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Light
17.1 What is Light?
• Light is an important source of energy.
• It enables us to see.
• The Sun is the most important source of
natural light.
• Without sunlight, it is impossible for living
things to survive on earth.
Speed of light
• Light travels at a very high speed.
• During a storm, lightning and thunder
occur at the same location, but we see
lightning first before we hear thunder.
• This shows that light travels faster than
sound.
• Light has the fastest speed in the
Universe. It travels at 300,000,000 m/s in
a vacuum.
Speed of light
• Comparison of the speed of light with the
speeds of other things:
Sound
invacuum
air
Light
in
Earth
in orbit
330
m/s
Space
shuttle
300,000,000
m/s
around
the
Sun
7,780 m/s
29,780 m/s
Cheetah
m/s
Garden31.0
snail
Jetplane
0.04m/s
m/s
350
How does light travel?
• Light travels in straight lines.
Experiment 1: How do we know light travels in straight
lines?
Now,
looking
Look try
at the
flame of a
through
a bent hose.
candle through
a
The
lighthose.
from the
straight
You
candle
should cannot
be ablereach
to see
your
eyes.
This shows
the light
through
the
that
light does not
hose.
bend, but travels in
straight lines.
How does light travel?
• Since light travels in straight lines,
shadows have the same shape as the
object blocking the light.
• If light did not travel in straight lines, it
would go round all objects and cast no
shadow.
Rays and beams
• We can trace the path of light from a source by
drawing straight lines to join one point to another.
• Such lines are called light rays.
• A bundle of light rays is known as a beam of light.
• Beams of light can be parallel, divergent or
convergent.
Parallel beam
Divergent beam
(fan outwards)
Convergent beam
(merge to a point)
Quiz 1
1. What is light?
• Light is an energy that enables us to see
2. How fast does light travel in a vacuum?
• It travels at 300,000,000 m/s in a vacuum.
3. Why are shadows of objects formed when
there is light?
• Because light travels in a straight line and
is blocked by the objects.
Quiz 1
4. What are light rays?
The straight paths of a light source
from one point to another.
5. State the three types of light beams.
• Parallel beam
• Divergent beam
• Convergent beam
17.2 Reflection
• When light rays hit the surface of
another medium, it can be
reflected.
• On a smooth surface such as the
surface of a mirror, regular
reflection takes place.
• A clear and undistorted image can
be seen in the surface.
A parallel beam
Reflected
of light
images
staysare very
parallel after
clear
regular
on a reflection.
smooth surface.
17.2 Reflection
• If the surface is rough such as the surface of a sheet of
paper, irregular reflection occurs.
• The surface can be seen but no reflected images are
seen in it.
• For example, when light shines on the paper, no
reflected image is formed although light is reflected.
Although this page appears
smooth, under a
microscope, you can see
how rough it is. Thus, there
is no reflected image.
A parallel beam of light hitting a rough
surface gets scattered or reflected in
an irregular manner.
Reflection in a mirror
• The flat mirror is a good reflector of light and is also
called a plane mirror.
• Look at yourself in a mirror. What you see is an
image of yourself.
• You can see your image in the mirror because light
rays from your body are regularly reflected by the
mirror into your eyes.
Reflection in a mirror
• Characteristics of plane mirror images
o The distance of the image from the mirror is equal
to the distance of the object from the mirror.
o The image and the object are the same size.
o The image is upright.
o The image is laterally inverted. This means the left
and right side of the image are reversed.
o The image is virtual. This means that the image
cannot be projected onto a screen behind the mirror.
Reflection in a mirror
• The position of an object’s image can be shown using
light rays.
• The light rays that bounce off the mirror into the eye of
the observer will appear to have originated from the
image from behind the mirror.
Tracing reflected light rays
The perpendicular line to
the surface NO is also
known as the normal.
N
B
A
A ray of light AO strikes the
mirror at point O, creating an
angle i with the
perpendicular line NO. AO is
called the incident ray, and
angle i is called the angle of
incidence.
