6.3 Image Formation by Lenses

Physics
Chapter 6 Light and Optics

Cahaya dan Optik

6.3 Image Formation by Lenses

Pembentukan Imej Oleh Kanta

Converging Lens Lenses CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES
Diverging Lens
Lens is a piece of
Biconvex Biconcave
transparent material such
Lens
as glass, perspex or

plastic which has two
surfaces with at least
one surface curved.
There are two main types
of lenses, convex lens
and concave lens.

Glass/perspex

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Lenses CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Convex Lens Concave Lens

Biconvex Convex meniscus Plano- Biconcave Concave meniscus Plano-
convex concave

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CHAPTER 6 LIGHT AND OPTICS Refracted ray
6.3 IMAGE FORMATION BY LENSES

Refracted ray

The above activity shows that parallel light rays which pass
through convex lens will converge at a focal point, F.
Therefore, convex lenses are known as converging lenses.

Parallel light rays passing through concave lens appear to be
diverging from a focal point, F. Therefore, concave lenses are
known as diverging lenses.

Chapter 6 Light and Optics

Understanding Lenses

Thickness Of Lens And Its Focal Length

Thickness of lens Focal length

A thicker convex lens has a shorter focal length while a thinner convex lens
has a longer focal length.

To differentiate between convex lens and concave lens,

convex lens : positive focal concave lens : negative focal
length length

focal length, f = +10.0 cm focal length, f = -20.0 cm

refers to a convex lens of focal refers to a concave lens of focal
length of 10.0 cm. length of 20.0 cm

Physics Form 4

Chapter 6 Light and Optics

Understanding Lenses

Power Of A Lens

❖ The power of a lens is defined as the reciprocal of its focal length
(measured in metres)

Power of lens,P = 1 or P = 100
f(m) f(cm)

❖ Unit = per metre (m-1) or Dioptre ( D ).

Examples Focal length, f Power, P
Type of lens + 5.0 cm
- 10.0 cm + 1 = +20D
convex 0.05

concave 1 = -10D
- 0.1

Physics Form 4

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CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Position and Characteristics of Images Formed by Lenses

As long as the focal length is
fixed as you wish, you can get
the image anytime by drawing.

Most of the situation, two rays are enough
to get the intersection and the image.

CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Position and Characteristics of Images Formed by Lenses

Convex Lens

Concave Lens

CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Difference between Real and Virtual Images

The image falls
onto any

screen/paper/hand

The image can only
be seen inside the

glass/mirror

Difference between Real and Virtual Images CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Real image: Image is Virtual Image: Image is
formed on screen/paper seen inside mirror/glass.
Can only view through
or any surface.
Can view from any mirror or glass.

position.

CHAPTER 6 LIGHT AND OPTICS

Linear Magnification, m 6.3 IMAGE FORMATION BY LENSES

When an object is seen Thin lens ray diagram – GeoGebra
through a magnifying glass
at a distance less than its
focal length, the image
formed is magnified.
The size of the image
formed by a convex lens
depends on the position of
the object

Lenses (bu.edu)

CHAPTER 6 LIGHT AND OPTICS
6.3 IMAGE FORMATION BY LENSES

Comparison between image size and object size is made
based on the ratio of image height to object height.
This ratio is known as linear magnification, m.

Ratio of image CHAPTER 6 LIGHT AND OPTICS
height to object 6.3 IMAGE FORMATION BY LENSES
height is also the
same as the Ratio Linear magnification, m
of image distance does not have a unit.

to object
distance.

1. magnified, upright, virtual 3. Place the object more than focal point.

2.
10 cm

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