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Refraction 2Lenses

Copyr ight Mark Jordan

Davitt College,

Castlebar

For non-commercial purposesonly.. Enjoy!

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Help

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C1 C2

C1 & C2 are the centres of the spheres of which the surfaces of thelens form a part

The line through C1 & C2 form the principal axis

Converging Lens

Principal axis

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C1 C2

Converging lens

FF

A ray of light on entering the lens is refracted towards the normal

and on leaving away from the normal. Since surfaces are inclinedtowards each other the ray is refracted towards the principal axis.

Rays parallel to the principal axis

converge towards a point called

the principal focus F.

Since light can travel equally

well in both directions, there

are two foci.

Normal

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F = Focal point

To locate the position of an image in a convex lens we use two

of the following rays of light

FF

3 through the focus emerging parallel to principal axis.

1 parallel to the principal axis emerging through focus

2 striking the centre of the lens passes straight through (if lens is thin)

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F = Focal point

object

Images in Convex lens

vuf

111

vu

f

Image

Real, inverted & diminished

Image formed in convex lens when the object is placed

outside twice the focal length

FF

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Image formed in convex lens when the object is placed at

twice focal length

F = Focal point

object

ImageReal, inverted & same size as object

Images in Convex lens

vuf

111

vu

f

FF

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Image formed in convex lens when the object is placed

between fand 2f

F = Focal point

object

Image

Real, inverted & magnified

Images in Convex lens

vuf

111

vu

f

FF

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Image formed in convex lens when the object is placed at

the focus

F = Focal point

object

Imageat Infinity

Images in Convex lens

vuf

111

u

f

FF

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Image formed in convex lens when the object is placed

inside the Focus

F = Focal point

object

Images in Convex lens

vuf

111

u

f

ImageVirtual, magnified & upright

FF

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rayboxlens

u v

Experiment to find the focal length of a convex lens

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F = Focal point

1. A ray which strikes the lens

travelling parallel to principal

axis is refracted as if it came

from focus

2. A ray striking the

centre of the lens

passes straight

through (if lens is thin)

3. A ray heading for

the focus on striking

the lens is refracted

parallel to principalaxis

To locate the position of an image in

a concave lens we use twoof thefollowing rays of light

Convave lens

FF

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Image formed in concave lens when the object is placed

in front of lens

F = Focal point

object

Images in Concave lens

vuf

111

ImageVirtual, upright &

diminished

v

u

f

FF

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Cornea

IrisPupil

Lens

Optic nerve

Retina

Suspensory

ligament

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Accommodation is the ability of the lens in the eye tochange its focal length so that light from close up and

distant objects always fall on the retina

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light from

distant object

falls short of

retina

light fromdistant object

falls on

retina

with help of a

diverging(concave) lens

Short-sight defect

Corrected

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light from

near object

falls behind

retina

Long-sight defect

Corrected

with help of a

converging (convex)lens

falls on

retinalight fromnear object