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    Unit 11  LIGHT

    Fig. 1 Have you ever seen a rainbow?

    Why can you see all the colors of the spectrum in a rainbow? What does

    water have to do with it?

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    Getting started You met the word spectrum in Unit 8. When you look at a spectrum you can see all the colors which make up the white light that comes from the Sun.

    In previous grades, you learned that objects which reflect all of the light appear white. Colored objects reflect some colors of light and absorb others. For example, objects that reflect just blue light appear blue, and objects that reflect just red

    light appear red.

    1. Does a green leaf absorb or reflect green light?

    2. Does a green leaf absorb or reflect blue light?

    3. What color of light does a banana reflect?

    4. If you take a photo of a white piece of paper in a forest, the paper will look green in the photo. Why?

     

    White light

    The light that comes from the Sun. It is made up of lots of colors, which you can see in the spectrum.

    Fig. 2 This shows the

    spectrum of ‘white’ light.

    Fig. 3 

    A green leaf.

    Absorb

    When a ray of light isn’t reflected from the object. It may be converted to heat energy.

    Reflect

    When a ray of light ‘bounces’ off an object and changes direction.

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    Concepts In completing this unit you will learn to:

    • Explain the properties and behavior

    of light (P3)

    Key Terms The meanings of these terms can be found in the glossary on pages 275–90.

    absorb

    blurred image

    image

    luminous

    non-luminous

    ray

    reflect

    refraction

    screen

    sharp image

     translucent

     transparent

    white light

    Investigating Scientifically S5, S11

    Success Criteria Learning outcome P3

    Here is what you might aim to achieve by  the end of this unit:

    • Emerging  – identify properties

    of light

    • Developing  – describe properties

    and behavior of light

    • Mastery  – explain properties and

    behavior of light

     What level do you think you will be able

     to achieve?

      Learning Outcomes

    I know what

    these words

    mean

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    1 Sources of light

    Tonight, if the sky is clear, find a dark place and look at the stars. Think about what you can see. Where are the stars? How can you see them?

    Seeing the stars can tell us quite a lot about the properties of light.

    In Unit 10 you learned that sound can’t travel in space. This isbecause space is a vacuum. Sound can only travel through a material, for example air or water.

    But we can see the stars – so this means that light can travel through a vacuum.

    P3

    Fig. 4 Here are

     just some of the stars

    you can see on any

    clear night.

    Fig. 5 These

    astronauts can see

    each other, but they

    can’t hear each

    other – unless they

    use their radios.

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    When you see stars you are looking at very distant objects. You can see a distant star because light travels from the star in a straight line. A light ray is the name we give to a line of light.

     l i g h t   f r o m

      s t a r

    The speed of light

    Alpha Centauri is the closest star to us. The light you might see tonight from Alpha Centauri was produced in the star almost four and a half years ago. It has taken that long to travel the distance between the star and the Earth.

    Alpha Centauri is about 41 000 000 000 000 kilometers away from us!

    We know the distance between Alpha Centauri and the Earth. We also know how long it takes light to reach us from this star. Using these facts, we can calculate the speed of light.

    The speed of light is approximately 300 000 000 meters per second. Light travels almost a million times faster than sound!

     Luminous and non-luminous objects

    Stars are luminous because they produce light. Most things

    we see every day don’t produce light. These objects are non- luminous.

    Ray

    Light travels in straight

    lines. These lines are rays.

    Fig. 6 If you could

    follow the ray of

    light from a star in a

    straight line you would

    eventually get to the star.

    Luminous

    Things which produce light are called luminous.

    Non- luminous

    Things which don’t produce

    light are callednon-luminous.

    Fig. 7  You see a

    luminous object when

     the light it produces

    reaches your eye.

    Fig. 8 You see a

    non-luminous object

    when the light it reflects

    reaches your eye.

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    Which of these are luminous, and which are non-luminous?

    Put a tick in the correct box.

      luminous non-luminous

    stars [ ] [ ]

    light bulb [ ] [ ]

    tree [ ] [ ]

    eye [ ] [ ]

    the Sun [ ] [ ]

    water [ ] [ ]

    the Moon [ ] [ ]

    Think about luminous and non-luminous objects. Explain how shadows show that light must travel in straight lines. The diagram in Fig. 10 will help.

     

    shadow shadow

    starts here

    shadow

    ends here

    Fig. 9 

    You can see

    shadows on the

    sand.

    Fig. 10  How the shadow of

    a wall forms.

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    2 Pinhole camera

    Activity 2.1

    What to do:

    1. Take the lid off the box so you can see inside.

    2. Make a small hole in the middle of one end of the box. Use a needle first, and then a knitting needle to make the hole larger.

    3. On the other end of the box, cut most of the end away.

    4. Stick a sheet of translucent paper over the end that you have cut away. This will be the screen.

    5. Put the lid on the box. Your camera is ready to use!

    6. Use your camera to look at some objects on the screen. You should be able to look at:

    • objects outside the window

    • luminous objects in the classroom.

    7. Take a photo of the pinhole camera’s screen.

    8. Describe the images you see on the screen.

     

    P3 S5 S11 Equipment:

    A shoebox,

    wax paper or other translucent

    paper, needle and knitting needle,

    black paint,

    sticky tape,

    camera,

    scissors

    Translucent

    A material that light can travel through, but which you cannot see through clearly.

    Screen

    A flat surface that an image appears on so that we can see it, for example on a smartphone.

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    To understand why the images are upside down, remember that light moves in straight lines.

    pinhole

    screen

    Activity 2.2

    In this activity you will improve your pinhole camera.

    What to do:

    1. You can improve the camera by painting the inside of thebox black. Remove the screen while you paint the box. Don’t forget to paint inside the lid.

    2. Use your camera to look at the same objects. Again, you can take photos. Are the camera’s images better? If they are, explain why.

     

    3. Take the lid off the box again to make the next step easier.

    4. Make a small hole in the tinfoil with a needle. Stick the tinfoil in front of the pinhole on the camera, so that the hole in the tinfoil lines up with the hole in the box.

    Image

    This is a picture of an object. When you see a reflection of a

    tree in water, the reflection is an image. When you see an actor in a film, you are really seeing an image of the actor.

    Fig. 11 Here is

    a pinhole camera.

    You can see the rays

    of light which pass

     through the pinhole.

    Equipment: Your pinhole camera,

    black paint,

     tinfoil,

    sticky tape,

    needle

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    5. Put the lid back on and try the camera again. What difference has this change made?

     

     Images through the pinhole camera

    Some images are sharp and some are blurred.

    Here are two images of an object seen through a pinhole camera.

    object imageobject image

    A large hole lets more light through. The light reflected from one point on the object travels in a straight line to many parts of the image. This makes the image blurred.

    A small hole lets less light through. The light reflected fromone point on the object can travel in a straight line to just a small part of the image. This makes the image sharp.

    Sharp image

    An image which is clear. We can see

    the shapeaccurately.