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© Boardworks Ltd 2003. A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97)

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Presentation on theme: "© Boardworks Ltd 2003. A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97)"— Presentation transcript:

1 © Boardworks Ltd 2003

2 A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000). Normal ViewNotes Page View Teacher’s Notes Flash Files A flash file has been embedded into the PowerPoint slide wherever this icon is displayed – These files are not editable.

3 © Boardworks Ltd 2003 Light : What is Light? Light carries energy and travels as a wave. Light travels at m/s or km/s (much faster than sound). Light waves travel in straight lines.

4 © Boardworks Ltd 2003 Light : What materials will light travel through?  Hold different materials between the lamp and the screen. Use the results table and shading chart on the next slide to estimate the opacity of different materials.

5 © Boardworks Ltd 2003 What materials let light through?  Opaque objects do not allow light to pass through them - transparent ones do. Translucent materials only let through part of the light. Shading chart. 75 % 100% 0% 15 % 2.5 % 50 % 30 % 10 % 25 % 20 % 5 %

6 © Boardworks Ltd 2003 How does light travel? 1)Fill a clear glass trough or empty fish tank with smoke. 2)Shine rays of light through the tank and describe what you see.  LIGHT WAVES TRAVEL IN STRAIGHT LINES.

7 © Boardworks Ltd 2003 Seeing objects How do we see an object? The light from the object enters our eye. Do we see all objects in the same way? There are two ways we see objects….. We see some objects by reflected light. We see other objects because they are light sources.

8 © Boardworks Ltd 2003 Light sources How do light rays from a light bulb and other light sources reach our eye? They travel in a straight line directly into our eye.

9 © Boardworks Ltd 2003 Reflected light How do light rays from a book and other such (non-luminous) objects reach our eye? Light from the lamp or another light source strikes the book and some of the light is reflected into our eye.

10 © Boardworks Ltd 2003 Reflection : Good or Poor?  Objects which reflect light well have smooth, shiny surfaces and are usuallypale colours.  They give clear images because they reflect the light regularly.  Mirrors are excellent reflectors.  Objects which do not reflect light well have rough, matt surfaces and are usually dark colours.  They give no or diffuse images because they reflect the light irregularly.

11 © Boardworks Ltd 2003 Reflection : Good or Poor? Arrange these items along the arrow: White Paper Red Roses Polished Black Shoes Al foil Yellow Banana Blue Car Tree Bark Tangerine Green Leaf Tarmac Road Best Reflectors Worst Reflectors

12 © Boardworks Ltd 2003 What happens to the light which isn’t reflected? Some of this light may be absorbed (e.g. as heat). Some of this may also be transmitted (e.g glass reflects a small amount of light, absorbs some of the rest and allows most of it to pass through.

13 © Boardworks Ltd 2003 Reflection: Investigating Reflection. These activities allow you to investigate the main laws of reflection.  You should summarise each investigation with a law you have written based on that exercise. 1)Reading in mirrors. 2)How far away is the image? 3)The maths of reflection. 4)Reflecting without mirrors.

14 © Boardworks Ltd 2003 Reflection: 1. Reading in Mirrors. In small groups, take it in turns to read the list of words on the next slide with your back to the screen using a mirror. You can only move on to the next word when you read the first word correctly.  Put your results in a table like this:

15 © Boardworks Ltd 2003 Reflection : 1. Words DogManBall BatBikeAnts ParkFinsPink LitterSandyShark

16 © Boardworks Ltd 2003 Reflection : 1. Results A)Who read the words in the quickest time? B)Plot a bar chart of you r results like: C)What was the average time taken in your group?

17 © Boardworks Ltd 2003 Reflection : 1. Lateral inversion The way plane mirrors reflect light regularly gives a clear image which is the same size as the object - but what is different about the image ? This is called lateral inversion.

18 © Boardworks Ltd 2003 Reflection : 2.How far away is the image? 1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it. Place a pin as the object in front of the mirror. 2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for 3 more positions of the ruler. 3. Remove the mirror and ruler. Where the lines cross is the image position. What are the distances between the mirror and object and its image? I

19 © Boardworks Ltd 2003 Reflection : 3. The Maths of Reflection  Fix a plane mirror to a piece of A5 paper and draw around it. Angle i Angle r  Draw a normal line (at 90º) through the middle of the mirror outline.  Use a ray box to shine an incident ray at the mirror - plot the incident and reflected rays.  Measure the angles of incidence [i] and reflection [r] and write the answers in the table on the right. Repeat for another 5 angles of incidence.

