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PHYSICS 3

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Convex LensesLesson objectives

Understand what effect convex

lenses have on light rays.Be able to draw ray diagrams

for convex lenses.

Tuesday, 03 June 2014

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A lens canbe thought ofas a series ofprisms.

The lensrefracts all the

rays to a pointcalled thepr inc ipal focus[F].

The distancebetween the

centre of thelens and F iscalled the focalleng th [ ] .

Imagineparallel rays

of light froma distantobject hittingthe lens.

Draw normal

lines [at 90 tothe surface] foreach ray.

Use the first

refraction ruleto work out theray direction.

Draw normallines where therays enter theair [at 90 to thesurface].

Work out thedirection of therefracted raysusing the secondrefract ion rule .

When light enters aless d ense medium[e.g. air], it bends awayfrom the normal.

Using Refraction : lenses

When light enters a moredense medium [e.g.glass], it bends towardsthe normal.

F

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Biconvex LensesBiconvex lenses are converging lenses.

When incoming parallel light rays are incident upon abiconvex lens...

The rays arefocussed to theprinciple focus

f focal length, distance from F to lens

F principle focus

F

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Drawing Ray Diagrams (1)

Draw one ray from the top of the objectparallel to the centre axis. This is refractedthrough the principal focus.

Draw a second ray through the centre of thelens. This passes straight on it is notrefracted at all!

Where the rays meet the image is formed.

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Drawing Ray Diagrams (2)

Two light rays leave the object O and pass throughthe lens. Where they meet an image I is produced.This is a REAL, DIMINISHED and INVERTED image.

Lens

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Your Ray Diagram 1

A convex lens has a focal length of 4 cmand an object 3 cm high is placed 8 cmin front of the lens (i.e. at 2F).

Find the position, size and nature of theimage formed.

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Convex Lens Ray Diagrams Object at 2F

Im age is real , inv erted and th e sam e size as th e objec t .So i t s 8 cm f rom the lens and 3 cm h igh .

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Your Ray Diagram 2

A convex lens has a focal length of 4 cmand an object 2 cm high is placed 6 cmin front of it.

Find the position, size and nature of theimage formed.

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Convex Lens Ray Diagrams ObjectBetween 2F and F

The image i s 12 cm f rom the lens , 4 cm high and i sreal , inv er ted and m agnif ied.

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Your Ray Diagram 3

A convex lens has a focal length of 6 cmand an object 1 cm high is placed 4 cmin front of it.

Find the position, size and nature of theimage.

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Convex Lens Ray Diagrams ObjectBetween F and C

The image i s 12 cm in f ron t of the lens , 3 cm highand i s v i r tual , up r igh t and m agni f ied .

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1. Which statement is true?

A. Virtual images can be projected onto screens

B. Erect images are upside-down

C. Concave mirrors are diverging

D.Biconcave lenses are diverging

Optics Test

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2. Which statement is true?

A. Diminished images are smaller than the object

B. Convex mirrors are converging

C. Biconvex lenses are diverging

D.Real images can not be projected onto

screens

Optics Test

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Optics Test

3. What optical device is shown?

A. Biconvex lens

B. Biconcave lensC. Convex mirror

D. Concave mirror

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4. What do you think happens when

Parallel light rays strike a convex lens?They pass through thefocal point of the lens.

Diverging light rays?Emerge as a parallel beam if theypass though the focal point (F).

F

F

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Summary

Objectposition

ImagePosition

Real orvirtual

Magnifiedor

diminished

Inverted orerect

>2F

at 2F

between 2Fand F

at F

between Fand lens

between Fand 2F

at 2F

> 2F

at infinity

same sideas object

virtual

real

real

real

magnified

magnified

same size

diminished

upright

inverted

inverted

inverted

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Use a ruler tomeasure the distancebetween the lens andthe screen - this isthe focal length [].

Using Refraction : lenses - finding

Chose adistant object[to get parallelrays of light].

Hold a plainwhite screenin one hand.

Hold the lens in the otherhand and move it closer tothe screen until a clearimage appears.

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Refraction : lenses

1. Find the focal length [] of your lens.

2. Fix the lens to the centre of a metre rule and mark the distancesF and 2F either side of the lens.

2F F F 2F

3. Place the candle >2F away from the lens and move the screenuntil an image appears and record observations.

4. Repeat for the candle at 2F, between 2F and F, at F and betweenF and the lens.

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Results

Objectposition

ImagePosition

Real orvirtual

Magnifiedor

diminished

Inverted orerect

>2F

at 2F

between 2Fand F

at F

between Fand lens

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Refraction : lenses

Object >2F away

O

2F F F 2F

I

The image [ l ] is formed between F and 2F away from thelens, is inverted and diminished.

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Object at 2F

O

2F F F 2F

I

The image [ l ] is formed at 2F away from the lens, isinverted and the same size.

Refraction : lenses

f l

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Object between 2Fand F away

O

2F F F 2F

IThe image [ l ] is formed further than 2F away fromthe lens, is inverted and magnified.

Refraction : lenses

f i l

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Object at F away

O

2F F F 2F

The image [ l ] is formed at infinity - the rays never meet[we use this set-up for searchlights].

Refraction : lenses

R f i l

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Object between F and lens

O

I

The VIRTUAL image [ l ]

is formed on the sameside of the lens as theobject, is the right way up and magnified.

2F F F 2F

Refraction : lenses

R f i l

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2F F F 2F

Magnification = Distance from lens to image

Distance from object to lens

Refraction : lenses

U i f i l

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Using refraction : lenses summary

There are two main types of lens:

Convex Concave

Convex lenses work by bending [refracting] raysof light to a principal focus.

The distance from the centre of the lens to theprincipal focus [F] is called the focal length [].

The image formed by a convex lens is inverted[back-to-front and upside-down].

The thicker the lens, the shorter the focallength[].

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Convex Lens Ray Diagrams Object Beyond 2F

Image is real, inverted and diminished

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Your Ray Diagrams

Draw ray diagrams with your objectpositioned:

a) At 2F describe the image,b) Between F and 2F describe the image,c) Between the lens and F describe the

image.

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Optics Test

3. What optical device is shown?

A. Biconvex lens

B.Biconcave lens

C. Plane mirror

D. Concave mirror

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