Oh no! I can’t
see a thing. I
think I lost
my eye
sight!!
Before we move on,
We have…
Representing the video clips
available
Representing the applets
available
Representing the websites
available
Introduction to Light
Lesson objectives
Pupils should be able to

recall and use the terms for reflection, including normal, angle of
incidence and angle of reflection.

state that, for reflection, the angle of incidence is equal to the angle of
reflection and use this principle in constructions, measurements and
calculations.
Lesson Trigger
What is light?
How do you know this is true?
What evidence do you have to
show this property?
Teacher demonstration
In these demonstrations, your teacher will use a laser pointer to illustrate some
interesting phenomena related to light.
Pls turn to PB page 63 for detail.
Properties of Light
1 Light travels in a straight line.
2 Light travels at a speed of 3  108 m/s.
3 Light can travel through vacuum.
4 Light is a wave that carries energy (light energy) from one
place to another.
Reflection of Light
Seeing the Light
When light meets any surface, reflection of light occurs.
Reflection always involves two rays – an incoming or incident ray that strikes a
surface, and an outgoing or reflected ray that leaves the same surface.
Reflection of Light
Normal
This is an imaginary line perpendicular to the surface at the point where the light
strikes.
It is drawn to indicate the angle of incidence i and angle of reflection r.
What can you say about the angle of
Laws of Reflection:
incidence and angle of reflection?
normal
1 angle of incidence = angle of reflection
i = r
i
r
What can you say about the orientation of the
2 The
incident
reflected
ray and
normal at the
incident
ray, ray,
reflected
ray and
the normal?
point of incidence all lie on the same plane.
Go to Exp11a.mpg
Specular Reflection
Reflection is great! The fact that light reflects off objects allow us to see them.
smooth surface
When light reflects from a smooth surface, it maintains its geometry.
Incident parallel rays are reflected as parallel reflected rays.
This is called specular reflection .
Diffuse Reflection
When a reflecting surface is rough, diffuse reflection occurs.
rough surface
The law of reflection still holds, but incident parallel rays do not reflect as parallel
rays any more.
In diffuse reflection, the reflected rays leave the surface at so many different
directions such that the image is disrupted.
Specular and diffuse Reflection
On a mirror or a calm water surface, reflection is specular. The image formed on
such surfaces are clear and sharp.
However, if the surface is rough, or the water surface is disturbed, diffuse
reflection occurs. The image formed is blur.
Representing Light
A directed straight line is normally drawn to represent a ray, which is the path taken
by light from source to target.
target
source
A beam of light is a stream of light rays, and is represented by a number of directed
straight lines.
Lesson Closure
Reflect on your daily encounters with light,
(a)
generate one other possible evidence to support the
statement that 'light travels in a straight line', and
discuss with your partner how you can demonstrate
your statement with a safe, simple activity.
(b)
list two examples each for diffuse and specular
reflections around you.
Mirror mirror on the wall
We look at mirror everyday. What we see on the mirror is called our image. How
do you describe your image on a plane mirror?
Characteristics of image on a plane mirror
•
Same size as object
•
As far behind the mirror as the object is
in front.
•
Virtual, as it cannot be captured on the
screen
•
Laterally inverted
Go to Exp11b.mpg
Image on a plane mirror
The image of an object is formed on a plane mirror when light ray from the object
incidents on the mirror.
object
i
r
plane mirror
Light will reflect at a mirror surface such that the angle of incidence and reflection
are equal.
Drawing Ray Diagrams
There are many light rays reflected from an object to reach the mirror surface.
However, only some light rays will be captured by the eyes.
object
plane mirror
To view the image of an object in a mirror, the eye should be positioned along the
direction where the reflected rays from the mirror can be captured by the eye.
Drawing Ray Diagrams
To the eye, the light ray reaching the eye appears to come from the image behind
the mirror.
object
plane mirror
This type of image is called a virtual image because it is formed at a place where
there is no light from the object. The mirror simply makes the light appear to be
coming from behind it.
Drawing Ray Diagrams
Steps involved in drawing ray diagrams
Supposing a triangular object is placed in front of a mirror. We can draw a ray
diagram to show how the eye sees the image in the mirror.
plane mirror
object
x cm
x cm
image
1st - draw the image of the triangle, such that it is of the same size and same shape,
and as far behind the mirror as the object is in front.
