# KS3 Light

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KS3 Light
08/04/2010

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 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:

Normal

Incident ray                               Reflected ray

Angle of       Angle of
incidence      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
____ _____ 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
Refraction in a
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rectangular block
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Newton’s disc animation
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Splitting white light animation
Colour
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White light is not a single
colour; it is made up of a
mixture of the seven
colours of the rainbow.

We can demonstrate
this by splitting white
light with a prism:

This is how rainbows
are formed: sunlight is
“split up” by raindrops.
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Splitting white light with a prism
1. Shine a ray of bright white light at a prism and move
the prism until colours appear.

2. Draw a diagram to show what you observed.
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The colours of the rainbow:

Red
Orange
Yellow
Green
Blue
Indigo
Violet
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Splitting white light into colours
A prism splits a ray of white light into a spectrum of colours.
This is known as dispersion.

When white light is split, the colours always follow the
same order.
Use this phrase to remember the order of colours:

Richard Of York Gave Battle In Vain
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Red

Magenta      Yellow

White

Blue                     Green
Cyan
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White light can be split up to make separate colours.
These colours can be added together again.

The primary colours of light are red, blue and green:

makes magenta                            green makes cyan
(purple)                                       (light blue)

and green                                    three makes
makes yellow                                  white again
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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                           Only red light
light                           is reflected
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A pair of purple trousers would reflect purple light
(and red and blue, as purple is made up of red and blue):

Purple light

A white hat would reflect all seven colours:

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:

Shirt looks red

White
light

Shorts look blue
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In different colours of light this kit would look different:

Red
Shirt looks red
light

Shorts look black

Shirt looks black
Blue
light

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

Yellow   Cyan   Magenta
Refraction through a glass block:
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Wave slows down and bends
towards the normal due to
entering a more dense medium

Wave slows down but is
not bent, due to entering
along the normal

Wave speeds up and bends
away from the normal due to
entering a less dense medium
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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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