20 © Boardworks Ltd 2003 Reflection : 4.Reflecting without mirrors Mirrors are good reflectors but not perfect - they give 2 reflections. We use glass prisms instead of mirrors in good quality binoculars and other instruments.

21 © Boardworks Ltd 2003 Reflection : 4.Reflecting without mirrors Shine rays of light into a prism as shown in the ray diagrams below.  Copy and complete the ray diagrams using a ruler and pencil.  Don’t forget to include arrows on your rays!

22 © Boardworks Ltd 2003 Using plane mirrors By positioning two plane mirrors at 45° to each other at either end of a tube we can make a _________. periscope Periscopes are used in _________. submarines

23 © Boardworks Ltd 2003 Reflection : Summary You should now know that : Pale and shiny surfaces are good reflectors, dark and rough surfaces are not. The image in a plane mirror is laterally inverted. The image is the same distance behind the mirror as the object is in front. The image in a plane mirror is the same size as the object. angle of incidence =angle of reflection ¡ =r

24 © Boardworks Ltd 2003 Refraction

25 © Boardworks Ltd 2003 Refraction : Bending light The speed of light waves depends on the material they are travelling through. If the light waves enter a different material [e.g. from glass into air] the speed changes. This causes the light to bend [or refract]. Air = FastestDiamond = slowestGlass = slower Glass

26 © Boardworks Ltd 2003 Refraction - at the air-glass boundary

27 © Boardworks Ltd 2003 Refraction :Investigating Refraction 1. Place a rectangular glass block on a sheet of paper and draw around it. 2. Draw a normal line [90º] along the top surface of the block. 3.Shine rays of light with incident [i] angles of 30º, 60º and 0º into the block, making sure they all hit where the normal line crosses the glass surface]. Measure angle “r” each time and record your answers. Angle i Angle r

28 © Boardworks Ltd 2003 Refraction : What happened?  Put your answers into the table below:  Write 2 ‘rules’ to describe: a) what happens to the ray as it enters the glass. b) what happens to the ray as it re-enters the air.

29 © Boardworks Ltd 2003 Air to Glass: angle of incidence > angle of refraction  I >  r As the light ray moved from air into glass it moved towards the normal. If light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.  i >  r

30 © Boardworks Ltd 2003 Glass to Air angle of incidence < angle of refraction  i <  r As the light ray moved from glass into air it moved away from the normal. If light rays move from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal.  i <  r If the surface the light ray is incident upon is parallel to the surface it emerges from than the emergent ray makes the same angle with the normal as the incident ray. (If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end).

31 © Boardworks Ltd 2003 Angle of incidence = 0° When the angle of incidence is 0  the light ray is not deviated from its path. Un-deviated light ray

32 © Boardworks Ltd 2003 Refraction in a rectangular block

33 © Boardworks Ltd 2003 Revision tip Remember the word: TAGAGA Towards (normal) Air Glass Away (from normal) Glass Air

34 © Boardworks Ltd 2003 Fast and slow  If you were running along a beach and then ran into the water when would you be moving slower, in the water or on the beach? In a similar way as light moves from one medium to another of different density the speed of light changes. Do you think light moves faster or slower in a more dense medium? In the water. Light moves slower through a more dense medium.

35 © Boardworks Ltd 2003 The speed of light Light travels at km/s in a vacuum. As it enters denser media the speed of light decreases. Looking at the chart, which do you think is denser, Perspex or water? Perspex must be denser because light travels more slowly through Perspex than water.

36 © Boardworks Ltd 2003 Why does light change direction as it enters a material?  Imagine a car driving from the road into a muddy field.  In the muddy field it slows down as there is more friction.  If it enters the field at an angle then the front tyres hit the mud at different times.  Tyre one hits the mud first and will move more slowly than tyre two.This causes the car to turn towards the normal.  When the car leaves the mud for the road, tyre one hits the road before tyre two and this causes the car to turn away from the normal. Tyre 1 Tyre2 Mud Road

37 © Boardworks Ltd 2003 No change in direction  If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time.  The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

38 © Boardworks Ltd 2003 Same for light When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field. Part of the light ray s____ d___ before the rest and this causes the change of d______. If the light enters a new medium along the normal (  i = 0  ) then it does not ‘bend’ because all of the light ray slows down at the s___ t___. ormal lows own irection ame ime

39 © Boardworks Ltd 2003 Refraction : Effects of Refraction  Many visual effects are caused by refraction.  This ruler appears bent because the light from one end of the ruler has been diffracted, but light from the other end has travelled in a straight line.  Would the ruler appear more or less bent if the water was replaced with glass?

