I went to Spain for a short holiday last week. I was hoping to get a break from science but I failed.
The weather was beautifully sunny and I spent many an hour in or near the water at the little coastal village of Cadaquès. I read, swam, strolled, laughed with my kids, took a few photographs and did nothing. Lots of nothing.
Just the ticket for unwinding after another draining year of teaching and research. But I couldn’t quite get science out of my mind. Physics in particular was bothering me, even though my main interests these days are more biochemical.
In part I was preoccupied by a public lecture that I will be giving in the autumn on X-ray diffraction. This is a powerful technique for working out the structures of molecules but it’s hard to explain to people because we don’t have everyday experience of diffraction phenomena.
Diffraction occurs when waves — whether made of water or light or X-rays or whatever — encounter a sharp edge or a small opening. When they do, the wave fronts bend around the edge or fan out as they pass through the narrow aperture, becoming distinctly curved in the process.
At school your physics teacher may have demonstrated diffraction using a specially designed wave tank. I thought I might be able to see the effect in the wild, so to speak, by watching the waves of the Mediterranean as they pulsed into the rocky shore at Cadaquès. But as you can see from the video that I took, it’s not so easy. My experiments lacked control, since the sea was stirred in different directions by the wind and most of the rocky obstacles interrupting their paths were irregular. The reflection of the water waves by the rocks is fairly obvious, but I couldn’t convince myself I had seen any diffraction. The video is at least rather soothing.
I will have to look elsewhere for examples of diffraction but I realised as I trawled through my holiday snaps that I had captured plenty of other wave phenomena. You can’t really get away from physics when you go outside.
First off there is the long-established principle that light wave travels in straight lines (at least when not in the vicinity of very massive objects), visible — just — as the suns rays burst though gaps in the morning cloud.
Of course the path of light waves can be bent markedly when they are reflected. This was in evidence (though the rays themselves were not visible) as the colours of the evening sky bounced off the waters of the bay, the image broken and flickering because of the waves rippling across the liquid surface.
The clouds in that photograph, and their reflection, are tinged with sunset reds and oranges because of the way that the air of the atmosphere scatters light. This effect — Rayleigh scattering — occurs because the electromagnetic waves of the sun’s rays have much longer wavelengths (the distance between successive crests – around 500 nm for visible light) than the atoms and molecules (about 0.3 nm) that they collide with in the Earth’s atmosphere.
The scattering — deflection of the rays into all directions — is strongest for shorter wavelengths which are at the blue end of the spectrum. That’s why the sky is blue during the day: what you see is the light from the sun that has been most strongly scattered down towards the surface of the planet as it streams though the air above your head.
The sun itself is a tad yellowish if you dare to look directly at it during the day (not recommended) because a proportion of the blue light in the visible spectrum has been removed.
This effect is much more pronounced at sunrise and sunset since the light then passes though a much greater length of atmosphere to travel to your eye (think about the angles — I reckon it’s about 600 times more). This means that much more blue is scattered out leaving a deeper red-orange hue.
The scattering effect varies as you look up, away from the sun. As you lift your line of sight, the illuminating light is more scattered rather than transmitted. You can just about see all the colours of the spectrum as you tilt your head to the blue above, though seeing the greenish tinges merging with the yellow is a bit of a challenge for me. I may be a bit colour-blind.
These effects can also be seen when the moon rises because the light it reflects from the sun also has to pass through a thick section of atmosphere.
But as the moon rises higher into the night sky, the coloration disappears and it turns pale.
Here’s a good question: why isn’t it blue?
I could easily have broadened my scientific interests in Spain to include chemistry and biology. My son and daughter were both stung by jellyfish. Was that defence or attack? Why did their skin redden and blister shortly afterwards? And why did the application of vinegar soothe the pain?
But that would have been a bit too close to home. After all, I was on holiday.
@Stephen_Curry is a professor of structural biology at Imperial College.