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Lenses. Eyeglasses. Peep-hole. Watch. Camera. Security mirror. Magnifying glasses. 1 In daily life, you may have come across lenses that are thicker or thinner in the middle. Which of the following items do you think may contain lenses ?. . . . . .

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slide2

Eyeglasses.

Peep-hole.

Watch.

Camera.

Security mirror.

Magnifying glasses.

1 In daily life, you may have come across lenses that are thicker or thinner in the middle.

Which of the following items do you think may contain lenses?

slide3

2 When you see through lenses,

do the objects appear the same as they are?

No !

introduction
Introduction
  • Lenses are very useful.
  • e.g. in cameras, projectors, telescopes, microscopes and eyes
  • The most common lenses:
  • spherical lenses

(i.e. the surface is spherical)

1 convex and concave lenses
1 Convex and concave lenses

Convex lens - thicker at the centre than at the edge.

1 convex and concave lenses6
1 Convex and concave lenses

Concave lens - thinner at the centre than at the edge.

1 convex and concave lenses7
1 Convex and concave lenses

a Converging or Diverging?

convex lens

concave lens

(converging lens)

(diverging lens)

1 convex and concave lenses8
1 Convex and concave lenses

a Converging or Diverging?

convex lens

concave lens

(converging lens)

(diverging lens)

1 convex and concave lenses9
1 Convex and concave lenses

a Converging lens

bend the light inwards

You may remember ‘converging’ as ‘concentrating’.

1 convex and concave lenses10
1 Convex and concave lenses

a Diverging lens

bend the light outwards

You may remember ‘diverging’ as ‘dividing’.

1 convex and concave lenses11
1 Convex and concave lenses

a Converging or diverging?

Recall that light travels more slowlyin glass than in air.

Light converges or diverges as some parts of the wavefronts lag behind.

converges

diverges

slide12

b Key features of lenses

  • Parallel rays are refracted inwards.

principal focus

  • Refracted rays meet at a point called the principal focus F.
slide15

F'

F

F'

F

  • A convex lens has 2 principal foci.
  • 1 on each side

C

principal axis

  • The line through the optical centre and 2 foci is called the principal axis.
slide16

Parallel rays are refracted outwards.

principal focus

  • Refracted rays appear to spread from a point called the principal focus F.
slide19

F'

F

F'

F

  • A concave lens has 2 principal foci.
  • 1 on each side

C

principal axis

  • The line through the optical centre and 2 foci is called the principal axis.
q1 a light ray is incident on a
Q1 A light ray is incident on a…

A light ray is incident on a convex lens.

Which one represents the path of the light ray?

X

Y

Z

A Path X.

B Path Y.

C Path Z.

q2 a light ray is incident on a
Q2 A light ray is incident on a…

A light ray is incident on a concave lens.

Which one represents the path of the light ray?

X

Y

Z

A Path X.

B Path Y.

C Path Z.

2 image formation by a lens
2 Image formation by a lens

O

screen

When the light from a point of an object enters our eyes,

we can see the point.

I

2 image formation by a lens24
2 Image formation by a lens

I

O

a Real images

Light rays converge to a point.

2 image formation by a lens25
2 Image formation by a lens

I

O

screen

Image can be captured by a screen.

Hence called ‘real’.

2 image formation by a lens26
2 Image formation by a lens

I

O

b Virtual images

Light rays diverge from a point.

No rays actually come from the image.

Hence called ‘virtual’.

convex lens

2 image formation by a lens27
2 Image formation by a lens

I

O

b Virtual images

Light rays diverge from a point.

concave lens

slide28

I

O

screen

Since only convex lensesconverge light rays, real images can only be formed by convex lenses.

slide29

Light rays through lenses

Video

Video

Video

Simulation

Simulation

3 graphical construction of image formation
3 Graphical construction of image formation

a Construction rules

In ray diagrams, we use symbols to represent lenses.

convex lenses

concave lenses

slide31

Construction rules for convex lenses

Rule 1

A ray parallel to the principal axis...

F

C

…is refracted through F.

slide32

Construction rules for convex lenses

Rule 2

A ray passing through F'...

F

F'

C

…is refracted parallel to the principal axis.

slide33

Construction rules for convex lenses

Rule 3

A ray passing through C...

F

F'

C

…travels straight on.

slide34

Construction rules for concave lenses

Rule 1

A ray parallel to the principal axis...

C

F'

F

…is refracted so that it appears to come from F'.

slide35

Construction rules for concave lenses

Rule 2

A ray directed towards F...

C

F'

F

…is refracted parallel to the principal axis.

slide36

Construction rules for concave lenses

Rule 3

A ray directed towards C...

C

F'

F

…travels straight on.

3 graphical construction of image formation37
3 Graphical construction of image formation

b Drawing ray diagram

Click the following to see how to draw ray diagrams.

convex lens

concave lens

q1 which of the following
Q1 Which of the following...

A

B

F

F’

F

F’

C

F

F’

Which of the following incident rays is mentioned in the construction rules?

q2 which of the following light
Q2 Which of the following light…

A

F’

F

B

C

Which of the following light rays is correct ?

q3 how to find the focus of the
Q3 How to find the focus of the...

