Light

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Light
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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.

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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.

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Speed of light

• Comparison of the speed of light with the speeds
  of other things

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How does light travel?

• Light travels in straight lines.
• Experiment 1 How do we know light travels in
  straight lines?

Now, try looking through a bent hose. The light
from the candle cannot reach your eyes. This
shows that light does not bend, but travels in
straight lines.
Look at the flame of a candle through a straight
hose. You should be able to see the light through
the hose.
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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.

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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)
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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.

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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

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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.

Reflected images are very clear on a smooth
surface.
A parallel beam of light stays parallel after
regular reflection.
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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.
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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.

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Reflection in a mirror

• 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. This means the
  left and right side of the image are reversed.
• The image is virtual. This means that the image
  cannot be projected onto a screen behind the
  mirror.

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Reflection in a mirror

• The position of an objects 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.

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Tracing reflected light rays
The perpendicular line to the surface NO is also
known as the normal.
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.
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.
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Types of mirrors

• Plane mirrors
• Plane mirrors are usually used to check ones
  appearance.
• They are also used as a form of decoration to
  make a room appear more spacious.
• Plane mirrors are used in cars as rear-view or
  side-view mirrors.

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Types of mirrors

• Concave mirrors
• These are mirrors with a curved surface that
  bends inwards.
• When the object is near the concave surface,
  images formed are virtual and magnified.

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Types of mirrors

• Convex mirrors
• Mirrors that bend outwards are called convex
  mirrors.
• The back of a spoon is an example. Side-view
  mirrors of cars are sometimes convex mirrors.
• Convex mirrors cover a wider field of vision, and
  the images formed are distorted and smaller.

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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.

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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.

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Quiz 2

• 6. The diagram below shows the reflection of
  light on a smooth surface.
• (a) Name the light ray AO.
• Incident light ray
• (b) Name the
• light ray OB.
• Reflected light ray

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Quiz 2

• (c) Name the perpendicular line NO.
• Normal line
• (d) Name the angle AON.
• Angle of incidence
• (e) Name the
• angle BON.
• Angle of reflection

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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.

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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.

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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.
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.
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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.

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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.

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Quiz 3

• 4. The diagram below shows the refraction of
  light from air through a glass block.
• (a) Name the light ray RA.
• Incident light ray
• (b) Name the
• light ray AB.
• Refracted light ray

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Quiz 3

• (c) Name the light ray BS.
• Emergent light ray
• (d) Name the perpendicular
• lines NA and NB.
• Normal lines

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Quiz 3

• (e) Name the angle RAN.
• Angle of incidence
• (f) Name the angle NAB.
• Angle of refraction

N
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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.

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17.4 Dispersion of White Light

• Sunlight appears white, but it is made up of
  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.
1. When a beam of white light passes through a
prism, it is split into different colours.
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.
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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.

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17.5 Colours
Green
Cyan
Yellow
White
Blue
Red
Magenta
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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.

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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 white light is shone on these balls, each
ball will reflect their respective colours. This
is because white light contains all the colours
of the spectrum.
When red light is shone on the balls, only the
red ball will reflect the red light. The green
and blue balls appear black since they can only
reflect green and blue light respectively.
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17.5 Colours

• Experiment 3 Coloured light
• What happens when a yellow ball is used in this
  experiment?

Under white light, a yellow ball appears yellow
But if red light is shone on the yellow ball, 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.
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Quiz 4

• 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

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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

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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.

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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

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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.

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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.