Number 378, June 24, 1998 by Phillip F. Schewe and Ben Stein
THE PHYSICS OF FOOTBALL (soccer) is au courant now that the 1998 World Cup has begun in France. The June 1998 issue of Physics World anatomizes a great moment in recent soccer history---the penalty kick taken by Brazilian player Roberto Carlos in a game last year against France. Carlos kicks the ball with high speed (70 mph) and imparts a high spin (10 rev/sec). Airflow past the rocketing ball is at first in the low-drag, turbulent regime. However, about 10 m along its trajectory (just as it shoots wide of a wall of poised defenders) the ball slows enough for it to enter into a smooth-airflow (laminar) phase. This entails an ever increasing degree of drag, which in turns brings the Bernouilli principle (and a hefty sideways force, or "lift") into play, dramatically curving the ball past the goalie into the net.
SINGLE-SHOT FAR-INFRARED PICTURES. Conventional photography records the visible world at wavelengths of 300-800 nm. Two dimensional images at near-infrared wavelengths (around 1 micron) can also be rendered. Doing spectroscopy or making pictures (and movies) at even longer wavelengths (10-1000 microns or, equivalently, terahertz frequencies) is difficult but desirable. Difficult because there have been no reliable detectors or coherent (laserlike) sources for this undervalued part of the electromagnetic spectrum; desirable because many molecules (in pollutants, diseased tissue, explosives, etc.) rotate and vibrate at energies corresponding to these far infrared (FIR) frequencies (Science, 8 May 1998). Progress is happening on several fronts. For instance, new coherent sources are at hand: scientists at Dartmouth produce FIR radiation by sending the beam from a scanning electron microscope (SEM) across a diffraction grating (Urata et al., Physical Review Letters, 19 January 1998). And at Rensselaer Polytech (Xi-Cheng Zhang, email@example.com, 518- 276-3079) researchers have been able to make an image of a single THz pulse. That is, they measure the actual waveform---the electric field as a function of time (over a 25 ps period) and space (over a 10 mm span)---of a terahertz burst of radiation. They do this by encoding the THz signal onto a light pulse at optical wavelengths which has been stretched from 200 fs to 20 ps in a process called chirping. The combined signal is later decoded and imaged with a video camera, yielding (for the first time) a real-time image of a THz pulse and not just a sequence of stroboscopic samplings over a long period. This constitutes a step toward real imaging with THz light. (Jiang and Zhang, to be published in Optics Letters, 15 July.)
TURNING SOLID METAL INTO A TRANSPARENT, HIGHLY REFRACTIVE FLUID is what happens when a 120-fsec laser pulse strikes a sample at the Institute for Laser- and Plasma Physics at the University of Essen. Researchers there make a slow-motion movie (with a frame every tenth of a picosecond) of laser ablation, the process (important in many industrial and surgical applications) in which laser light quickly heats and removes material from a solid. After the initial laser onslaught, the material in the affected area (about 300 microns across) is first pulled into a fluid state of high temperature (several 1000 K) and pressure (several tens of GPa) and then is carried away from the surface by hydrodynamic flow. The clue as to what this fluid is doing (temperature, index of refraction, etc.) during the expansion is the appearance of "Newton rings," the optical pattern set up when light rays reflecting from the ablating portion of the sample interfere with light reflecting from the remaining material in the back. These rings (never seen before) were remarkably similar for the whole range of metals and semiconductors tested, implying that some universal ablation mechanism was at work. (Klaus Sokolowski-Tinten et al., 6 July Physical Review Letters; see figureat Physics News Graphics; firstname.lastname@example.org.)