CHAPTER 7 Light, Sight and Colour

8/9/2019 CHAPTER 7 Light, Sight and Colour

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CHAPTER 7

LIGHT, COLOURAND SIGHT


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Images formed by a planemirror

The information of image by a planemirror is a result of the reection oflight.


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Images formed by plane mirrors havethe following characteristics :

a) Laerally in!ered (the right sideand left side are interchanged)

b) "ir#al (images that cannot be

displayed on screen)$) Uprig% (images have the same

orientation as the actual object)

d) Same si&e as %e ob'e$e) Same disan$e of %e ob'e$ and

%e image from %e mirror


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Images formed by convex andconcave lenses

!"#!#$ lens is an object which is made of

transparent material such as glass orclear plastic.

It has two faces% of which at least oneis curved.

There are two types of commonlyused lenses& the $on!e( lens andthe $on$a!e lens.


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'oncave lenses


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'onvex lens


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hen parallel rays of light passthrough a concave lens% the rays oflight are di!erged from the focalpoint.


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'oncave lens


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Opi$alerm

Symbol eaning

rincipal axis + The straight line thatpasses through the focalpoint and the opticalcentre. It is perpendicularto the vertical axis of the

lens.,pticalcentre

, The centre of the lens.

*ocal point * The point of the principal

axis where the raystravelling parallel to theprincipal axis.•$re converged afterpassing through a convexlens


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Opi$al erm Symbol eaning

focal length f The distance between

the focal point and theoptical centre.

,bjectdistance u The distance betweenthe object and theoptical centre.

Imagedistance

- The distance betweenthe image and theoptical centre.


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ays diagram

ay diagram can be used to illustratethe formation of images by lenses.

ays diagram show the path ta/en bylight from an object to form animage.

The rays of light are represented by

straight lines. $rrows on the linesindicate the direction of light.


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ules for drawing ray diagrams fora convex lens


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ules for drawing ray diagrams forconcave lens


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*ormation of image by a convexlens


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*ormation of image by a concavelens

The characteristics of the imageformed by a concave lens are nota0ected by the object distance.

The image formed by a concave lensis always:

"ir#al

Uprig%Diminis%ed


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1etermining the focal length ofconvex lenses

The focal length of a lens is thedistance between the focal point andthe optical centre of the lens.

The focal length of a lens isrepresented by f .

The focal length of a convex lens

depends on the thic/ness of the lens.


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The thinner the convex lens% thelonger is its focal length.

This is because a thinner convex lensconverges parallel rays of light to apoint further from the lens than athic/er convex lens.


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2.3 The *ormation of Images by,ptical Instrument

inhole 'amera


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Condiion C%ara$erisi$s of %eimage

The object is placed closerto the pinhole

$ larger% real and invertedimage is formed

The object is placedfurther away from the

pinhole

$ smaller% real andinverted image is formed

The pinhole is madesmaller

$ sharp but dim image isformed

The pinhole is madebigger $ brighter but blurredimage is formed

$ convex lens is placed infront of the camera

$ bright and sharp imageis formed


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eriscope


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$ periscope is an opi$alinsr#men designed for !ie*ing%ings o!er an obsa$le or from a$on$ealed posiion.

$ periscope functions on the principleof the re+e$ion of lig%.

The image formed by a periscope is"ir#alUprig%

T%e same si&e as %e ob'e$


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Telescope


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$ telescope has two convex lenses ofdi0erent focal length (ob'e$i!e lensand eyepie$e lens)

The objective lens has a longer focallength (f

o) while the eyepiece lens

has a shorter focal length (fe).

"ormal adjustment 4 (f o 5 f e).


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The objective lens focuses parallel lightrays from a distant object at its fo$al poin(F o). The image formed is real% in!ered

and smaller %an %e ob'e$. The eyepiece lens is positioned so that the

image formed by the objective lens is at the

fo$al poin of %e eyepie$e lens (F e).

The eyepiece lens forms a !ir#al%in!ered and larger image a inniy.

The 6uman !yes and


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The 6uman !yes and'amera


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H#maneye

-#n$ion Camera

!ye lens To focus on the object 'amera

lens

upil The opening that allowslight rays to enter

$perture

!yelid To control the duration forlight to enter

#hutter

etina #creen that is sensitive tolight for the formation ofimage

*ilm

Iris To control the si7e of theopening

1iaphgram


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Ob'e$ .ye Camera

1istantobject

To focus on a distantobject:• The ciliary musclesrelax• The eye lens becomesthinner• The focal length

becomes longer

To focus on a distantobject:• The lens moves inwards


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Ob'e$ .ye Camera

"ear

object

To focus on a near

object:• The ciliary musclescontract• The eye lens becomesthic/er•

The focal lengthbecomes shorter

To focus on a near object:• The lens movesoutwards


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2.8 ight 1ispersion

ight dispersion : %e separaion of*%ie lig% ino is $omponen$olo#rs.


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To understand light dispersion% wehave to understand that:

a) ight travels in a straight lineb) ight is refracted when its travel

from one medium into another

medium with di0erent densitiesc) hite light is actually a mixture of 2

di0erent colours of light

d) 1i0erent colours of light arerefracted at slightly di0erent angles.


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1ispersion of light through a prism


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Spe$r#m : a band of colours hen a beam of white light passes

through a prism% its di0erentcomponent colours are refracted atslightly di0erent angles.

Red light has the greaes speed%hence it is refra$ed %e leas.

"iole light has %e slo*es speed%

hence it is refra$ed %e mos.


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ecombining the colours in thespectrum


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The formation of a rainbow

$ rainbow is a natural phenomenon causedby the dispersion of sunlight by raindropssuspended in the air.

hen sunlight (white light) enters raindrop%its component colours are refracted anddispersed at di0erent angles.

These dispersed lights undergo total

internal reection. hen these lights leavethe rainbow% they are refracted againbefore entering the eyes of the observer.


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#ometimes a primary rainbow and asecondary rainbow are formed.

The secondary rainbow has violet on

the top curve and red on the bottomcurve.

The formation of the secondary

rainbow is due to the reason that thelight in the water droplets is reectedtwice.

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CHAPTER 7 Light, Sight and Colour