Unit 12 - Light IV - Lens Power Point

8/14/2019 Unit 12 - Light IV - Lens Power Point

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Unit12:Light


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Learning Objectives:

, Describetheactionofathinlens(bothconverg ngan verg ng ona eamo g . Definethetermfocallengthforathinconverging

lens Constructraydiagramforimagesformedbythin

converginglens

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The Use of Lens In Our Daily LivesTelescope Contact lenses

Binoculars Magnifying glass

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What is a lens?

surfaces. ,hitting the different parts of the lenssurface will be refracted differently.

Depending on the curvature of the lens,g rays w e er converge or verge.

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2 Types of Lens

Converging Lens Diverging Lens Cause parallel rays to converge

Cause parallel rays to divergebehind

e in t e ens.

To Converge: To tend to meet in a point

t e ens.

To Diverge: To move, lie, or extend inor line different directions from a common

point; branch off.

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We will only study

THIN CONVERGING LENS!

A thin conver in lens focuses a beam of li ht

to a point.

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How does light refract through lens?All the ra s on to will bend downwards.

will go straight.

converge to one pointAll the rays below will bend upwards.

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Features of Thin Converging Lens

Principal axis

Focal length ( f )Length between the optical centre and the principal

or zon a ne w c passessymmetrically through the optical

centre

Optical centre (C)

The oint in the lens that all Principal focus/ Focal Point ( F )

8rays will pass throughundeviated.

Point on the principal axis where all rays parallel to the principalaxis will converge to after passing through the lens.


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Light passing through a thinner lensbendf

less giving a longer focal length

Light passing through a thicker lensbend more thus giving a shorter focal length

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Ray Diagram of Image formed by Converging Lens

Step #1: Draw a horizontal line to represent the principal axis.

Step #2: Draw a vertical line with arrow heads to represent the lens.

Step #3: The point of intersection of the horizontal and vertical lines is the optical centre. Label it as C.

Step #4: Mark the principal focus/ focal point F on the principal axis. The distance between C and F is thefocal length, f. The value of f is usually given in the question.

Step #5: Draw in the object from the left side of the lens. Mark the object as O. The height and theposition of the object is usually given in the question.

Step #6: Draw two rays from the object(a)From the object passing through the optical centre.b From the ob ect arallel to the rinci al axis. When the ra hits the lens ass it throu h the rinci alfocus/focal point F.

Ste #7: The ima e is at the osition where these two ra s intersect. Draw the ima e and mark it as I.

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How to draw a ray diagram of image formedby thin converging lens?y thin converging lens?

The following slides will not be found in

our notes. Please a attention to theprocess of constructing ray diagram.

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In General, How to draw???

Step #1: Draw a horizontal line to represent the principal axis

This is the principal axis

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Step #2: Draw a vertical line with arrow heads to represent the lens

This line with two arrowheads re resents a thin

converging lens.13


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Step #3: The point of intersection of the horizontal and vertical lines is the opticalcentre. Label it as C.

C

Optical Center C

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: .between C and F is the focal length, f. The value of f is usually given in the question.

Principal Focus/ Focal Point

f f

C

The focal length

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Step #5: Draw in the object from the left side of the lens. Mark the object as O. Theheight and the position of the object is usually given in the question.

ject,

f f

C

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Step #6: Draw two rays from the object(a)From the object passing through the optical centre.

A ray of light passing through theoptical centre of a very thin lens is

straight.,

C

f f

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:(b) From the object parallel to the principal axis. When the ray hits the lens, pass itthrough the principal focus/focal point F.

the principal axis will pass

through the principal,

through the lens.

f f

C

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: .and mark it as I.

.start from the top of the

object. Where they,your image.

C Image, I

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What are the characteristics of suchimages?mages?

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Characteristics Of An Ima e Formed by Lens

When you describe an image formed by lens, you state the following things:---. .

Check where the object and image are.

If they are on the same side of the lens, then the image is virtual. Thevirtual image appears to converge on the same side of the object.

If the object and image are on the opposite sides of the lens, thenthe ima e is real.

2. The position of the image from the lens. (Image distance v = distance ofmage rom t e ens. ect stance u = stance o o ect rom t e ens.

3. Whether the image is upright or inverted.

4. When the ima e is ma nified, diminished or same size as ob ect.

Magnification Image Height

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Object is placed exactly 2ffromthe lens

real image : Use solid line

Object O

u = 2f v = 2f

mage

Object

Distance u

Image

Distance v

Image is Uses

u = 2f v = 2f

Image andobject on

Real Inverted Same size

Same size copyformed byphotocopier

oppos e s e

of lens

as o ec

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Object is placed more than 2faway real image : Use solid line

from the lens

Object O

Image I

u > 2f f < v < 2f

Object

Distance u

Image

Distance v

Image is Uses

u > 2f f < v < 2fImage andobject on

Real Inverted Diminished

Camera

oppos e s e

of lens23


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Object is placed between f and 2ffrom the real image : Use solid line

lens

Object O

f < u < 2f v > 2f

Image I

Object

Distance u

Image

Distance v

Image is Uses

f < u < 2f v > 2f

Image andobject on

Real Inverted Magnified

Projector

opposite side

of lens24


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Object is placed exactly ffrom the

Virtual image : Use dotted line

Image at infinity

Object O

u = f Parallel rays

Object

Distance u

Image

Distance v

Image is Uses

infinity

Image and

Virtual Magnified

produce aparallel

side of lens

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Object is placed less

Object

Distance u

Image

Distance v

Image is Uses

than f from the lens u < f Image isbehind object

Image and

Virtual Upright Magnified

Magnifyingglass

object on sameside of lens

mage

Object O

u < f

Eye looks through the lens to see

v, image behind object

t e v rtua mage

26Virtual image : Use dotted line


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Object is placed very far away from the lens

Parallel rays from a distant object

Image I

v = f

Object

Distance u

Image

Distance v

Image is Uses

u = v = f

Image andobject on

Real Inverted Diminished

Telescope

oppos e s e

of lens27


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Lesson Summary

Describetheactionofathinlens(bothconverginganddiverging onabeamolight.

converginglens Constructraydiagramforimagesformedbythinconverginglens

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Unit 12 - Light IV - Lens Power Point

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