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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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Shadows

Shadows are places where light is “blocked”:

Rays of light

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QuizQuiz1. Which travels more quickly, light or

sound?2. True or false. Light can bend round

corners.3. What causes shadows to form?4. Would a cup or a glass give a darker

shadow?5. Are your shadows larger in the

middle of the day or in the evening?

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Copy and complete …Copy and complete …OPAQUE objects absorb the light, don’t let it pass through. This causes a _________. E.g. a _______ is opaque.

____________ objects absorb some of the light but some can still pass through. This causes a pale shadow. E.g. a china ________ is translucent.

TRANSPARENT objects absorb no light letting it all pass through. This prevents a solid shadow from forming. E.g. a _________ is transparent.

shadow, translucent, glass, cup, rock.

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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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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 ____ _____ 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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Pin Hole CamerasPin Hole CamerasA pinhole camera is a simple device for recording

an image. It uses two screens separated by a tube or box one of which is sensitive to light and one of which has a pin hole.

The light passes in through the tiny hole and hits the back screen.

The image you see is inverted (upside down). Using your knowledge of light try and explain why.

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Pin hole camerasPin hole cameras

As the light from the bottom of the tree travels in a straight line through the hole it then travels up to the top of the screen.

As the light from the top of the tree passes through the hole it travels in a straight line to the bottom of the screen.

This results in the final image recorded on the light sensitive paper being upside down. You just need to turn the paper the other way up to view it!

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Images from pinhole Images from pinhole camerascameras

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Copy and complete the Copy and complete the followingfollowing

Light travels in ___________ lines at 300,000km/s. When it hits an opaque object it causes ___________ to be formed.

When light hits a plane mirror it is ___________. The angle of ________ equals the angle of ___________.

When light hits an uneven surface it is ____________.

21/04/23Unjumble the following Unjumble the following wordswords

Reflection

Refraction

Normal

Angle of incident

Plane mirror

Diffusion

Spectrum

nfleecirto

tefniorcar

lmorna

Gnela fo cidinetn

Ealpn romirr

sniufifdo

tmpcesru

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Pin hole camerasPin hole cameras

Draw a diagram of a pin hole camera and using labels explain how it works to record an image.

1. Are they as good as a normal camera?2. Why?

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Now complete worksheet 8Kb7

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Refraction through a glass Refraction through a glass block:block:

Wave slows down and bends towards the normal due to

entering a more dense medium

Wave speeds up and bends away from the normal due to 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 pencil is placed in water it looks like this:

In this case the light rays are slowed down by the water and are _____, causing the pencil to look odd. The two mediums in this example are ______ and _______.

Words – speed up, water, air, bent, medium

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LensesLenses

Lenses are used to focus light or disperse light

This changes the direction that the light was travelling.

There are two types of lenses

Diverging – spreads light rays out

Converging – focuses light rays together

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Converging LensesConverging Lenses

Converging lenses are convex in shape

Focal pointLight rayscoming in arefocussed by the lens

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Diverging lensesDiverging lenses

Light rayscoming in aredispersed by the lens

Light rays spread out

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Lenses and refractionLenses and refractionIt is easier to understand what is going on with a lens if you think of it in small pieces.

Each time the light is being REFRACTED through a piece of lens and so is changing speed. This causes the light to bend and change direction.

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LensesLensesWhen light travels through a lens it is ________. The medium of the lens is more ______ than air so the light bends _______ the normal in the lens then _____ from the normal when it moves back into air.

With a convex or ________ lens the result focuses the light to a _____ _____. The amount of curve on the lens changes the focal length.

With a concave or ________ lens the result spreads out the rays of light.

towards, focal point, dense, diverging, converging, away, refracted

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ColourColourWhite 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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The colours of the spectrum:The colours of the spectrum:

RedOrangeYellowGreenBlue

IndigoViolet

Video on colour

YouTube - Sci Eye Colour

Video on light

YouTube - Scientific Eye Light & Reflection

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Questions on the videoQuestions on the video1. Explain fully why the sky is blue

and the sun appears yellow.2. List the primary colours.3. What do colour filters do to white

light?4. Explain why a red t-shirt looks red

in white light.5. Explain why a red t-shirt looks

black in green light.

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Red

GreenBlue Cyan

YellowMagenta

White

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Adding coloursAdding coloursWhite 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 all three makes white again

Adding red and green makes yellow

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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

21/04/23Some 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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