This beautiful photo (one of the winners of the 2006 ‘Visions of Science Photographic Awards’) is quite pleasing to the eye. Apparently, it was shot using a grille in front of the light source, so that the deformation of the water around the clip can be seen.
I’m not sure I understand what’s causing the ‘concentric’ curves around the paper clip. There are no waves in the water, and the clip is, I presume, stationary. Are we seeing bands of equal densities of water? If so, it’s not obvious why they should be arranged in such a manner. Is the clip vibrating (at a microscopic level)? Again, it’s not apparent to me, at least, why we get the resultant pattern.
I think you’re seeing curves that represent tha gradient of the meniscus that forms between the water and the paperclip. I think it’s done by using a polarised light source and shooting through a polarised lens - the different angles of water refract the light in different ways, changing the angle of polarisation for some regions.
Actually, I think it’s simpler than I stated above - I think the bands are just the reflection of a grille on the water surface, but the water surface is still being distorted by the meniscus formed against the paperclip edges.
In any case, I realized that I neglected to tell you what was actually being photographed (although it sounds like you’re not having any trouble figuring it out). Still, to be clear, let me say that it’s a close-up photo of a paperclip floating (or being supported by) water.
The water surface is deformed by the paperclip (like this
Above the surface of the water, and parallel to it, is a grating consisting of black bars with spaces between them.
Above that is a diffused light source.
The camera is probably offset slightly to one side, so that it doesn’t capture its own reflection in the water surface, but what is essentially happening is that the image of the black bars against the white diffused background is being distorted when it reflects off the bent surfaces of the water. it the paper clip was not there, the reflection would just show straight bars.
The study of this is called photoelasticity, and it’s very useful in studying actual global stress states (rather than strains at individual points measured by a strain guage or rosette). This image is showing surface tension stresses in the water. Note the busy areas around stress discontinuities like the ends of the clip and places where two pieces of wire come together. Mangetout’s explanation is correct (or at least, right enough for the non-technical reader).
The Wiki link, in particular, seems to well describe what’s going on.
But, two follow-up questions.
According to the article, “photoelastic materials exhibit the property of birefringence only on the application of stress”. Why is that?
Why do we get that particular pattern of stress? I would have guessed that the distribution of surface tension stresses would be much less complex (and less asthetic) than what we’re seeing.
The surface of the water is deformed, and the paper clip is held up by surface tension. I’ve “floated” sewing needles and wire baskets this way, and I’ve seen similar photos of “water strider” insects “walking” on the water.
The bands might be due to reflection of a regular grid, but they look a biy more like interference fringes to me (You can get them without color separation in various ways, chief among them using a monochromatic source – not necessarily a laser). It’s certainly not photoelasticity – the liquid won’t have any stresses in it.
Never worked out how exactly it works, but this appears to be very similar to patterns seen when doing a ronchi test on a telescope mirror. The mention of the grating fits also. The setup is that the light source, AND the camera are both behind the grating. Check out the last link on the wikipedia article above.
It doesn’t look like a Ronchi test. The classic Ronchi test places a grating at the center of curvature of the (nearly perfect) piece under test and you see the grating and its image together. The pattern is the Moire fringe pattern of the grating and its image “beating tigether”. This doesn’t look like a Moire pattern.
This is a very crude approximation of what’s happening in the picture.
In my model, a grating of black bars is suspended over a mirrored surface that contains a dimple (we’re pretending the rusty metal ball made the dimple). The reflection of the black bars (against the sky) is distorted by the various angles of the inside surfaces of the dimple.
The only reason it looks like stress patterns on the paperclip picture is that surface tension tends to result in a dimple that curves away sharply, also, what appears to be stress discontinuities at the ends of the clip is nothing more than the point at which the axis of the grating is perpendicular to the curved end of the paperclip.
I altered my model so that the dimple is elongated, rather than circular, then rotated the dimple so that it was about 30 degrees offset from the grid, narrowed the black bar grating and the setup looks like this, and when I placed the camera very nearly above the dimple, so that it was just peeping through the grating, looking almost straight down at the subject, it looks like this.
You find the explanation on the visions_of_science webpage: on the Menu, select “Winners 2005” and scroll down to “Einstein Year Award”, the photograph shown won the 1st price
That surface curvature is proportional (although not linearly) to the tension and stress. Turning that into actual numbers is a difficult computational and measurement problem, but that’s what causes the distortions it the first place.
You can float a paperclip on water if you’re very, very careful (and the water is distilled and depolarized) but I would suspect that they’ve actually used oil or glycerine for greater consistancy and elastic properties that are closer to linear.
