8 k light (whs)

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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.