LightLightLens

Types of converging lens:A converging

(or convex) lens

is thicker in the

middle than

at the edge. (a) biconvex (b) plano-convex(c) concavo-convex

principalaxis

Optical centre, C

Technical Terms:The optical centre, C, is the centre of the converging lens.

principalaxis

Technical Terms:The principal axis is a horizontal line passing through the optical centre

Optical centre, C

Technical Terms:The principal focus, F, of the converging lens is the point on the principal axis to which all rays originally parallel and close to the axis converge, after passing through the lens.

F

F: focus

Focal length

C F

Technical Terms:

F

Focal length f

C F

The focal length, f, of the lens is the distance between the optical centre and the principal focus.

Standard rays(For construction of ray diagram)

F CF

step 1 step2 step 3

F F

object

image

Object distance u

Image distance v

Technical Terms:The object distance, u, is the distance between the optical centre and the object. The image distance, v, is the distance between the image and the optical centre.

Construction rules

Incident ray through the optical centre, C

Incident ray parallel to the principal axisIncident ray directed towards focus, F’

F’F C

object

Image

Formation of image by a converging lens:

Converging lens of focal length = fObject distance = uImage distance = v ( 6 Cases)

Given:

(continue on next slide)

2F F F image

Object at infinite position

Image nature:real, diminished and inverted image Image distance: v = fUses: telescopev=f

u=f

Case 5a:

2F F F

object

image

u

2f

v

u > 2f

Nature of image: real, inverted and diminishedImage distance: f <v<2fUses: In a camera, in your eye at this moment

Case 5b:

2F F F 2F

object

image

u

2f

v

u = 2f

Case 5c:

Nature of image: real, inverted and same sizeImage distance: v = 2fUses: photocopier

F F

object

image

u

f

v

f<u<2fNature of image: real, inverted and enlargedImage distance: v>2fUses: slide projector, film projector, OHP

Case 5d:

F F

object

u

f

u=fNo image is formed. Image distance : (Image is formed at infinity) v = ∞Uses: Theatres, spotlights

Case 5e:

F F

object

image

u

f

v

0<u<f

Nature of image: virtual, erect and enlargedUses: magnifying glass.

Case 5f:

Simple Magnifying GlassSimple Magnifying Glass

TELESCOPE

Formation of virtual images by a convex lens: (Case 5)When an object is placed within

the focal length of a convex lens, the image formed is virtual, upright and enlarged.

This principal is used in a magnifying glass.

Virtual Image:The image formed by this way is a virtual image, please explain?

To find the focal length of a convex lens:Approximate method

Place a screen at the back of a convex lens. Adjust the position of the lens until a clear image of distance object is obtained on the screen. The distance between the lens and the screen gives the focal length of the convex lens.

Approximate method to find the focal length of the convex lens

Focal length

Accurate method: In this experiment, an illuminated

object is obtained by using a piece of cardboard with a cross wire at its centre to cover the face of a torchlight.

Steps to measure the focal length:Place a plane mirror at the back of a convex

lens.Adjust an illuminated object in front of the

convex lens until a clear image is obtained at the same position as the object.

Measure the distance between the convex lens and the object(image). It gives the focal length of the convex lens.

F F

object

image

Worked example 1:Given: focal length of convex length, f, object distance, u and its size.Find by graphically the size and the nature of its image produced.

The image obtained:Enlarged, inverted andreal.Image distance, v > f

F F

image

object

Worked example 2:Given: focal length of convex length, f, image distance, v and its size.Find by graphically the size and the position of the object.

Graphically:

object

image

F

focal length f

Given: the size and position of distance of an object and its image .Find by graphically the focal length, f, and the position of the lens.

Worked example 3:

Graphically:

GCE O-LevelGCE O-Level Past Examination PaperPast Examination Paper

Science (Physics)

Nov 1998

10. The human eye has a converging lens system that produces an image at the back of the eye. If the eye views a distant object, which type of image is produced?

A real, erect, diminished B real, inverted, diminished C virtual, erect, diminished D virtual, inverted, diminished

B

November 1989

10. In the diagram, XY is a convex lens and F are the principal foci.An object is placed at O. At which point is the base of the image formed?

D

X

Y

GCE O Nov 1997

11. The diagram shows the action of a magnifying glass. Which point is the principal focus of the lens ?

D

Nov 1996

13(a) Draw a labelled diagram to show what is meant by the focal length of a thin converging lens. [2]

Focal length is distance from centre of lens to focus.

(continue in next slide)

13(b) A thin converging lens can be used to produce either a real or a virtual image. Explain the

difference between these types of image. [2]

(Cont. …) Q. 13 Nov 1996

Real image is the image that can project on the screen. Virtual image is the image that produce by reflection of light and cannot projected on the screen. Draw two ray diagrams, one to show the formation of a real image, the other to show the formation of a virtual image, using a thin converging lens. [6]

(continue in next slide)

(Cont. …) Q. 13 Nov 1996

13(b)

When f > u,image:Virtual, upright andEnlarged.

When u > f,image:Real, invertedand reduced.

6(a) Draw rays on the diagram below to show what is meant by the focal length of a converging lens.

GCE Nov 1991

lens

Rays parallelto axis

axis

Focal length

(continue on next slide)

6(b) The diagram below shows the image I of an object O produced by a converging lens. Complete a ray diagram to show two rays passing from the object to the image. Mark the positions of the lens L and the relevant focal point F on the diagram. [4]

(Cont. …) Q. 6 Nov 1991

O

I

lens

F

L

F

Nov 1990

2. The diagram below represents a thin converging lens L (shown as a vertical line) which is being used to produce an image I of a point object O.

On the diagram, complete the path of the ray shown and draw a second ray to locate the position of the principal focus (the focal point) of the lens. Label this point F. [4]

F

L