40 © Boardworks Ltd 2003 Refraction : Apparent Depth The rays of light from the stone get bent [refracted] as they leave the water. Your brain assumes they have travelled in straight lines. Your brain forms an image at the place where it thinks the rays have come from - the stone appears to be higher than it really is. Actual location Image

41 © Boardworks Ltd 2003 The Archer fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, say on a leaf. The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey. image of prey prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

42 © Boardworks Ltd 2003 Refraction : Magic coins Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin. Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?

43 © Boardworks Ltd 2003 Refraction : Summary When light bends this is called refraction. Refraction happens because the light changes speed [or velocity]. When light enters a more dense medium [e.g. glass], it bends towards the normal. When light enters a less dense medium [e.g. air], it bends away from the normal. If the incident ray hits a surface at 0º, no refraction occurs.  Remember that the angle of reflection [r] and angle of refraction [r] have the same symbol. In reflection,I=r In refraction,I  r

44 © Boardworks Ltd 2003 Colour

45 © Boardworks Ltd 2003 Lack of colour Imagine you could only see in black and white. What are the possible implications this could have on your life? Would it rule out any careers for you? What dangers could there be? You must give a two minute presentation to the rest of the group on your ideas. Activity

46 © Boardworks Ltd Shine a ray of bright white light at a prism, as shown above, and move the prism until colours appear. 2.Draw a diagram to show what your observed. Colour : splitting white light up

47 © Boardworks Ltd 2003 Explanation What happens? The white light ray is split into a spectrum of colours. This is known as DISPERSION. Why? The different colours of light have different wavelengths. Different wavelengths are refracted different amounts. Red light is refracted least. Violet light is refracted the most. Which colour is refracted the most? How do you remember the order of the colours? Richard Of York Gave Battle In Vain

48 © Boardworks Ltd 2003 Colour : splitting white light up

49 © Boardworks Ltd 2003 Until now we have used the title ‘splitting white light up’. However this process has a scientific name. The splitting of white light into a spectrum of colours is called DISPERSION. But why does white light get dispersed? Colour : splitting white light up

50 © Boardworks Ltd 2003 Dispersion Each of the colours [ROYGBIV] has a slightly different waveform - what is different? They each have a different wavelength [ ].

51 © Boardworks Ltd 2003 Because the different colours of light have different wavelengths they are bent (refracted) by different amounts. But which colour do you think is bent the most? Red light is refracted least because it has a long wavelength. Violet light is refracted the most because it has the shortest wavelength. Dispersion

52 © Boardworks Ltd 2003

53 Colour : joining the colours back together Remember how you dispersed white light to give a spectrum of colours? Now do the opposite to it - you’ll need 2 prisms! A similar effect can also be seen using a colour wheel [or Newton’s disc].

54 © Boardworks Ltd 2003 Newton’s disc Colour in a paper or card circle with the colours of the spectrum. Using string or a pencil spin your disc around. What do you predict you will see? What did you observe? What does this tell you?

55 © Boardworks Ltd 2003 Newton’s disc

56 © Boardworks Ltd 2003 Seeing colours We see objects by light striking an object and then reflecting into our eye. How do we see colour? Why does a red dress look red? Why does a green apple look green? How do we see objects like a book?

57 © Boardworks Ltd 2003 Primary Colours Animation

58 © Boardworks Ltd 2003 Primary Colours You can make any colour by mixing three colours. Do you know what they are? Red Green Blue The colours labelled in white text are the PRIMARY COLOURS. The colours labelled in blue text are called SECONDARY COLOURS. They can be made by mixing the primary colours Cyan Yellow Magenta

59 © Boardworks Ltd 2003

60 Red objects Why does a red snooker ball look red in white light? Remember white light is made of a spectrum of colours. The snooker ball absorbs all the colours of the spectrum EXCEPT red, so red light is reflected into our eye. The snooker ball appears red.

61 © Boardworks Ltd 2003 Green objects Why does a green snooker ball look green in white light? The snooker ball absorbs all the colours of the spectrum EXCEPT green, green light is reflected into our eye. The snooker ball appears green.

62 © Boardworks Ltd 2003 Black objects Why does a black snooker ball look black in white light? The snooker ball absorbs all the colours of the spectrum. NO light is reflected into our eye. The snooker ball appears black.

63 © Boardworks Ltd 2003 White objects Why does a white snooker ball look white in white light? The snooker ball doesn’t absorb any of the colours of the spectrum. The whole spectrum of light is reflected into our eye. The snooker ball appears white.

64 © Boardworks Ltd 2003 Magenta objects Why does a magenta ball look magenta in white light? The ball absorbs all the colours of the spectrum EXCEPT red and blue, red and blue light is reflected into our eye. The ball appears to be magenta, a mixture of red and blue light.

