Malus through the looking glass
I'm still playing with the little bits of polarising plastic I bought from eBay. I was wondering if I could verify Malus' law by photographing my computer screen through a piece of polariser. My first attempt, covering the lens with the filter and rotating the camera failed but my second, sticking the filter on the screen worked quite well. The image shows the filter in various orientations.
To measure the intensity I first copied the photos from my phone to a folder on my computer, then I did a screen shot copied into paint and made the thumb sheet in the image. Took the thumbs into PaintNet and used the colours sampler to get the V value. This is related to intensity.
I also used paintNet to measure angle. I cropped the image to give an individual thumb then rotated it (rotate zoom in layers menu). I rotated until the piece was the same orientation as the first image and noted the angle.
The results were plotted in loggerPro (of course) to give the graph below. I then tried to fit a cos2 curve which wasn't entirely successful. I think It'd make a nice lab but needs a computer + camera. I think it could be done with just the computer's webcam and a mirror but I haven't tried yet.
As a side experiment I tried photographing the filter held over a white piece of paper. I then measure the V value on the photo of the filter and the paper to the side. The value of the filter was roughly 1/2. OK, it was very roughly 1/2 but I did it quite quickly.