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KS3 Light
W Richards
Worthing High School

2. Light travels in straight lines:
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Properties of Light
Light travels in straight lines:
Laser

3. Light travels VERY FAST – around 300,000 kilometres per second.
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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.

4. Light travels much faster than sound. For example:
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Light travels much faster than sound. For example:
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.

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

6. Luminous and non-luminous objects
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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

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

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

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

10. Angle of incidence = Angle of reflection
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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 !!!

11. Clear vs. Diffuse Reflection
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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

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

13. 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.

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

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Red
Magenta
Yellow
White
Blue
Green
Cyan

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Adding colours
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:
Adding blue and red makes magenta (purple)
Adding blue and green makes cyan (light blue)
Adding red and green makes yellow
Adding all three makes white again

17. Only red light is reflected
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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:
Homework
White
light
Only red light is reflected

18. A white hat would reflect all seven colours:
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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

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

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

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

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

24. Red Blue Green White Yellow Cyan Magenta
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Red
Blue
Green
White
Yellow
Cyan
Magenta

25. 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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27. Words – speed up, water, air, bent, 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