27_LectureOutline

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Copyright © 2010 Pearson Education, Inc.

Lecture Outline

Chapter 27

Physics, 4th Edition

James S. Walker




Chapter 27

Optical Instruments




Units of Chapter 27

• The Human Ee an! the Camera

• Lenses in Com"ination an! Correcti#e

Optics

• The $a%nifin% &lass

• The Compoun! $icroscope

• Telescopes

• Lens '"errations




27() The Human Ee an! the Camera

Li%ht passes throu%h the cornea of the human

ee an! is focuse! " the lens on the retina. Theciliar muscles chan%e the shape of the lens* so

it can focus at !ifferent

!istances. The #itreous

an! a+ueous humorsare transparent. ,o!s

an! cones on the retina

con#ert the li%ht into

electrical impulses*

-hich tra#el !o-n the

optic ner#e to the "rain.





The ee pro!uces a real* in#erte! ima%e on the

retina. Wh !ont thin%s look upsi!e !o-n to us/The "rain a!0usts the ima%e to appear properl.





The ciliar muscles a!0ust the shape of the lens

to accommo!ate near an! far #ision.





The near point is the closest point to the ee thatthe lens is a"le to focus. 1or those -ith normal

#ision* it is a"out 2 cm from the ee* "ut

increases -ith a%e as the lens "ecomes less

fle3i"le.

The far point is the farthest point at -hich the

ee can focus4 it is infinitel far a-a* if #ision is

normal.





The simplest camera consists of a lens an! film

in a li%ht(ti%ht "o35





The camera lens cannot chan%e shape4 it mo#es

closer to or farther a-a from the film in or!er tofocus.

The f (num"er characteri6es the si6e of the

aperture5

The com"ination of f (num"er an! shutterspee! !etermines the amount of li%ht that

reaches the film.




27(2 Lenses in Com"ination an!

Correcti#e Optics

In a t-o(lens sstem* the ima%e pro!uce! " thefirst lens ser#es as the o"0ect for the secon!

lens.





Correcti#e Optics

To fin! the ima%e forme! " a com"ination of

lenses* consi!er each lens in turn* startin% -ith

the one closest to the o"0ect.

The total ma%nification is the pro!uct of the

ma%nifications of each lens.





Correcti#e Optics

' nearsi%hte! person has a far point that is a

finite !istance a-a4 o"0ects farther a-a -ill

appear "lurr. This is !ue to the lens focusin%

too stron%l* so the ima%e is forme! in front ofthe retina.





Correcti#e Optics

To correct this* a !i#er%in% lens is use!. Its focallen%th is such that a !istant o"0ect forms an

ima%e at the far point5





Correcti#e Optics

The stren%th of correcti#e lenses is usuall+uote! as refracti#e po-er* -hich is the in#erse

of the focal len%th5





Correcti#e Optics

' person -ho is farsi%hte! can see !istanto"0ects clearl* "ut cannot focus on close

o"0ects the near point is too far a-a. The lens

of the ee is not stron% enou%h* an! the ima%e

focus is "ehin! the retina.






27(8 The $a%nifin% &lass

' ma%nifin% %lass is a simple con#e3 lens.

Workin% in con0unction -ith the ee* it

makes o"0ects appear "i%%er "ecause it

makes them appear closer. Similar to a

correcti#e lens for farsi%hte!ness* it "rin%s

the near point closer to the ee.





The an%ular si6e of

an o"0ect is the

an%le it su"ten!s on

the retina* an!

!epen!s "oth onthe si6e of the

o"0ect an! its

!istance from the

ee.





This an%le* assumin% it is small* is %i#en " the

hei%ht of the o"0ect !i#i!e! " its !istance fromthe ee.

If the o"0ect is mo#e! closer to the ee* its

an%ular si6e increases.

If it is place! at the near point* its si6e is5





9o-* place acon#er%in% lens -hose

focal len%th is less

than N #er close to

the ee* an! place the

o"0ect at the focal

point of the lens. This

%i#es the o"0ect alar%er an%ular si6e.





The an%ular ma%nification is then %i#en "5





The ma%nification can "e ma3imi6e! " ha#in%the ima%e at the near point5




27(: The Compoun! $icroscope

' compoun! microscopehas* in its simplest form*

t-o con#er%in% lenses.

