Optics made fun

This is in no way an original topic: countless blogs and web pages celebrate the importance of scientific outreach initiatives. Last week the Department of Physics at the University of Oxford hosted a new “Stargazing Oxford” event, an afternoon of activities related to astronomy and beyond.

When I arrived I saw an inflatable planetarium, a variety of telescopes brought for the occasion by local amateur astronomy groups, and hands-on stalls covering a range of topics from particle physics (where a pinch of creativity allows for a brilliant analogy between constituents of matter and coloured LEGO bricks) to optics.

Yes, optics – a “new entry” conceived by a bunch of PhD students in atomic and laser physics. Most of them are in my research group, which is why I knew I would find them there. This was the first time that I saw an “all-optical stall” at “Stargazing Oxford” – which made me unreasonably proud of studying optics myself.

My creative colleagues had selected three experiments for the event: “Build your own cd spectrometre” (already proposed in past years – it is surprising how well this works, and it additionally provides with nice souvenirs that children can take home — see http://www.youtube.com/watch?v=ZowYVDQDDZ4), “Why is the sky blue?” (such a neat experiment! One may be surprised to learn that it only requires a box filled with water and milk powder or soap, together with a torch — see http://www.youtube.com/watch?v=gUgDtWANJ18) and “Guiding light with water” (another simple and clever activity, although the use of a laser pointer did not fully convince me (1) — see http://www.youtube.com/watch?v=bPGsI0EtBdI for an overview on optical fibres and the physics behind them).

I test one of the handmade spectrometres – the coloured lines are the spectral components of the ambient artificial light!

I was told that a good number of visitors young and old had stopped by the optics stall, the youngsters displaying a big smile when they found out that they could keep their handmade spectrometre. The marvels of optics were successfully unveiled – not only during the event, but also while preparing for it. In fact, my colleagues had spent some time gathering materials and testing the experiments earlier that morning: one could see them running around the office with cardboard tubes and transparent plastic boxes, trying torches and looking for batteries, scissors and tape. Spontaneously, a few other members of the group would stop by and participate in the action by giving advice or asking a question. “Why is the sky blue?” seemed to be the trickiest activity: it soon became clear that the proportion of water and milk powder was critical. Too much powder would make the water medium too opaque, preventing the experiment to work – just like pollution or thick smoke can temporarily obscure a sunny sky.

It took us (I use “us” as, by the second attempt, finding the way to explain why the sky is blue – and sunsets are red – had become a collective effort) a while to identify the right combination of elements. Eventually, one of us switched on the torch and cried “It’s red!”. We thus gathered around the plastic box filled with water and milk powder. When someone shone the torch in a direction parallel to the bottom of the box, one could see two effects: if observed from above (i.e., from an angle between forty and ninety degrees, roughly speaking), the light passing through the whitish water exhibited a blue glow. Powder particles in the water mainly scatter short wavelengths: the whitish water in the box plays the role of air on the earth, and “selective scattering” of wavelengths present in the light coming from the sun is the reason why the sky appears blue to us. On the other hand, if one bends so as to be in line with the light from the torch (looking at the opposite side of the transparent box, therefore observing the components of the light which “survive” all the way through the water and powder), one sees a warm orange-red luminous source – a stunning artificial sunset (2). We took turns to witness the beauty of this simple yet effective experiment – PhD students, postdocs, everyone crawling on the floor like five-year-old children, shouting “Wow!” and then standing up again to comment on the experience.

This is why I love science, among other reasons.

Science is a quest for the curious – science is fair (and humbling, I should add): it speaks to everyone and treats everyone equally. Scientists should do so too, and sometimes they actually do: while I was talking to my colleagues at their stall, I noticed Dame Jocelyn Bell Burnell wearing a bright red helper’s t-shirt and welcoming visitors. That was a nice evening.

(1) I stumbled upon a video which proposed exactly the same experiment as the one performed by my colleagues. However, I was concerned by a sequence in the video in which a bright green laser pointer is shone directly onto the recording camera; I feel urged to point out that this is not an example of good and safe practice, as no laser light should be directly pointed towards the eyes (or any other body part, for that matters). So, if you are curious to try this out at by yourself, please use laser pointers (the green ones being more dangerous than the red ones) with due care.

(2) See http://www.sciencemadesimple.com/sky_blue.html for helpful diagrams.


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