By pure coincidence, there had been a Horizon programme on a few weeks before our Café and I feel that they stole our thunder to a certain extent; but I will forgive them for that because I do in general really like Horizon (the episode covering Fermat's Last Theorem is one of my favourite pieces of science broadcasting ever). Regardless, we had probably the highest attended Café to date and a large number of very decent and patient Birmingham citizens stood for a lot longer than, and a lot closer to other decent and patient citizens for a couple of hours on a Tuesday night to talk about science.

Let me start with the basics. Light is a form of energy and can be categorised in many ways, such as brightness, intensity, frequency etc but one of the most useful is to talk about the wavelength. The wavelength of light simply refers to the length of the repeating unit that makes up the wave pattern, but it is the wavelength that also determines what sort of light you have and specifically, what colour it is. 'Light' is just one part of the larger electromagnetic spectrum. In one part of this spectrum, we have a number of wavelengths that make up visible light and if you separate out those wavelengths, you will get a rainbow of colour - this is effectively what a rainbow is. If you go to longer or shorter wavelengths though, there is a huge variety of other 'colours' of light that you cannot see, such as ultra-violet, X-rays, microwaves, radio waves, infra-red etc. These are all exactly the same as 'light', just longer and shorter versions of them and that is why I refer to them as being colours. Does it matter that you cannot see them? So this in effect answers the question, what is colour; it is the specific wavelength of an electromagnetic wave. Yes this is sort of true, but we have forgotten the human element - you and I, and more importantly, our eyes!

How do we know that a nice green apple is actually green, what does that even mean? It turns out that it means completely different things to different people. The interesting parts of our eyes, when it comes to colour, are structures called cones. Men tend to have three cones but women can much more commonly have 4 cones and presumably a different sense of vision accordingly. This raised an interesting discussion in terms of trying to imagine what vision might appear like through someone else's eyes. I am sure that this highly philosophical discussion has taken place many times before in many different pubs, but we actually came up with a solution - possibly! One of the audience members nonchalantly mentioned that they perceive colour differently between their two eyes and after discussing how they came to find out this wonderful natural variation and everyone else in the audience has finished blinking alternately, we decided that this was in fact a very common occurrence and actually a unique insight into how colours can look differently to different people. Try it for yourself if you don't believe me; look at an object with a strong colour and then alternately cover each eye, backwards and forwards and see if there is a change in the richness of the colours that you see.

This led onto a bit of a theme for the evening, with lots of activities to go home and try. I mentioned earlier on that electromagnetic waves come in varying shapes and sizes and one of these is X-rays; we are all familiar with the concept of X-rays in a medical context but on this night we found a new source of X-rays - envelopes! If you take a modern self-adhesive envelope (the sort that already has the glue on) and repeatedly stick and unstuck it in a dark room, you will see little flashes of light given off when you do the unsticking. I have personally tried this and it does genuinely work, but I was amazed to find out that apparently it releases X-rays as well - brilliant. By all accounts, sellotape and even breaking polo mints does the same thing! We also talked about a test to show that the light sensitive parts of the eye (rods and cones) have different sensitivities to colour and that the rods only see in black in white whilst all of your colour comes from the cones. If you sit and stare at a point on the wall in front of you and then someone approaches from the side, by all accounts you cannot tell what colour clothes they are wearing (as long as you keep staring at your point on the wall). The rods are very good at picking up movement in your peripheral vision - so you know someone is there, but they only 'see' in black and white.

This is why I love talking about science with people from a variety of backgrounds. You never know what sort of strange but fascinating titbits of information might just come your way. Granted, no-one suggested a cure for cancer, or reversing climate change, but I don't think anyone would believe me if I promised that and scientific discussions don't always have to be hard hitting in order to be interesting or worthwhile. What the Birmingham Cafes Scientifique do promise though is some good banter and if you choose to dine at the Jekyll and Hyde as well, some very good food (especially the sweet potato chips). The Cafes are interesting, challenging, surprising, enjoyable and informal and that is not bad for a Tuesday night!