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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.
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?
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
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
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?