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THE LAST WORD Shivering timbers This photo was taken at Vieux Port in Marseille, France. Can anybody explain why the distribution of ripples varies so much that some of the mast reflections have large “waves” in them, whereas others are almost straight?
■ In any harbour, there are always waves of some description. These are caused by swells from the open sea and the wakes of passing boats, such as the ferries to the Frioul Islands and the Château d’If in Marseille, where the picture was taken. These set up a chaotic pattern of small wave trains as they are refracted and reflected off quays and the hulls of moored boats. As with radio or sound, in any mixture of wave trains that varies in wavelength and direction, there will be times and places at which the waves reinforce each other, and others at which they cancel out. Mast reflections will appear more wobbly in the former case and relatively straight in the latter. Butch Dalrymple-Smith Yacht designer La Ciotat, France ■ The key is that the masts, which are dead vertical, show relatively little distortion, whereas the diagonally oriented forestays show a great deal of rippling. The explanation for this lies in what the rippling water does to the light optically. In essence, the curved surface
of the water acts like a mirror at a funfair, vertically distorting whatever it reflects. Any point on its surface may reflect things either above or below the spot where they would be reflected by a flat mirror. For the masts, this matters little because those spots, whether higher or lower, all result in a reflection that looks more or less the same. For the forestays, on the other hand, the diagonal creates a greater distortion on the water surface than a vertical mast would, due to the angle at which the viewer has taken the photograph: there is a greater transverse component from the photographer’s angle. This explanation is complicated by the fact that the waves on the water are not entirely perpendicular to the perspective of the photo, meaning that even the straight vertical masts ripple
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slightly. However, the picture’s perspective (looking out across the water towards the horizon, rather than down at the water from directly above) helps to minimise this effect by visually compressing the waves vertically. Ben Haller Ithaca, New York, US ■ The reflections of the masts and rigging of the boats appear to be disturbed differently by the ripples in the water because of the different angles at which the observer sees them. In the slightly rippled water, the reflections of the vertical masts do not vary greatly from straight lines due to the foreshortening effect, which minimises the appearance of any breaks or distortions in the images. However, the reflections of the diagonal forestays have small transverse components
from the observer’s perspective, and disturbances in those lines are more visible. By comparison, the almost completely transverse roof line of the buildings has been fully disrupted because of the same ripples. If you “fan-fold” a sheet of paper to simulate waves and draw “masts” running the length of the sheet, and “forestays” at an angle to them, you can see that when viewed from a low angle, the masts still appear relatively straight, whereas the forestays are broken up as they run across the page. Chris Daniel Colwyn Bay, Conwy, UK
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Why, when I transfer my low-fat spread on to my knife, does it form this pattern, especially when it’s taken from a new tub? Alicia Bryant Chelmsford, Essex, UK
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