The new syllabus contains some "required experiments" that means that your students have to do them and they must be recorded on the good old 4psow. Well I suppose they had to have some reason for holding onto the paperwork. Most of these are quite standard like measuring g but there are a few oddities like investigating the performance of a simple compound telescope. The problem is that in Normal adjustment both object and image are at infinity so they are quite difficult to measure. What we mean by angular magnification is actually how much bigger does the image appear. I tried estimating this with my eye but realised that it might be better using a camera. I took the two photos with my mobile phone. One through a telescope and one from the same point without. The objective was 200mm and the eyepiece 100mm, so the angular magnification should be 2 (at normal adjustment). I found that the photo taken through the telescope gave a bigger image of the SMARTboard so I adjusted the image size so that the board was the same in each. By drawing lines in paint I found that the width of the unmagnified image was 30 pixels and the magnified 50 pixels. That's not bad agreement especially since the object isn't actually at infinity so m = f_o/f_e doesn't exactly apply.  You can see in the photo that the image doesn't coincide with the object (the board and black rectangle aren't both in focus) so the angular magnification isn't quite equal to the ratio of the sizes of the rectangles but I think its a neat way of getting a feel for what's going on.

I tried the same thing with a microscope which has an angular magnification equal to the product of the linear magnification of the two lenses. The set up doesn't look like a real microscope unless you have a very short focal length objective so can get the object very close. The photograph analysis worked fine but I think my set up was giving a final image at infinity so I couldn't check my result. I'll have to try again tomorrow. I think I should buy some short focal length lenses before next year.

And after all that the IB have now taken these two off the list of required practicals. :-(