Week 21 Lens Presentation(1) (1)

8/17/2019 Week 21 Lens Presentation(1) (1)

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Debjani Goswami

Lens

Week 21

07/02/2016


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Learning Objective

LENS

ON!E"#$NG %ND D$!E"G$NGLENS

ON!E& %ND ON%!E LENS

LENS #O"'(L%


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Learning O)tcome

*now t+e )ses o, -ens

om.are conve an concave -enses

Draw t+e ra iagram ,or ierent .ositions o, t+e

So-ve .rob-ems b )sing -ens ,orm)-a


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O.tics

4+e overa-- st) o, +ow -ig+t be+aves is ca--e o.tics3

4+e branc+ o, o.tics t+at ,oc)ses on t+e creation o,

images is ca--e geometric o.tics5 beca)se it is base

on re-ations+i.s between ang-es an -ines t+atescribe -ig+t ras3


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% -ens is a .iece o,trans.arent materia-5s)c+ as g-ass or.-astic5 t+at is )se

to ,oc)s -ig+t an,orm an image3

Lense

s

Eac+ o, a -enss two,aces mig+t be eit+erc)rve or at3

% -ens is an o.tica-evice t+at is )se toben -ig+t in a s.eci8c

wa3


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onve -enses are t+ickerin t+e mi-e an t+)s t+e

converge -ig+t ras3

oncave -enses are t+innerin t+e mi-e an t+)s t+eiverge -ig+t ras3

onverging an iverging -e


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onve Lense

The lens shown in thefgure is called a conve -ens because it is thicker

at the center than at theedges.

A convex lens oten iscalled a converging lensbecause when surrounded

by material with a lowerindex o reraction, itreracts parallel light raysso that the rays meet at apoint.


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oncave Lenses

4+e -ens s+own in t+e8g)re is ca--e a concave-ens beca)se it is t+innerin t+e mi-e t+an at t+e

eges3

% concave -ens o,ten isca--e a iverging -ensbeca)se w+en s)rro)ne

b materia- wit+ a -owerine o, re,raction5 ras.assing t+ro)g+ it s.reao)t.


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onvergingLens

4+e ,oca- .oint o, a

c)rve mirror was t+e

image .oint o, a istant

star

$t is t+e same ,or a -ens

4+e ,oca- .oint o, a

converging -ens is w+ere

t+e incoming ras ,rom a

istant star a-- intersect3% istant star is )se to

g)arantee t+at t+e

incoming ras are .ara--e-Focal point

Focal distance


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"a iagrams ,or

-enses

"a iagrams are rawings o, t+e

ierent sit)ations ,or -enses3

#or t+e ra iagrams5 ass)me

t+at t+e -enses are t+in3


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Foca ength

Principa

focus or Foca

point

Optica centre

Foca p ane

Principa axis

onverging Lens


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The .o-e o the lens is the centre o the lens. Rays drawn passing through this point are notdiverted, they continue in a straight line.

The ,oca- -engt+ o a lens is the distance between the pole o the lens and the ocalpoint O" the perpendicular distance between the axis o the lens and the ocal plane.

The ,oca- .oint or .rinci.a- ,oc)s o, a conve -ens is point through which rays o light

travelling near to and parallel to the principal axis pass ater reraction by the lens. Thepoint all emerging rays pass through!.

The ,oca- .oint or .rinci.a- ,oc)s o, a concave -ens is point rom which rays o lighttravelling near to and parallel to the principal axis seem to emerge rom ater reraction by

the lens. The point all emerging rays can be "traced back" through!.

'ore abo)t conve -ens


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%,ter being re,racteb t+e -ens5 t+e rasconverge at t+e ,oca-.oint5 F5 o, t+e -ens3

4+e 8g)re s+ows two,oca- .oints5 one oneac+ sie o, t+e -ens3

>o) co)- t)rn t+e -ensaro)n5 an it wi-- workt+e same3

onve Lenses an "ea-$mages


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FF

"a Diagram


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Click

4+e c+aracteristics o, t+e image,orme b a converging -ens e.en ).on t+e -ocation o,t+e object3

4+ere are si ?strategic? -ocations w+ere an object ma be.-ace3#or eac+ -ocation5 t+e image wi-- be ,orme at a ierent.-ace an

wit+ ierent c+aracteristics3 We wi-- i--)strate t+e siierent-ocations an -abe- t+em as %SE;1 to %SE;63

Case-1: Object at infinity

Case-2: Object just beyond 2 F’

