AN INVESTIGATION INTO THE PROPERTIES
OF CONVEX LENSES
- This investigation is very affected by the accuracy of your
measurements. This is not obvious at the start, but precision in your
measuring is always a good idea, and this investigation is no
exception. Of course it is very difficult to decide when the image is
exactly in focus, but try hard to judge it. (For example, you could
make the adjustments, measure the distances, write them down without telling
your partner, and then move the lens (or screen, but not both); now your
partner repeats the reading, and writes down her distances. Then you
can compare these two independent readings - record both of them, and use
the average.
- Try hard to work out what the general trends are, as the lens is
moved closer to the object (i.e. as u gets smaller). I don't
mean what the detailed shape of the graph is - just the overall
tendencies. This will help you to make your readings more systematic,
so aren't jumping about taking too many readings in some ranges of distance,
and not enough in others.
- Why do you suppose that you were given a lens marked with a B?
Presumably there are other lenses in the laboratory, marked with different
letters. How might they be different? What would that do to your
results?
- Why did you do your preliminary experiment, which probably took a
couple of lessons? (a) To get confident with the apparatus,
practiced at finding the focus.. (b) So you can get a
preliminary graph of v against u, from which you
can take an informed guess what the real shape of the graph is. Your main
experiment is then to test whether this really is the shape.
- You really should study this applet
very carefully. Firstly it shows what the light is doing; secondly it
provides you with a complete set of theoretical data, that you can compare
with your experimental data (e.g. on a graph). This is a good
alternative applet.