I am teaching one of the things I most like to teach at the moment, and on Saturdays running Open Days. I am teaching signal-to-noise discrimination in the night vision of us and other animals, like the cat shown. Improving signal-to-noise is the reason why the eyes of cats and other animals that hunt at night reflect light.

At the back of cats’ eyes, behind the retina, there is a reflective coating that bounces back any of the photons of light that have passed through the retina and these photons travel back through the retina, giving a second chance for the retina to absorb and detect these photons. The retina is where light is detected and this reflective coating gives the retina two chances at detecting photons which is important at low light levels where they are few photons flying around.

If we had done a lot of hunting at night on the African savannah where we evolved, we too would have reflective eyes. The advantage of a reflective coating over just making a thicker retina that absorbs more light is that the thicker the retina the larger the noise in the signal it generates, whereas a coating increases the signal without increasing the noise.

On Saturday, my nuclear and radiation physics colleagues ran lab demos to give prospective students and their parents some idea of what university physics is. Hopefully this will demystify it a bit for those prospective students whose parents are not scientists, and may not have gone to university and so do not really know what we do.

My favourite one of their demos is measuring the radioactivity of bananas. It is cool, they just put the banana on their germanium detector and wait. A small fraction of the potassium that is present in all living things is radioactive and they detect this. Bananas are rich in potassium (I think this is why tennis players eat them) and so bananas are particularly good for this. Indeed so popular has measuring the radioactivity in bananas that there is even a Banana Equivalent Dose (BED).

The BED and our vision at night have something in common: They are both examples of an application of physics to something everyone is familiar with, but typically does not associate with physics. I love the moment when it dawns on someone that something they took for granted requires a nifty bit of physics to work or to understand.