Case-: Object at 2F’

Case-!: Object bet"een 2F’ and F’

Case-#: Object at F’

Case-$: Object "ithin focal length %f&

Lenses


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Click

C'()-2 : Object just beyond 2F’

age is real %fored by refracted rays&

n0erted %upside do"n&

3educed %saller than object&

4ocated bet"een F and 2F

Object age

,ote-1

' ray that coes parallel

is refracted through F

,ote-2

' ray that goes through the

0ertex goes right through

,ote-

' ray that goes through F’

is refracted parallel

,O.)

n order to establish

an iage point/ all

"e need are t"o

intersecting rays

.his ray is extra

in locating the

iage

onve or onverging Lens


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Click

C'()- : Object at 2F’



(ae si5e as object

4ocated at 2F

Object age

'gain:

n order to establish

an iage point/ all

"e need are t"o

intersecting rays

.his ray is extra



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Click

C'()-! : Object bet"een 2F’ and F’



6agnified %larger than object&

4ocated beyond 2F

Object age



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Click

C'()-# : Object at F’

,o iage is fored

%rays refract parallel&

Object ,o iage



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Click

C'()-$ : Object is "ithin focal length

age is 0irtual

%fored by extended rays&7pright

6agnified4ocated on sae side as

object

Object

age



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Object istance 4.e o, image (ses

) @ $nverte5 sma--er5 rea- 4e-esco.e

) A 2, $nverte5 sma--er5 rea- amera5 ee

) @ 2, $nverte5 same siBe5 rea- C+otoco.ier

, ) 2, $nverte5 magni8e5 rea- Crojector

) @ , ).rig+t5 magni8e5 rea- S.ot-ig+t

) , ).rig+t5 magni8e5 virt)a- 'agni,ing g-ass

$mage ,orme b -ens


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Sign onvention

%-- istances are meas)re ,rom t+eo.tica- centre o, t+e -ens3

4+e istances meas)re ,rom t+e o.tica-centre in the direction of incident lightare taken as .ositive3

4+e istances meas)re ,rom t+e o.tica-centre against the direction o, t+eincient -ig+t are taken as negative

4+e istance meas)re ).war an

.er.enic)-ar to t+e .rinci.a- ais aretaken as .ositive

4+e istances meas)re ownwar an.er.enic)-ar to t+e .rinci.a- ais aretaken as negative3


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$m.ortant .oints ,or meas)ringistances ,or 9

A convex lens when a real imageis formed :

4+e istance o, t+e object ,romt+e o.tica- centre )F is a-wasnegative3

4+e istance o, t+e image ,romt+e o.tica- centre vF is a-was

.ositive3


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Lens ,orm)-a erivation

Let % re.resent an object .-ace at rig+t ang-es tot+e .rinci.a- ais at a istance greater t+an t+e ,oca--engt+ , o, t+e conve -ens3 4+e image %11 is ,orme

beon 2#2 an is rea- an inverte3O% @ Object istance @ )O%1 @ $mage istance @ vO#

2 @ #oca- -engt+ @ ,


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'agni8cation mF is sim.- t+e image+eig+t ivie b t+e object +eig+t3

'agni8cation


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The power P o a lens is the inverse o its ocallength f .#ince f is measured in meters "m" the units o lenspower are m$%.

The power also depends on the type o

lens. Convex lenses have positive powers,while concave lenses all have negative powers.&or example, a %' cm ocal length convex lens has apower o (%' m$%) while a *' cm ocal length concave

lens has a power o $+ m$%

.

Cower o, a -ens


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13%n object 2H cm awa ,rom a -ens .ro)ces a,oc)se image on a 8-m 1H cm awa3 W+at ist+e ,oca- -engt+ o, t+e converging -ensI J3KcmF

23$, t+e ,oca- -engt+ o, t+e -ens in o)r camera is2 cm5 at w+at istance m)st objects be .-aceso t+at a ,oc)se image is .ro)ce on a .iece

o, 8-m set cm ,rom t+e -ensI 6 cmF

Eercise


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Determine t+e +eig+t o, t+e image3

'n object "hose height is 818 is placed

18 fro the focal point of a con0erging

lens "hose focal length is 8#8

Eercise


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Construct a ray diagra to locate the iage fored by the lensillustrated belo"

Cli k

Top (object) point

Top (image) point

Bottom (object) point

Bottom (image) point

Image

.his is the sybol for a con0erging lens

Cractice

Week 21 Lens Presentation(1) (1)

Documents

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