Posted on 24 January 2012 at 18:11h

This morning I wrote a problem sheet for my HL students on the photoelectric effect. The first question was about a filament lamp so I illustrated it with a photo of the lamp behind my head. I was interested to notice how the brightness of the lampshade changed from the inside to the outside and wondered if I could use the photo to show that the intensity was proportional to 1/r² . A couple of months ago I wrote a blog post about using Paint.NET to measure albedo so thought I'd try the same technique.

I realised that because of the angle of the shade relative to the camera I needed to place a scale in the lamp. You can see I used a piece of mecano, the holes are clearly seen at regular intervals.  then sampled the photo and plotted a graph of position against intensity (units unknown).

You can see the trend is in the right direction but its hardly a 1/r² curve. I think I would have to calibrate the intensity measurement and do something about those stripes. The shape of the shade might also have some influence. Maybe I'll try this with a bulb in the middle of the floor. On the other hand maybe not.