Drawing Ray Diagrams
2nd – draw two diverging rays from any point on the image towards where the eye is
positioned.
object
plane mirror
image
3rd – draw two diverging rays from the corresponding point on the object to the
mirror to meet the reflected rays.
Go to E-SimPhy_308.exe
Ray diagram Practice 1
(a) Which
Sarah direction
S went shopping
withray
Caroline
one Sunday
is the light
actuallyCcoming
from?afternoon. Both girls stood
in frontdirection
of a shop
admiring
theray
window
Which
does
the light
seemsdisplay.
to reach Sarah’s eye?
(i)
(ii)
Mark Caroline’s image on the glass window, at appropriate position.
Draw ray diagrams to show how Sarah can see her friend by reflection on
the shop window glass.
Where should you mark Caroline’s image on the glass window? Why?
S
wall
C
Shop window
wall
Ray diagram Practice 1
(b)Which
Caroline
movesis
away
from
Sarah
to a new
C’ to look
direction
thethe
light
ray
actually
coming
from?
does
light
ray
seems
to location
reach
Sarah’s
eye?at the display at the
next shop.
S
C
i
wall
(i)
(ii)
(ii)
Shop window
C’
r
wall
Mark Caroline’s new image position
Show by ray diagram, whether Sarah can still see her friend by reflection.
Explain how your ray diagram helps you conclude on whether Caroline
can be seen by reflection.
Light rays from Caroline can still be reflected on the
glass window, obeying the Laws of Reflection, where
i = r.
Beyond classroom activity
Fun with Billiard
Visit the website below to practise your billiard skill! Have fun playing and
learning the Physics of billiard, and see how the Laws of Reflection can be
applied in playing billiard.
Lesson Closure
Enrichment / Extension
Think about the various type of mirror you encounter in your daily life.
(a)
Look at the image formed. Can you explain the difference in the images
formed from the one you use everyday in your bathroom?
(b) Compare and contrast the type of surfaces of such mirrors, and explain
why the images formed are different from one another?
Refraction of Light
Lesson objectives
Pupils should be able to
◙
recall and use the terms for refraction, including normal, angle of
incidence and angle of refraction.
◙
recall and apply the relationship, sin i  sin r = constant to new
situation or to solve related problems.
◙
define refractive index of a medium in terms of the ratio of speed
of light in vacuum and in the medium.
Refraction of Light
Behaviour of light
When a beam of light encounters an obstacle in its path, a number of things can
happen:
1
Reflection -
2
Refraction -
3
Absorption -
The degree of each effect depends on the nature of the materials the light is
incident upon.
Refraction of Light
Using
a laser
pen,
a beam
of light to
is sent
from
What
do you
think
will happen
its path
of water
travelinto
as itair.
strikes the air-water
boundary?
When light strikes such transparent boundary, both reflection and refraction
occur.
refraction
refraction
reflection
water
reflection
laser pointer
glass
refraction
air
reflection
Refraction of Light
When
will refraction
Why does
refraction occur?
occur?
1
Refraction occurs whenever light passes
between transparent media of different
optical densities.
2
Refraction occurs because light travel with
different speed when in media of different
optical densities.
Refraction of Light
The change in speed at the transparent boundary between two media causes light
Optical density of glass > water > air > vacuum
to change direction.
Air has lower optical
density – faster.
Glass has higher optical
density – slower.
glass
air
Air has lower optical
density – faster.
The more optically dense the material, the slower the speed of light in that material
Refraction Terminology
i = angle of incidence
r = angle of refraction
i
r
incident ray
air
refracted ray
air
glass
water
refracted ray
r
incident ray
i
Go to E-SimPhy_304.exe
Refraction Rules
Light bends away from the normal when emerging from water into air, which is from
an optically denser medium to an optically less dense medium.
Optical density of glass > water > air > vacuum
bends away
from the
normal
bends
towards
the
normal
Light bends towards normal when entering into water from air, which is from an
optically less dense medium to an optically denser medium.
