Transcript
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KS3 LightKS3 Light
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Properties of LightProperties 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 Properties of Light summarysummary
1) Light travels in straight lines2) Light travels much faster than sound3) We see things because they reflect
light into our eyes4) Shadows are formed when light is
blocked by an object
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ReflectionReflectionReflection from a mirror:
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
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The Law of ReflectionThe Law of Reflection
Angle of incidence = Angle of Angle of incidence = Angle of reflectionreflection
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 ReflectionClear 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 mirrorsUsing mirrorsTwo examples:
1) A periscope
2) A car headlight
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ColourColourWhite 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:The colours of the rainbow:
RedOrangeYellowGreenBlue
IndigoViolet
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Adding coloursAdding coloursWhite 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 colourSeeing colourThe 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 lightUsing 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
lightShirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
10/04/23Some further examples:
Object Colour of lightColour 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 filtersUsing filtersFilters can be used to “block” out different colours of
light:
Red Filte
r
Magenta
Filter
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Investigating filtersInvestigating 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 Refraction through a glass block: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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RefractionRefractionRefraction 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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REFRACTIONREFRACTIONRefraction 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.
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