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KS3 Light

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Properties of Light
Light travels in straight lines:
Laser

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Light travels VERY FAST – around
300,000 kilometres per second.
At this speed it can
go around the world 8
times in one second.

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Light travels much faster than sound. For example:
1) Thunder and lightning
start at the same
time, but we will see
the lightning first.
2) When a starting pistol
is fired we see the
smoke first and then
hear the bang.

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We see things because they
reflect light into our eyes:
Homework

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Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects Reflectors

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Shadows
Shadows are places where light is “blocked”:
Rays of light

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Properties of Light summary
1) Light travels in straight lines
2) Light travels much faster than sound
3) We see things because they reflect light
into our eyes
4) Shadows are formed when light is blocked
by an object

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Reflection
Reflection from a mirror:
Incident ray
Normal
Reflected ray
Angle of
incidence
Angle of
reflection
Mirror

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The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at
the SAME angle it hits it.
The
same !!!

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Clear vs. Diffuse Reflection
Smooth, shiny surfaces
have a clear reflection:
Rough, dull surfaces have
a diffuse reflection.
Diffuse reflection is when
light is scattered in
different directions

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Using mirrors
Two examples:
1) A periscope
2) A car headlight

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Colour
White light is a mixture of
the seven colours of the
rainbow.
White light can be split
by a PRISM
Rainbows form when
sunlight is split by
raindrops.

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The colours of the rainbow:
Red
Orange
Yellow
Green
Blue
Indigo
Violet

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Adding colours
White light can be split to make separate colours.
These colours can be added together again.
The primary colours are red, blue and green
Adding blue and red
makes magenta
Adding blue and
green makes cyan
Adding all
three makes
white again
Adding red
and green
makes yellow

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Red
GreenBlue
Cyan
YellowMagenta
White

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Seeing colour
The colour an object appears depends on the colours
of light it reflects.
For example, a red book only reflects red light:
White
light
Only red light
is reflected

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A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light
(and red and blue, as purple is made up of red and blue):
Purple light
White
light

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Using coloured light
If we look at a coloured object in coloured
light we see something different. For
example, consider a football kit:
White
light
Shorts look blue
Shirt looks red

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In different colours of light this kit would look different:
Red
light
Shirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue

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Some further examples:
Object Colour of light
Colour object
seems to be
Red socks
Red Red
Blue Black
Green Black
Blue teddy
Red Black
Blue
Green
Green camel
Red
Blue
Green
Magenta book
Red
Blue
Green

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Using filters
Filters can be used to “block” out different colours of light:
Red
Filter
Magenta
Filter

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Investigating filters
Colour of filter Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta
Yellow

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Red
Magenta
White
Yellow
Blue Green
Cyan

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Refraction through a glass block:
Wave slows down and bends
towards the normal on
entering a more dense medium
Wave speeds up and bends
away from the normal on
entering a less dense medium
Wave slows down but is
not bent, due to entering
along the normal

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Refraction
Refraction is when waves ____ __ or slow down due to
travelling in a different _________. A medium is
something that waves will travel through. When a pen is
placed in water it looks like this:
In this case the light rays are slowed down by the water
and are _____, causing the pen to look odd. The two
mediums in this example are ______ and _______.
Words – speed up, water, air, bent, medium

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REFRACTION
Refraction is when rays SPEED UP or SLOW
DOWN when travelling through a different
MEDIUM. E.g. a ruler placed in water:
The light rays are slowed down by the water and
are BENT. This makes the ruler look bent. The
two media in this example are WATER and AIR.