Chapter 22-specific PCK
PCK2204
Beer’s law: How much light is transmitted?
With reference to Learning module 2204 Beer's law: How much light is transmitted?
Here is an activity that surprises most people at first. It can often induce a level of cognitive dissonance. But the resolution of the surprise can provide a meaningful understanding of Beer's law - if the students are 'ready' for the resolution.
It is directly related to the student module 2204 Beer's law, and is demonstrated near the end of the video in that module.
It could hardly be more simple to arrange and conduct.
The activity
- Use two identical 100 mL graduated cylinders (or any identical, parallel-sided high containers, such as pasta tubes).
- Measure 10 mL of a solution of food colouring (or red wine or cordial) into each. [You may need to experiment with concentrations to maximize the effectiveness of the activity.] Looking side-on to the cylinders, the intensities of colour should look indentical.
- Add water to one of them,to make the volume up to 100 mL.
- Compare the intensities of colour looking side on. The colour intensity of the solution to which water is added is much less than the other. Not surprising.
- Compare the intensities of colour looking down from above the cylinders, placed side by side - preferably after asking the students to commit to a prediction. The colour intensities are the same. Not expected!
- Engage the students in developing an explanation for the observations, with reference to Beer's law. A key hint is to think about comparing the (absolute) number of dye molecules with which photons interact as they travel the path across each of the solutions.
The self-check questions in Module 2204: Beer's law may be helpful.
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A matter of perspective ....
I recall that long long ago, the great Australian science education researcher Professor Peter Fensham reported findings that some people (including students) conceive of the notion of speed as the distance an object travels in a unit of time, while others are more comfortable to think of speed in terms of the time taken to travel a defined distance. Different ways of thinking about the same concept.
One is not better nor worse than the other, but if the teacher uses only one of those frameworks in their explanations/discussions, it seems likely that those students with the other perspective might find it difficult to come to an understanding - no matter how clearly we might think our explanation is, it may be inconsistent with the student's frame fo reference. Ah, the demands on teaching!!
What a classic piece of PCK is that!
I have no evidence to support what I am about to propose: it seems quite likely that some students will think about Beer's law in terms of the percentage of incident light that is transmitted (by a defined pathlength of a given solution), while others might think more comfortably in terms of the percentage of light that is absorbed.
So there is a challenge for teachers. And perhaps a research question for any teacher engaging in classroom 'action research'.
I recall that long long ago, the great Australian science education researcher Professor Peter Fensham reported findings that some people (including students) conceive of the notion of speed as the distance an object travels in a unit of time, while others are more comfortable to think of speed in terms of the time taken to travel a defined distance. Different ways of thinking about the same concept.
One is not better nor worse than the other, but if the teacher uses only one of those frameworks in their explanations/discussions, it seems likely that those students with the other perspective might find it difficult to come to an understanding - no matter how clearly we might think our explanation is, it may be inconsistent with the student's frame fo reference. Ah, the demands on teaching!!
What a classic piece of PCK is that!
I have no evidence to support what I am about to propose: it seems quite likely that some students will think about Beer's law in terms of the percentage of incident light that is transmitted (by a defined pathlength of a given solution), while others might think more comfortably in terms of the percentage of light that is absorbed.
So there is a challenge for teachers. And perhaps a research question for any teacher engaging in classroom 'action research'.
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