Step 1: Draw a light ray A parallel to the principal axis from the tip of the arrow.

O

I

q3 how to find the focus of the41
Q3 How to find the focus of the...

Step 2: By the construction, the ray A appears to come from the ______ and the extended part ________ the tip of the image.

focus

passes

O

I

q3 how to find the focus of the42
Q3 How to find the focus of the...

Step 3: The point where the extended refracted light ray A cross the ______________ is the focus.

principal axis

O

I

F

3 graphical construction of image formation43

F

I

C

F'

O

3 Graphical construction of image formation

c Magnification

magnification m

height of image (image size)

=

height of object (object size)

3 graphical construction of image formation44

u

v

F

I

C

F'

O

3 Graphical construction of image formation

Also,

image distance

v

 m =

magnification m =

u

object distance

4 position and nature of image
4 Position and nature of image

f

Convex lens refracts the rays to form an inverted image on a plane at the principal focus.

convex lens

focal plane

Such a plane is called the focal plane.

slide47

construction ray

C

F'

F

I

We can find the position and nature of image by drawing ray diagram

∴ The 2 parallel rays also converge toI.

4 position and nature of image48
4 Position and nature of image

I

Images formed by a convex lens

F

2F

C

2F'

F'

at infinity

object:

image:

at F , real, inverted and diminished

slide49

I

O

2F

F

C

2F'

F'

object:

beyond 2F ’

image:

between F and 2F

real, inverted and diminished

slide50

I

O

2F

F

C

2F'

F'

object:

at 2F ’

image:

at 2F

real, inverted and same size

slide51

O

I

F

2F

C

2F'

F'

object:

between F ’ and 2F ’

image:

beyond 2F

real, inverted and magnified

slide52

O

2F

F

C

2F'

F'

object:

at F ’

image:

at infinity

slide53

I

O

2F

F

C

2F'

F'

object:

< F ’

image:

on the same side as the object

virtual, erect and magnified

slide54

Images formed by a convex lens

Simulation

Simulation

4 position and nature of image55
4 Position and nature of image

I

Images formed by a concave lens

2F

F

C

F'

2F'

object:

at infinity

image:

at F’

slide56

O

I

F

2F

2F'

C

F'

object:

within 2F ’ (or near object)

image:

between F ’ and 2F ’, on the same side of object

virtual, erect and diminished

example 6
Example 6

O

2F

F

2F'

F’’

An object is positioned between F’ and 2F’ of convex lens.

Complete the rays.

(a) Locate the image.

(b) Is it a virtual or a real image?

example 659
Example 6

O

(a)

2F

I

F’’

F

2F'

(b) It is a real image.

example 7
Example 7

O

An object is positioned between F ’ and 2F ’ from concave lens.

Complete the rays.

F’’

F

2F

2F'

(a) Locate the image.

(b) Is it a virtual or a real image?

example 761
Example 7

O

I

(a)

F

2F

2F'

F’’

(b) It is a real image.

example 8
Example 8

O

F

2F

F'

C

5 cm

5 cm

An object of height 5 cm is placed at 15 cm from a convex lens of focal length 10 cm.

Find the position and magnification of the image.

example 863

O

I

10 cm

5 cm

=

Example 8

height of image

= 10 cm

height of object

= 5 cm

5 cm

5 cm

F

2F

F'

C

m

= 2

example 864

I

30 cm

=

15 cm

Example 8

Alternatively,

image distance

= 30 cm

object distance

= 15 cm

5 cm

5 cm

F

2F

O

F'

C

m

= 2

q1 a boy holds a magnifying
Q1 A boy holds a magnifying...

A boy holds a magnifying glass at arm’s length.

He looks at a poster through the glass and sees a magnified erect image.

What happens to the image if he moves the lens closer to his eyes?

q1 a boy holds a magnifying66
Q1 A boy holds a magnifying...

What happens to the image if he moves the lens closer to his eyes?

A It gets larger till it gets totally blurred at some distance.

B It gets larger, keeping erect all the way.

C It gets smaller and becomes totally blurred at some distance.

D It gets smaller, keeping erect all the way.

q2 if you can capture an image
Q2 If you can capture an image...

If you can capture an image of a doll on a screen using a lens,

which of the following may NOT be correct?

A The lens you use is a convex lens.

B The image is magnified.

C The image is real.

D The image is erect.

q3 in the diagram a ray parallel
Q3 In the diagram, a ray parallel...

10 cm

In the diagram, a ray parallel to the principal axis of the lens is reflected backwards.

What is the focal length of the cylindrical convex lens?

A 5 cm

B 10 cm

C 20 cm

D 40 cm

q4 what happens to the image
Q4 What happens to the image…

What happens to the image when the plane mirror is moved backwards?

A The image becomes blurred.

B The image becomes smaller.

C The image does not change.

q5 both convex and concave
Q5 Both convex and concave...

Both convex and concave lenses can produce _______ images, which must be _________ than the object if convex lenses are used.

larger

virtual