e!t+

The !hosen o-8e!t is a "erti!a# #ine ith to %oints .e+g+

arro or !and#e/ To #o!ate the image of the o-8e!t e m$st

#o!ate the image of -oth the -ottom and the to% %oint+ The -ottom of the arro is %#a!ed a#ong the %rin!i%a# a'is so that

its image is a#ays at some %oint a#ong the %rin!i%a# a'is+

The image of the to% %oint is #o!ated -y the interse!tion ofto ref#e!ted rays+ The to image %oints are 8oined together

to %rod$!e the image+



(+ O-8e!t -eteen F and P

Image is.i/ 3ehind the mirror

.ii/ ;irt$a#

.iii/ ?%right

.i"/ Larger than the o-8e!t+

4+ O-8e!t at F

Image is at infinity



&+ O-8e!t -eteen F and C

Image is

.i/ 3eyond C

.ii/ Rea#

.iii/ In"erted

.i"/ Larger than o-8e!t

:+ O-8e!t at C

Image is

.i/ At C

.ii/ Rea#

.iii/ In"erted

.i"/ Same si9e as o-8e!t



@+ O-8e!t -eyond C

Image is

.i/ 3eteen F and C

.ii/ Rea#

.iii/ In"erted

.i"/ Sma##er than o-8e!t

+ O-8e!t at infinity

Image

.i/ At F

.ii/ Rea#

.iii/ In"erted

.i"/ Sma##er than o-8e!t



Images in a !on"e' mirror

E'%eriments sho that the image formed in a !on"e' mirror

is a#ays

.i/ ;irt$a#

.ii/ ?%right

.iii/ Sma##er than o-8e!t

Regard#ess of here the o-8e!t is #o!ated+

Therefore If the image is REAL the mirror is CONCA;E

If the image is ;IRT?AL the mirror is

CONCA;E IF THE IMAGE IS LARGER AN5

CON;E> IF THE IMAGE IS SMALLER



S$mmary Mirrors Con!a"e

a/5istant

o-8e!t

Rea#

In"erted

Sma##er than o-8e!t

At F

-/ O-8e!t

-eyond C

Rea#

In"erted

Sma##er

3eteen C and F

!/ O-8e!t at C

Rea#

In"ertedSame si9e as o-8e!t

At C

d/ O-8e!t

-eteen C

and F

e/ O-8e!t

-eteen F

and P

Rea#

In"erted

Larger

3eyond C

;irt$a#

Ere!t

Larger than o-8e!t

3ehind mirror

f/ O-8e!t at F No imageRef#e!ted rays

are %ara##e#

Ima%e) '!rme& by a &"er%n% )p*er(al mrr!r

regard#ess of o-8e!t

%osition IMAGE IS

;irt$a#

Ere!t

Sma##er than o-8e!t3ehind mirror -eteen F



and P

MIRROR E+,ATION

A## distan!e "a#$es are meas$red ith the mirror as the origini+e+ the %o#e P 6e $se the fo##oing sym-o#s

$ = distan!e from the o-8e!t to the mirror

" = distan!e from the image to the mirror

f = fo!a# #ength distan!e from fo!$s to mirror

For a s%heri!a# mirror it !an -e %ro"ed that

f vu

(((=+

and a#so that the magnifi!ation m

u

v

m ==

o-8e!tof height

imageof height

A !on"ention that e !an $se for mirrors is the REAL IS

POSITI;E !on"ention

It is that any distan!e from a rea# o-8e!t is ta1en as a %ositi"e

"a#$e and any "irt$a# distan!e is gi"en a negati"e "a#$e+

REAL B;E AN5 ;IRT?AL ;E

Con!a"e mirror Rea# fo!$s so f is B"e

Con"e' mirror ;irt$a# fo!$s so f is 0"e



PRO3LEM SHEET MIRROR FORM?LA

D$estion (+ A !on!a"e mirror has a radi$s of !$r"at$re of :)!m+ A rea# o-8e!t @ !m high is %#a!ed &) !m in front of the

mirror .i/!a#!$#ate the %osition and height of the image+ Is the

image rea# or "irt$a# .ii/ #o!ate the image $sing ray tra!ing

.iii/ re%eat the !a#!$#ations if the o-8e!t is %#a!ed ) !m from

the mirror .ii/ re%eat for a !on"e' mirror+

D$estion 4+ An o-8e!t is %#a!ed 4@ !m from a !on!a"e mirror of radi$s *) !m+ .i/ 5etermine the %osition re#ati"e si9e and

ty%e of image that is %rod$!ed .ii/ re%eat for a !on"e' mirror

.iii/ sho the ray tra!ing

D$estion &+ .i/ 6hat ty%e of mirror sho$#d -e $sed in order to

gi"e an ere!t image (@ th the si9e of an o-8e!t %#a!ed (@ !m

in front of it .ii/ !a#!$#ate the fo!a# #ength of the mirror+

D$estion :+ It is desired to !ast the image of a #am%

magnified @ times $%on a a## (4 m from the #am%+ 6hat

1ind of mirror is re2$ired and hat is its fo!a# #ength

lecture 11 a reflection

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