Incident
ray
Reflected
ray
O
The light ray bounces off the mirror and leaves along the
path OB. OB is called the reflected ray. This ray makes
an angle r with the normal. This angle is known as the
angle of reflection.
Types of mirrors
• Plane mirrors
o Plane mirrors are usually used to check one’s
appearance.
o They are also used as a form of decoration to make
a room appear more spacious.
o Plane mirrors are used in cars as rear-view or
side-view mirrors.
Types of mirrors
• Concave mirrors
o These are mirrors with a curved surface that bends
inwards.
o When the object is near the concave surface, images
formed are virtual and magnified.
Types of mirrors
• Convex mirrors
o Mirrors that bend outwards are called
convex mirrors.
o The back of a spoon is an example.
Side-view mirrors of cars are sometimes
convex mirrors.
o Convex mirrors cover a wider field of vision,
and the images formed are distorted and
smaller.
Quiz 2
1. What is reflection of light?
• It is the bouncing of light when it hits the
surface of another medium.
2. When does regular reflection occur?
• When light reflects on a smooth surface
such as the surface of a mirror.
3. When does irregular reflection occur?
• When light reflects on a rough surface
such as the surface of a sheet of paper.
Quiz 2
4. What is a plane mirror?
• A flat mirror which is a good reflector of light.
5. State five characteristics of plane mirror images.
• The distance of the image from the mirror is equal
to the distance of the object from the mirror.
• The image and the object are the same size.
• The image is upright.
• The image is laterally inverted.
• The image is virtual.
Quiz 2
6. The diagram below shows the reflection of
light on a smooth surface.
(a) Name the light ray AO.
Incident light ray
N
(b) Name the
B
A
light ray OB.
Reflected light ray
O
Quiz 2
(c) Name the perpendicular line NO.
Normal line
(d) Name the angle AON.
Angle of incidence
N
(e) Name the
A
angle BON.
Angle of reflection
O
B
Quiz 2
7. Name the three types of mirrors.
• Plane mirrors
• Concave mirrors
• Convex mirrors
8. State two characteristics of concave mirror images.
• The images are virtual.
• The images are magnified.
9. State two characteristics of convex mirror images.
• The images are distorted.
• The images are smaller than the objects.
17.3 Refraction
• Light bends when a light ray travels
from one medium to another medium
of different density (such as from air
to water).
• This bending of light is known
as refraction.
17.3 Refraction
• When light travels from air to glass, it gets refracted.
At A, the point where the ray enters,
a normal is drawn. When light enters
the glass block, it is moving from
a less dense medium to a denser
medium. Hence, the ray of light
will bend towards the normal.
incident ray
Normal
A
Normal
B
emergent ray
glass block
When the ray of light exits the glass
block at B, it is moving from a denser
medium to a less dense medium.
Hence, it will bend away from
the normal.
Effect of refraction: Apparent depth
• Refraction of light causes the coin
to appear higher than it really is.
• When viewed from the surface, the depth
of the basin of water will always seem less
than what it really is.
• We say the apparent depth is less than
the real depth.
• This explains why a pool appears
shallower than it is.
Quiz 3
1. What is refraction of light?
• The bending of light when it travels from one
medium to another medium of different density.
2. When does regular reflection occur?
• When light reflects on a smooth surface such as
the surface of a mirror.
3. When does irregular reflection occur?
• When light reflects on a rough surface such as the
surface of a sheet of paper.
Quiz 3
4. The diagram below shows the refraction of
light from air through a glass block.
R
(a) Name the light ray RA.
N
Incident light ray
A
(b) Name the
light ray AB.
N
Refracted light ray
B
S
Quiz 3
(c) Name the light ray BS.
Emergent light ray
(d) Name the perpendicular
lines NA and NB.
Normal lines
R
N
A
N
B
S
Quiz 3
(e) Name the angle RAN.