Summary
glass
air
i
from an optically less dense
medium to an optically
denser medium, i > r
bends towards
the normal
bends away
from the
normal
glass
air
r
from an optically denser
medium to an optically less
denser medium, i < r
Ray diagram Practice 2
For each of the diagram below, complete the path of blue light as it emerges
from the transparent medium.
Refractive Index, n
The refractive index of a medium is the ratio of the speed of light in one
medium relative to the speed of light in the other medium.
For example, the refractive index of glass at the air-glass boundary is given as:
air
glass
vair
vglass
vair
v air
n= v
glass
where v air and v glass are the
speed of light in air and glass
respectively.
Refractive Index, n
if 1 represents the angle of incidence in the less dense medium, and 2 represents
the angle of refraction in the denser medium;
1
2
glass
air
1
Then:
sin 1
n = sin 
2
2
The ratio of the two sin  s
gives the same refractive
index , and this relationship is
called Snell’s Law.
Go to Exp11c.mpg
Sample Problem 1
A ray of light approaches a glass-air boundary at an angle of incidence i = 30.
What is the refractive index of the glass if the angle of refraction r = 49 ?
glass
air
49
n
sin 1
=
sin 2
sin 49
=
sin 30
angle in less
dense medium
angle in less
dense medium
= 1.5
Go to E-SimPhy_306.exe
Sample Problem 2
A ray of light approaches a water-air boundary at an angle of incident i = 35.
(i)
Complete the path of the light as it crosses the water-air
boundary.
(ii)
What is the angle of refraction if the refractive index of water n = 1.3?
n
air
water
r
sin 1
=
sin 2
sin 35
1.3 =
sin r
r = 26
Refraction of Light
Lesson closure - Think about it
Prism disperses white light into its 7 component colours as the refractive
index of each colour light is different in the glass prism.
Analyse the diagram of dispersion, and infer the colour light thathas the
greatest refractive index.
Total Internal Reflection
Lesson objectives
Pupils should be able to:
•
explain the terms critical angle and total internal reflection.
•
describe the action of a thin lens on a beam of light.
•
define the focal length for a converging lens.
•
draw ray diagrams to illustrate the formation of real and virtual
images of an object by a thin converging lens.
Lesson Review
Refraction occurs as light passes across the boundary between two transparent
media.
refraction
But why does light refract?
What is the cause of such
behaviour?
reflection
glass
refraction
air
reflection
Think – Pair share (5 min)
Review previous lesson on refraction. Take turn to share with your partner
facts about refraction. Jot done your discussion in your Physics notebook.
Total Internal Reflection
The picture below be easily reproduced with a laser pointer and a transparent semicircle glass block in a darkened room. As light enters the glass block, it bends at the
surface instead of traveling its original path.
What do you think will happen if the angle of incidence in the glass block is
increased gradually?
Total Internal Reflection
A light ray from water is incident on the water - air boundary. The angle of incidence
is gradually increased.
(i)
(ii)
Calculate the angle of refraction if the refractive index of water is 1.3.
What do you observe about the angle of refraction as the angle of incidence is
gradually increased?
26o
41o
57o
air
air
air
glass
glass
glass
20o
30o
40o
As the angle of incidence increases, angle of refraction increases as well.
What is the maximum angle of refraction that can be produced?
Total Internal Reflection
At
a certain
critical angle
angle is
c,greater
a maximum
angle
of refraction
= 90
produced.
If the
next incident
than c,
no more
refraction
willisoccur.
A weak reflected ray is also produced.
The reflected ray becomes very strong and intense.
90
air
glass
weak reflection
c
c
air
glass
i> c
r
strong reflection
Go to Exp11d.mpg
if incident angle > critical angle:
 no light ray from the optically denser medium will be refracted.
 all light rays will be totally internally reflected into the optically denser
medium.
Critical angle and Total Internal Reflection
Critical angle c is defined as the angle of incidence from a denser medium which
produces an angle of refraction of 90.
90
air
glass
weak reflection
c
air
glass
i> c
c
r
strong reflection
Total internal reflection only occurs when:
 a light ray is travelling from an optically denser medium to an optically less
dense medium.
 the angle of incidence is greater than the critical angle.
Sample Practice 1
For each combination of media, which light ray (A or B) will undergo total internal
reflection if the incident angle is gradually increased? Explain your choice.