65 © Boardworks Ltd 2003 Look at the clothes below. What colour light is reflected by these clothes?

66 © Boardworks Ltd 2003 What colours are absorbed by this frog’s skin? What colours are reflected into your eyes? This part of the skin absorbs all the colours of the spectrum but reflects red light. This part of the skin absorbs all the colours of the spectrum and none are reflected.

67 © Boardworks Ltd 2003 What colours are absorbed by this flower? What colours are reflected into your eyes? This part of the flower absorbs all the colours of the spectrum but reflects yellow (a mixture of red and green) light. This part of the flower absorbs no colours, it reflects them all.

68 © Boardworks Ltd 2003 Filters Filters let certain colours of light pass through, but absorb all other colours. Using different coloured filters placed in front of your eye, look around the classroom and see what effect they have on your vision. Object filter

69 © Boardworks Ltd 2003 Red, blue and green filters Red filters absorb all colours…. Blue filters absorb all colours…. Green filters absorb all colours…. … apart from red light… apart from blue light … apart from green light

70 © Boardworks Ltd 2003 Magenta, cyan and yellow filters Magenta filters absorb all colours…. Cyan filters absorb all colours…. Yellow filters absorb all colours…. … apart from red and blue light … apart from blue and green light … apart from red and green light

71 © Boardworks Ltd 2003

72 But why do colours look different in different coloured light? Lets start with the example of a red ball in red light. The red light shines on the ball. The red ball reflects red light and so appears red.

73 © Boardworks Ltd 2003 What about the red ball in green light? The green light shines on the ball. The red ball only reflects red light so it absorbs the green light and reflects nothing. Because it doesn’t reflect any type of light it appears black. So what colour does a green ball appear in blue light? The green ball only reflects green light so it absorbs the red light and reflects nothing. Therefore it appears black.

74 © Boardworks Ltd 2003 But what if the filter you are using lets through more than one type of light. For example what will our red ball look like in magenta light. The magenta light shines on the ball. Remember that magenta is a mixture of blue and red light The ball reflects only red light. Therefore it absorbs the blue light and reflects the red light. It will appear to be red.

75 © Boardworks Ltd 2003 Over the next two slides you will be shown the same girl as in the previous example. However, she will be standing in a different coloured light each time. The colour of this light is written at the top of the slide. Your task is to drag the correct shirt and trousers onto the girl to represent what those clothes would look like in this coloured light.

76 © Boardworks Ltd 2003

77

78 Object (Colour)Colour FilterAppearance red ballred red ballblue blue bookgreen blue bookmagenta green applecyan green applemagenta red and blue tiered black blue green black red and black Complete the table below by adding in the colour that each object would appear to be in the conditions listed.

79 © Boardworks Ltd 2003 Flag colours For the flag shown, draw what it will look like in: a) Red light b) Green light c) Blue light Actvity

80 © Boardworks Ltd 2003 Multiple Choice Questions

81 © Boardworks Ltd 2003 Which of the following is not a light source? A.The Sun B.A star C.Traffic lights D.A book

82 © Boardworks Ltd 2003 Which of the following is not seen by reflected light? A.Your hand B.Jupiter C.Light bulb D.The Moon

83 © Boardworks Ltd 2003 Which of the following is the most dense? A.Air B.Water C.Glass D.Lead

84 © Boardworks Ltd 2003 When light changes direction as it moves from one medium to another we call this effect what? A.Reflection B.Refraction C.Diffraction D.Total internal reflection

85 © Boardworks Ltd 2003 If a ray of light moves from a more dense medium to a less dense medium at an angle to the normal what happens? A.It continues with no change of direction B.It bends towards the normal C.It bends away from the normal D.It stops

86 © Boardworks Ltd 2003 Which colour has the longest wavelength? A.Blue B.Indigo C.Yellow D.Red

87 © Boardworks Ltd 2003 Which colour below is refracted (bent) the least by a glass prism? A.Red B.Orange C.Yellow D.Green

88 © Boardworks Ltd 2003 Which of the following is not a primary colour? A.Red B.Blue C.Cyan D.Green

89 © Boardworks Ltd 2003 Which two primary colours make magenta? A.Red and cyan B.Red and yellow C.Red and blue D.Blue and violet

90 © Boardworks Ltd 2003 If white light passed through a magenta filter and then a blue filter, what colour would emerge? A.Red B.Red and blue C.Blue D.Black

91 © Boardworks Ltd 2003 What colour would a red dress look in cyan light? A.Red B.Green C.Blue D.Black


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