You get some new toys, you have to play with them. The toys in this case are a nice new laser pointer and a convenient mini-tripod for the camera: necessary for the 4-second exposure. Both presents courtesy of my eldest child; many thanks, J.


The laser is a 1-mW job: puny compared to those balloon-popping 500mW things you can get, but the most powerful type that you can use in lectures without running into trouble with the health and safety regulations. And it has a lot more grunt than the feeble things that my employer provides in lecture theatres, with their tiny batteries that run out after five minutes.

You can see the beam in the air due to scattering from whatever particles are in the air; that bit was in the lectures I did before Christmas. The beam shows up better with this laser than any I have had before. This may be because green light has a shorter wavelength than red and (as I was at pains to point out in last semester’s lectures), if the scattering particles are small enough then shorter wavelengths are scattered more. But I think it is mostly because the laser has more power than its predecessors.

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