Angle of incidence
(f) Name the angle N’AB.
Angle of refraction
R
N
A
N’
N
B
S
Quiz 3
5. State two effects of refraction of light.
• An object appears higher in water than it
really is.
• When viewed from the surface, water in a
swimming pool appears shallower than its
actual depth.
17.4 Dispersion of White Light
• Sunlight appears white, but it is made up of
different colours.
1. When a beam of white
light passes through a
prism, it is split into
different colours.
2. This is because the different
colours of white light bend
towards the normal through
diffferent angles when entering
and leaving the prism.
3. A spectrum of colours is produced,
which is made up of seven colours:
red, orange, yellow, green, blue, indigo
and violet. This splitting of white light is
called dispersion.
17.5 Colours
• Primary colours are basic colours that
cannot be obtained by mixing other
colours of light.
• Primary colours of light are red, green and
blue.
• Secondary colours of light are colours
that are formed by mixing primary colours.
• Secondary colours are cyan, yellow and
magenta.
17.5 Colours
Green
Cyan
Yellow
White
Blue
Red
Magenta
17.5 Colours
• We can see the colour of an object as it reflects that
colour into our eyes.
• When white light falls on a green object, the object
absorbs all other colours and reflects only green light.
• This is the case for the leaves of the strawberries below.
• The strawberries appear red because they absorb all
other colours except red.
White
Green
White
Red
17.5 Colours
Experiment 3: Coloured light
Take three different coloured balls: red, blue and green.
Let us see what happens to these coloured balls under
different colours of light.
When red
white
light
light
is is
shone
shone
onon
thethese
balls,
balls,the
only
each
redball
ballwill
willreflect
reflecttheir
the red
respective
light.
The green
colours.
andThis
blueisballs
because
white light
appear
black
contains
since they
all the
can
colours
only of
the spectrum.
reflect
green and blue light
respectively.
17.5 Colours
Experiment 3: Coloured light
What happens when a yellow ball is used in this
experiment?
But if red
Under
white
lightlight,
is shone
a yellow
on the
ball
appears
yellow
ball,
yellow
it appears red.
Yellow is a secondary colour, which
consists of red and green. Thus,
a yellow ball can reflect both red
and green light.
Quiz 4
1. How many colours does the sunlight consist
of?
• Seven
2. What is dispersion of white light?
• This splitting of white light into seven rainbow
colours.
3. State the colours of sunlight.
• Red, Orange, Yellow, Green, Blue, Indigo,
Violet
4. Which colour of light is the fastest?
• Red light
Quiz 4
5. Which colour of light is the slowest?
• Violet light
6. Which colour of light will be refracted the
most?
• Violet light
7. Which colour of light will be refracted the
least?
• Red light
Quiz 4
8. What are primary colours of light?
• Primary colours are basic colours that
cannot be obtained by mixing other
colours of light.
9. State the primary colours of light.
• Red, Green and Blue.
10. What are secondary colours of light?
• Secondary colours of light are colours
that are formed by mixing primary
colours.
Quiz 4
11. State the secondary colours of light.
• Cyan, Yellow and Magenta
12. How do you produce cyan light?
• By adding green light to blue light
13. How do you produce yellow light?
• By adding red light to green light
14. How do you produce magenta light?
• By adding red light to blue light
Quiz 4
15. Why are we able to see the colour of an object?
• We can see the colour of an object as it reflects
that colour into our eyes.
16. When white light falls on a blue table, why are
we able to see only a blue table and not other
colours?
• When white light falls on a blue table, the object
absorbs all other colours and reflects only blue
light.
Quiz 4
17. What happens to a red light when it is shone on
a blue chair?
• The red light will be absorbed by the chair.
The chair will appear black as it will reflect blue
light only and absorb other colours of light.
18. What happens to a red light when it is shone on
a yellow shirt?
• The red light will be reflected by the shirt.
The shirt will appear red as it will reflect red and
green light only and absorb other colours of
light.

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