Objectives
Objectives
• Define the terms normal, angle of
incidence and angle of reflection
• State the angle of incidence is equal
to the angle of reflection and use
this principle in construction,
measurements and calculations.

Nature of light
Nature of light
• A form of energy which enables us to see
• It travels in straight line
• It travels at a very high speed
(3.0 x 108
ms-1
) in vacuum.
• It is emitted by luminous sources such as
SUN, STARS, CANDLES,
SUN, STARS, CANDLES,
LAMPS,
LAMPS, etc.
etc.

Nature of light
Nature of light
• White light is a mixture of seven
White light is a mixture of seven
different colours namely Red,
different colours namely Red,
Orange , Yellow, Green, Blue,
Orange , Yellow, Green, Blue,
Indigo, Violet.
Indigo, Violet.
• Light is a transverse wave and
Light is a transverse wave and
forms part of the electromagnetic
forms part of the electromagnetic
spectrum
spectrum.
.

Light ray
Light ray
• The path along which light energy travels
is called a RAY.
• It is represented by a straight line with
an arrow to show its direction of motion.

Beam of Light
Beam of Light
• A beam of light is a bundle of light rays.
Parallel beam Converging
beams
Diverging beams

The abrupt change in the
direction of propagation of
light rays that strike the
boundary between different
.
medium
Reflection of light
Reflection of light

plane
p
normal
u
normal
center
An imaginary line perpendicular to a surface
.
where the reflection occurs
Normal :

Reflection of Light(1)
Reflection of Light(1)
i r
Incident
ray
normal
plane mirror
( )
The ray striking the surface incoming ray
( )
The ray reflected fromthe surface outgoing ray
Reflected
ray

Reflection of Light(2)
Reflection of Light(2)
The angle between incident ray and normal
The angle between reflected ray and normal
i r
Incident
ray
normal
plane mirror
Reflected
ray
∠ i = angle of incidence
∠ r = angle of reflection
∠ i ∠ r

:
Laws of Reflection
• Law 1: The incident ray, the reflected
ray and the normal at the point of
incidence all lie on the same plane.
• Law 2: The angle of incidence is equal
to the angle of reflection.
Anglei=Angler

The diagram shows a ray of light being
The diagram shows a ray of light being
reflected by a plane mirror MN.
reflected by a plane mirror MN.
Which one of the following statements is
Which one of the following statements is
completely correct ?
completely correct ?
A LP is the incident ray, PR
A LP is the incident ray, PR
is the reflected ray and x is
is the reflected ray and x is
the angle of reflection.
the angle of reflection.
B LP is the reflected ray, x is the angle of
B LP is the reflected ray, x is the angle of
incidence and y is the angle of reflection.
incidence and y is the angle of reflection.
C LP is the incident ray, x is the angle of
C LP is the incident ray, x is the angle of
incidence and y is the angle of reflection.
incidence and y is the angle of reflection.
D PQ is the normal, x = y, and PR is the
D PQ is the normal, x = y, and PR is the
incident ray.
incident ray.
C
1
Example1
Example

Reflection by
the plane mirror
i r
Plane mirror
Note: Normal always perpendicular
to the mirror
i r
Incident
ray
normal
plane mirror
Reflected
ray
∠ i = angle of incidence
∠ r = angle of reflection
∠ i ∠
r

2
Example2
Example
Angle incidence = 900
- 400
= 500
Angle of reflection = Angle of incidence
= 500
The diagram shows a ray of light reflected in a
plane mirror.
What is the angle of reflection?

3
Example3
Example
Angle of incidence = 900
- 100
= 800
Angle b = 100
A ray of light is reflected from a plane
mirror as shown in the diagram below:
• What is the angle of
incidence?
• What is the size of
the angle marked b?

