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KS3 Light

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. 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

6. Quiz Which travels more quickly, light or sound?
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Quiz
Which travels more quickly, light or sound?
True or false. Light can bend round corners.
What causes shadows to form?
Would a cup or a glass give a darker shadow?
Are your shadows larger in the middle of the day or in the evening?

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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.

8. 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

9. 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

10. 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

11. 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

12. 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 !!!

13. 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

14. 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

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Pin Hole Cameras
A 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 cameras
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!
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.

17. Images from pinhole cameras
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Images from pinhole cameras

18. Copy and complete the following
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Copy and complete the following
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 ____________.

19. Unjumble the following words
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Unjumble the following words
nfleecirto
tefniorcar
lmorna
Gnela fo cidinetn
Ealpn romirr
sniufifdo
tmpcesru
Reflection
Refraction
Normal
Angle of incident
Plane mirror
Diffusion
Spectrum

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Pin hole cameras
Draw a diagram of a pin hole camera and using labels explain how it works to record an image.
Are they as good as a normal camera?
Why?

21. SLR Cameras (digital and film)
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SLR Cameras (digital and film)
Now complete worksheet 8Kb7

22. 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

23. 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 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

24. Lenses Lenses are used to focus light or disperse light
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Lenses
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

25. Converging Lenses Converging lenses are convex in shape Light rays
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Converging Lenses
Converging lenses are convex in shape
Light rays
coming in are
focussed by the lens
Focal point

26. Diverging lenses Light rays coming in are Light rays spread out
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Diverging lenses
Light rays
coming in are
dispersed by the lens
Light rays spread out

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Lenses and refraction
It 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.

28. towards, focal point, dense, diverging, converging, away, refracted
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Lenses
When 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

29. 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.

30. The colours of the spectrum:
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Red
Orange
Yellow
Green
Blue
Indigo
Violet

31. Video on colour YouTube - Sci Eye Colour Video on light
YouTube - Scientific Eye Light & Reflection
Both good but both 20 minutes each. Don’t use them back to back!

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Questions on the video
Explain fully why the sky is blue and the sun appears yellow.
List the primary colours.
What do colour filters do to white light?
Explain why a red t-shirt looks red in white light.
Explain why a red t-shirt looks black in green light.
Video 138 colour and refraction

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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

35. 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

36. 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

38. 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

39. 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

40. 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

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

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

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