A
glass
air
air
water
B
B
glass
water
B
In both cases, light is approaching the boundary from an optically denser
medium to an optically less dense medium.
A gradual increase in the incident angle will produce an increasingly larger
angle of refraction.
Sample Practice 2
A ray of light approaches a water-air boundary at a angle of incidence c, which
causes it to undergo total internal reflection. Calculate the critical angle c if the
refractive index of water n = 1.3
90
air
water
c
n
sin 1
=
sin 2
sin 90
1.3 =
sin c
c = 50
Total Internal Reflection
Enrichment
A laparoscope is a medical equipment inside a hollow, thin tube. It is
connected to a camera and a high intensity light for doctor to see the
structure inside our body.
Perform an internet search to find out how
total internal reflection plays a part in
different fields.
Be prepared to share with your classmates
what you have learnt from your research
next week.
Thin converging lens
A ray of light that is incident at an angle on a transparent boundary will undergo
refraction.
glass
air
A converging lens
Converging lens is a piece of glass which is thicker at the centre and thinner
at the 2 ends. What do you think will happen to a light ray incident on its
surface?
A converging lens is simply a piece of glass which is thicker at
the centre and thinner at the edge.
Another imaginary line
used to mark the midpoint of the lens
Principal axis
- This imaginary line
divides the lens into equal
upper and lower halves.
Optical centre
-
geometric centre of the
lens
Parallel rays from distant object will all converge at a plane
called focal plane.
principal focus
focal plane
focal length
Quick Check 1
What are the names of the various parts of a thin converging lens?
Are you able to name each and every one of them?
4
2
1
3
5
Quick Check 2
What
happens
to each
of the
rays as
pass through
A parallel
beam
of light
notlight
parallel
to they
the principle
axisthe
will
converging
lens?
Canon
youthe
complete
the path they will take?
bend at some
point
focal plane.
F
focal plane
A beam of light parallel to the principle axis will bend at the
principle focus F.
For simplicity, a thin converging lens is represented by a double
arrowed line.
F
F
F
F
1 (a) Rays parallel to principal axis will meet at the principal focus.
F
(b) Rays passing though principal focus will emerge from
converging lens as parallel rays.
2 Ray passing through optical centre will emerge unbent.
F
Sample Practice 3
An object O is placed in front of a converging lens L.
(i) Draw ray diagram to show how its image is formed.
Label the image as I.
(ii) Describe the image formed.
L
The image formed is:
O
inverted, diminished,
real
F
I
Lesson Review
A converging lens is simply a piece of glass which is thicker at
the centre and thinner at the edge.
What do I know?
What have I learnt?
Think – Pair share (5 min)
Review previous lesson. Take turn to share
with your partner facts about converging lens.
Jot down your discussion in your Physics
notebook.
Trends and patterns on the images formed
The type of the image produced by a converging lens are
determined by the distance an object is placed away from the lens.
Camera
Same size photocopier
Slide projector
Spot light
Magnifying glass
Go to E-SimPhy_309.exe
Challenge yourself 1
The positions of an object and its image are as shown.
(i) How do you determine the position of the lens?
(ii) Where is the principle focus of the lens?
L
F
Challenge yourself 2
A thin converging lens casts an image a distance away from the
centre of the lens. Given the position of the principle focus as
shown below, where should the object be positioned?
Object O
Challenge yourself 2
Complete
path
of the rays
show how
anthe
image
Since boththe
blue
coloured
rays to
originate
from
tip ofcan
thebe
object
formed
by the
converging
lens.
arrow, both
rays
should end
at the tip of the image arrow.
L
Object O
F
F
Summary
By the end of this lesson pupils should be able to:
Use the terms normal, angle of incidence and angle of reflection (for reflection).
State the laws of reflection and use it in calculations and measurements.
Use the terms normal, angle of incidence and angle of refraction (for refraction).
Solve problems using sin i ÷ sin r.
Define refractive index of a medium in terms of the ratio of speed of light in
vacuum and in the medium.
Explain the terms critical angle and total internal reflection.
Summary
Describe the action of a thin converging lens on a beam of light.
Define the term focal length for a thin converging lens.
Draw ray diagrams to illustrate the formation of real and virtual images of an
object by a thin converging lens.