It is common knowledge that light carries energy. Less obvious is the fact that light also carries momentum. When we sit in sunlight, we are quite conscious of the heat from the light but not of any push. Nonetheless, it is true, that whenever ordinary light strikes an object, the collision gives rise to a small force on the object. This force is called radiation pressure.
The possibility that light could exert pressure goes back to Johannes Kepler, who in 1619 postulated that the pressure of light is what causes comets' tails to always point away from the sun. The corpuscular theory of light introduced by Isaac Newton made the idea of radiation pressure more plausible and stimulated many experimental attempts to measure it. During the 18th and 19th centuries all such attempts to detect the postulated pressure failed to reveal any force that could not be attributed to convection in the air caused by heat. In 1873 Sir William Crookes thought he had discovered radiation pressure in a partly evacuated chamber, only to find that he had invented the radiometer. (The little rotating-vane toy seen in many opticians' windows is a radiometer. It responds to the forces of molecular bombardment on surfaces heated by light rather than to radiation pressure. These thermal forces are called radiometric forees.) It was also in 1873 that James Clerk Maxwell predicted the magnitude of radiation pressure based on his new theory of electromagnetic waves. The predicted pressure was extremely small for ordinary light sources. In Crookes's experiment, for example, it was four or five orders of magnitude smaller than the observed radiometric forces.
The existence of radiation pressure, free of disturbing thermal effects, was finally demonstrated experimentally around the turn of the century by Ernest F. Nichols and G. F. Hull in the U.S. and by P. N. Lebedev in Russia. In both experiments the radiation pressure exerted by a light source was detected by a twisting motion of a vane suspended by a fine fiber in a high vacuum. The magnitude of the force measured in this way confirmed Maxwell's prediction. Commenting on the subject of radiation pressure in his presidential address to the British Physical Society in 1905, John H. Poynting said: "A very short experience in attempting to measure these light forces is sufficient to make one realize their extreme minuteness-a minuteness which appears to put them beyond consideration in terrestrial affairs." Poynting's view of the situation held true until just recently. The experiments described here, which show that the phenomenon of radiation pressure does indeed merit "consideration in terrestrial affairs," could not have been conducted before the invention of the laser in 1960.
The laser has opened up new fields of optical research and rejuvenated old OI)es. The special features of laser light that have brought about these changes are its high degree of spectral purity and its spatial coherence. These properties make it possible, among other things, to focus a laser beam to a spot with a radius close to the theoretical limit of one wavelength. Thus even with a power of a few watts one can obtain a light intensity at one wavelength that is some 10,000 times greater than the intensity available from the entire visible spectrum at the surface of the sun! Moreover, a laser beam operates with a simple, mathematically perfect, well-controlled intensity profile called a transverse mode. The most useful mode has a cross section with a simple Gaussian, or bell-shaped, energy distribution. Another important achievement has been the development of tunable lasers, with which one can select the wavelength of the light at will.
Radiation pressure has recently been reexamined in the light of these new laser sources and has been found to be a strong effect. Indeed, the forces exerted by laser sources have been shown to be large enough to move small particles around freely in various mediums. Accelerations as large as a million g (a million times the acceleration of gravity) are attainable on a continuous basis for tiny macroscopic particles as well as for individual atoms and molecules. These findings have given rise to new applications based on the physical motion of small particles driven by radiation pressure. This rather exotic force has also been found to have a number of unique features, some not realized before and some resulting from the nature of laser light itself. A few of the possible applications are the separation of particles in liquids, the optical levitation of particles in air and vacuum, the high-velocity acceleration of electrically neutral paltic1es, the separation of isotopes and the analysis of atomic beams.
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