The diagram shows a ray of light striking a
plane mirror. What is the angle of
incidence if the total angle between the
incident ray and reflected ray is 70°?
4
Example4
Example
Angle of incidence
= Angle of reflection
= 700
/ 2
= 350

5
Example5
Example
In the figure below, ray AB will be reflected twice.
Find the angle of reflection in the second reflection.
Copy the figure and complete the path of the ray.
C
300
r1
i2 r2
angle r1 = 300
; angle r2 = angle i2
= 900
- 300
= 600
u
300
A
B

MIRROR IMAGE(1)
BIKE
BOOK
BOX
CHOKE
DECIDE
CHICK

MIRROR IMAGE(1)
BIKE
BOOK
BOX
CHOKE
DECIDE
CHICK

MIRROR IMAGE(2)
SPEED
POLES
BED
LIGHT
POWER
CHECK

MIRROR IMAGE(2)
SPEED
POLES
BED
LIGHT
POWER
CHECK

Formation of images on a
:
plane mirror
• To the eye, all the reflected rays
appear to have come from I, the
image.
• Thus, the image formed is virtual.

Real and Virtual I mages
• A real image is one which can be
formed on a screen.
• It is produced by ray that converge
at the position of the image.
• A virtual image is one which cannot
be formed on a screen.
• It is produced by reflection of light.

I mage formed by
:
Plane Mirror
• It is virtual.
• It is upright.
• It is lateral inverted.
• It has the same size as the object.
• Distance of the image from the mirror
is equal to the distance of the object
from the mirror.

A person stands 2.0 m in front of the mirror as
shown. How far from the person is her image?
Example
Example
Image is 4.0 m
from the
person.

The diagram shows two divergent rays of
light from an object O being reflected from
a plane mirror.
At which position will the image be
formed?
Example
Example
B
B

i
r
Note:
i = r
object image
normal
eye
object
image
mirror
mirror
Ray diagramshowing the
formation of an image

I mage formed by
the plane mirror

Plane mirror
Regular reflection
Black surface
Absorption
Irregular surface
Diffuse reflection
Glass block
Transmission
Glass
Various cases of rays:

Two types of Reflection:
1. Regular reflection ( Image formed
by plane, a smooth surface, image
can see by our eyes)

2. Diffuse reflection ( Image formed by
white paper, with rough surface. Image
formed usually not clear)
(continue …)

mirror
mirror
:
Periscope
Periscopes are used
in submarines so
that people below
the water surface
can see what is
happening above.
(Type 1)

prism
prism
Periscopes are used
in submarines so
that people below
the water surface
can see what is
happening above.
:
Periscope
(Type 2)

Pe
ris
c
o
p
e
:[Co
nt.]
• The periscope consists of two parallel plane
mirrors (or prism) .
• Could you guess at what angle should each
mirror (or prism) be placed ?
• Why ?
• Which type of periscope is better ? Mirror or
prism ? Why ?

GCE O-Level
GCE O-Level
Past Examination Paper
Past Examination Paper
Science (Physics)

November 1989
8. Which of the following correctly describes the image
formed in a plane mirror when compared to the
object?
A virtual smaller
B virtual same size
C virtual larger
D real same size
B

Nov 1990
6. An object ‘O’ is placed in front of a plane mirror.
A viewer looks into the mirror as shown in the
diagram.
In which position is the image of O formed?
B

Nov 1991
10. The diagram shows a ray of light being
reflected by a plane mirror MN.
Which one of the following statements is
completely correct ?
A LP is the incident
ray, PR is the
reflected ray and
x is the angle of reflection.
B LP is the reflected ray, x is the angle of
incidence and y is the angle of
reflection.
C LP is the incident ray, x is the angle of
incidence and y is the angle of
reflection.
D PQ is the normal, x = y, and PR is the
C

4. The diagram below shows the position I of the image
formed by a plane mirror of an object O.
(a) Continue the two rays drawn leaving O to show
how they would be reflected at the mirror. [2]
GCE ‘O’ LEVEL Nov 1995
I O

4.(b) I is a virtual image. Explain the meaning of
this. [2]
(Cont. …) Q. 4 Nov 1995
Since the image, I, is produced by the reflection
of light and cannot project on a screen, there-
fore it is a virtual image.