Arago's spot

The story goes like this. Mr Poisson, like Isaac Newton, believed that light was made of a stream of particles, whereas Mr Fresnel theorised that it was a wave motion. Poisson (this is back 200 years ago) imagined an experiment that would prove Fresnel wrong - consider the shadow caused by a perfectly round object: diffraction (yes they had diffraction back then, how the particle people thought it worked I know not) would occur around the edge and, if light was wave-like, diffracted rays would constructively interfere only in the centre of the shadow because by symmetry the lengths of the rays to this point from the objects edge would be equal, and thus there would be a bright spot in the centre of the shadow, a spot almost as bright as if the round object were not there. As in the diagram below.  Wikipedia will give you the maths if you are interested. Poisson considered that this scenario was absurd -whoever heard of a bright spot in the middle of a shadow? And therefore light was corpuscular. This was 10 years after Young's double slit experiment which I had always thought was proof enough.

Mr Arago heard this debate and thought he would see for himself. He set up his experiment and demonstrated the spot of light which is now known as Arago's spot (or sometimes, out of spite, Poisson's spot), thus proving that light is indeed wavelike.

200 years later I thought I would repeat the experiment. You need a point source of light to ensure coherence around the object's edge, and a laser diode fits the bill. For my round object I chose a 4mm ball bearing which I superglued onto a microscope slide. The laser was about one metre behind the slide and the wall, as a screen, a similar distance in front of the slide.

And why did I go to all this bother? Because it is a really cool experiment which actually worked. And because I did 'O' and 'A' level Physics at school, then an Honours degree course in Physics at Oxford, and through all that Arago's spot somehow evaded me so that I only came across it a few weeks ago. Which is also kind of cool. And, honestly, these are my pictures below:

My Arago's spot 200 years after Arago's

My perfectly round object