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# Seeing the light

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### Transcript

• 1. Seeing the light February 4, 2011 Please stick in your quiz results, complete the target sheet by adding your score and colouring in the objectives.
• 2. First ideas
• In medieval times people thought that our eyes sent out rays that hit objects and returned with information – some people still do!
• Now we believe that light comes from a luminous source such as the Sun, stars or a light bulb, not our eyes
• 3.
• 4. Travelling at the speed of light
• The light that leaves a bulb travels outwards in all direction at a speed of 300 000 000 m/s
• This is so fast it seems instantaneous
• 5. Question (Level 4)
• When you switch on a light, why can you not tell that light hit closer objects before objects further away?
• 6. Worked example
• The Sun is about 150 000 000 000 m away from Earth. How long does it take for light from the Sun to reach us?
• REMEMBER: speed = distance / time
• 7.
• 8.
• 9. A model for light
• Scientists use models to describe light
• One model uses rays and ray diagrams to show the path light takes
• Only the important rays are drawn
• 10. Worked example
• Draw a ray diagram (pencil and ruler) to show a girl in a room with a window as the only light source looking at a clock
• 11.
• 12. Travelling through glass
• Most materials are opaque – light does not travel through them. The materials ‘soak up’ the light in a process called absorption
• Glass allows light to travel through it – this is called transmission
• If the light travels through in straight lines the material is said to be transparent
• 13. transparent
• 14.
• 15. translucent
• 16.
• 17. Experiment: Confusing light
• Sometimes we want glass to let the light in but not let people see in. Bathroom windows use obscuring glass. The trick is to allow the light through but to mix up the information the glass carries. This can be done by scattering the light rays.
• Scientists often use a model before starting experiments that use more expensive equipment. Here you will be using water and milk to model scattering by obscuring glass.
• 18. 1 Put 250 ml of water into the beaker. 2 One pupil secretly draws a letter about 2 cm high on a piece of scrap paper. 3 That pupil then puts the letter behind the beaker against the glass so that it can be seen through the water, as shown in the diagram above. 4 The other two pupils make a note of what letter they think they can see through the water. 5 The three pupils compare letters. If they are all the same, add five drops of milk to the water, stir, and try with another letter. Keep the letters the same size and thickness as the first letter. Continue, keeping a note of how many drops of milk have been added. 6 Continue this process until the letters do not agree. 7 Repeat the experiment twice more starting with fresh water each time. 8 Repeat the experiment one more time, but stop adding milk just before it starts to get difficult to read the letter, now move the letter backwards away from the beaker, what do you notice?
• marker pen
• scrap paper
• glass stirring rod
• 25 ml milk in test tube
• 5 ml pipette
• 250 ml glass beaker
water paper with letter (held against glass) beaker letter viewed through water
• 19. Considering the evidence 1 Explain why you have been asked to do the experiment three times. 2 How reliable do you think your results are? Explain why you think this. Extension 3 One engineer using this data decides to use a glass mix that is less obscuring, because the amount of light getting through is reduced by making it too cloudy. Another engineer wants to make it more obscure because he wants to ensure privacy in the bathroom. The director of the department decides that the glass should be less obscure. Do you think they are right? What might have made them make this decision? 4 Many types of obscuring glass are not frosted, but have a wobbly surface. Describe how this does the same job. Would this sort of glass be suitable for a bathroom screen? 5 Describe the difference between the way that the milk scatters the light and the surface unevenness scatters the light. Number of drops of milk to obscure letter Average Result three Result two Result one
• 20. 6.2 Seeing the light Questions
• Name two luminous objects.
• Use the particle model to explain why a yellow bucket, seen in yellow light, looks yellow.
• Use the particle model to explain why a white car looks white.
• Use the particle model to explain why a black snake looks black.
• If you shine green light onto a red card, the card appears black. Explain why.
1 of 2 (Level 4) (Level 4) (Level 5) (Level 6) (Level 7)
• 21. 6.2 Seeing the light Questions
• Draw a ray diagram to show what happens when a torch lights up a cup and saucer and you see both.
• Suppose you have a torch with a filter that blocks only red light and another torch with a filter that blocks every colour except red. You shine the torches at the same point on a sheet of white paper. What colour does the paper seem to be?
2 of 2 (Level 8) (Level 8)