One* the eepiece* is close

to the ee* -hile theo"0ecti#e is close to the

o"0ect.





The o"0ect is place! near the focal point of the

o"0ecti#e lens* %i#in% a ma%nification of5

The ima%e forme! is at the focal point of the

eepiece* -hich pro!uces an ima%e at infinit5





The total ma%nification

is %i#en a"o#e* an! is

the pro!uct of the

ma%nification of each

lens.




27( Telescopes

Telescopes are similar to microscopes in thatthe ha#e an o"0ecti#e an! an eepiece.

Ho-e#er* the o"0ects o"ser#e! are essentiall at

infinit* so the li%ht -ill "e focuse! at the focal

point of the o"0ecti#e.

The o"0ects themsel#es are #er lar%e* "ut their

an%ular si6e is #er small !ue to their %reat

!istance.




27( Telescopes

The ima%e forme! " the o"0ecti#e is at the focal

point of the eepiece.




27( Telescopes

The total ma%nification of the telescope is the

pro!uct of the ma%nification of each lens* an! is5

Telescopes usin% lenses are calle! refractors4

the first telescopes ma!e -ere of this tpe.




27( Telescopes

It is !esira"le to ha#e the o"0ecti#e of a

telescope "e as lar%e as possi"le* so that it macollect as much li%ht as possi"le. Each !ou"lin%

of the !iameter of the o"0ecti#e %i#es four times

as much li%ht.

;er lar%e lenses are !ifficult to han!le4 the are

thick an! hea#* must ha#e t-o precision

surfaces* an! a"sor" more of the li%ht the

thicker the are.




27( Telescopes

Therefore* lar%e telescopes are no- ma!e as

reflectors the o"0ecti#e is a mirror rather thana lens. The mirror has onl one surface* can "e

ma!e #er thin* an! reflects almost all the li%ht

that hits it.




27(< Lens '"errations

Spherical a"erration occurs -hen li%ht strikin%

the lens far from the a3is !oes not focusproperl. It can "e fi3e! " %rin!in% the lens to

a precision* non(spherical shape.





Chromatic a"erration occurs -hen !ifferent

colors of li%ht focus at !ifferent points.





Chromatic a"erration can "e impro#e! "

com"inin% t-o or more lenses that ten! tocancel each others a"errations. This onl -orks

perfectl for a sin%le -a#elen%th* ho-e#er.

Summar of Chapter 27





• The human ee is focuse! " the ciliar

muscles* -hich chan%e the shape of the lens.

• ' camera is focuse! " chan%in% the !istance

from the lens to the film.

•

The near point is the closest point at -hich theee can focus* tpicall 2 cm.

• The far point is the farthest point at -hich the

ee can focus* tpicall at infinit.

• f (num"er of a lens5






• In lens com"inations* the ima%e forme! " one

lens ser#es as the o"0ect for the ne3t.• 9earsi%hte!ness occurs -hen the ima%e is

focuse! in front of the retina* causin% the far

point to "e closer than infinit. It can "e correcte!

-ith a !i#er%in% lens.

• 1arsi%hte!ness occurs -hen the ima%e is

focuse! "ehin! the retina* causin% the near point

to "e more than 2 cm from the ee. It can "ecorrecte! " a con#er%in% lens.






• ,efracti#e po-er of a lens in !iopters* -hen

the focal len%th is in meters5

• ' ma%nifin% %lass is a con#er%in% lens. Its

ma%nification is %i#en "5






• ' compoun! microscope uses t-o lenses* the

o"0ecti#e an! the eepiece* to form an ima%e of a

small o"0ect place! close to the focal point of the

o"0ecti#e. Its ma%nification is %i#en "5





• ' refractin% telescope also uses t-o lenses to

form an ima%e of a #er !istant o"0ect. Itsma%nification is %i#en "5

• The len%th of the telescope -ill "e5

• ' telescope ha#in% a mirror instea! of a lens

as the o"0ecti#e is calle! a reflectin% telescope.






• Lens a"errations can !istort ima%es.

• Spherical a"erration occurs "ecause off(a3isras !o not focus at the focal point. It can "e

correcte! " precision shapin% of the lens.

• Chromatic a"erration occurs "ecause !ifferentfre+uencies of li%ht ha#e !ifferent in!ices of

refraction. It can "e correcte! " usin% multiple

lenses in an achromatic lens sstem.

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