Let me start with this shadow of a hand holding a coin.

If you look closely at the shadow of your hand on a really, really bright sunny day, you’ll see a thin bright halo around the shadow itself.

It’s bright for the same reason that the water is choppy round the end of a jetty in the sea: the light kind of bends round the edges of your hand and gets kind of caught up in itself.

So it proves that light is a bit like waves.

It’s dependent on something called the ‘diffraction limit’, which is the very finest, the very smallest light can go before it gets too choppy, like the ink from a felt tip pen bleeding out into thin paper. So it reveals the grain of the universe, the smallest resolution; like an inkjet printer, the universe has a maximum DPI. It’s one of the few times you can see that kind of deep physics visible in the day to day, and all you need to do is be observant.

[Image credit: Optics, Hecht]