Thursday, November 13, 2008

Revamping Intro Physics Laboratory - Part 7

{Note: If you wish to follow what has transpired so far in this series, here are Part 1, Part 2, Part 3, Part 3-Follow-up, Part 4, Part 5, and Part 6.}

This time, we do simple optics. Again, as a reminder of the whole point of this series is that the students should be given the ability to simply EXPLORE these things on their own (with minimal guidance), and come up with some sort of relationship between two different parameters, all based on the physics which they may or may not have learned. Each of these experiments do not require that they have encountered the corresponding material in class. So there really is no "theory" to "prove" or to verify here. It is pure "play".

Part 1

In this part, there is a lighted object, which will be the source, placed "far away". The students are given several convex lenses (with different focal lengths) and some papers as their screens. The object here (no pun intended) is to figure out how to get a focused image of the object onto the screen, without moving any closer to the object itself. The student can certainly vary the location of the screen and, to some extent, the lens itself. Ask them to record the position of the screen with respect to the lens when they find a focused image. Ask them to try it with the different lenses that they were given.

Part 2

Now ask them to start moving closer to the lighted object and see where they get a focused image on the screen using one of the lenses of their choosing. This time, they should record both the distance between the lens and the image and the distance between the lens and the object. Ask them to do this several times, each time getting closer to the object.

If they have the time or wish to explore some more, let them try this with a different lens.


1. Now, you tell them that they should put these numbers in a table, or a chart, or a graph, etc... anything where that might be useful for them to figure out if there is any systematic relationship between (i) the distance between the lens and the screen where the focused image is formed, and (ii) the distance between the lens and the object. Ask them if they can think of how these two parameters are related. Note that we don't expect them to be able to know the thin lens equation, and so, we should not expect them to be able to arrive at such a relationship. But they should be able to notice that as the move closer to the object (thus, the object distance is getting smaller), the focused image will be further away from the lens.

2. You tell them the focal lengths of the lenses that they used. Ask the students to think on how this number (which is a length) relates to all the data that they had collected. If the student is observant enough, he/she will notice that this number corresponds to the distance of the focused image when the objects is "far away", i.e. while they did Part 1 of the experiment. If the student notices this, then ask him/her what she would do if he/she is given a lens with an unknown focal length, and needs to make a quick determination, rough of the focal length.

3. When the students finally got introduced to the thin lens equation, ask them to re-analyze the data, plotting a graph, and using the data to extract the value of the focal length of the lenses. This would be a more accurate determination of the focal length.

Again, these experiments, the way they are designed, do not require any sophisticated knowledge of the accompanying theory. In fact, for this particular experiment, anyone off the street will be able to do it. It simply requires a bit of thinking and common sense to try and figure out the pattern in the observation and the data. The students may not be able to come up with the exact relationship between the parameters, but they need to notice the pattern on what happens to one and you vary the other.


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