Surprising Properties of Sunlight

Surprising properties of sunlight is another highlight of the Frontiers in Optics meeting. Greg Gbur, now at the University of North Carolina at Charlotte (gjgbur@uncc.edu), and his colleagues reexamined 19th-century-physics-based estimates for the coherence of sunlight and found that an assumption used in those estimates is technically inaccurate, but the results are surprisingly correct.

Generally, we only think of a highly controlled source such as a laser as producing coherent light. All light, however, including sunlight, has some degree of spatial coherence, i.e. some extent to which its fluctuations at different points in space are correlated (have precise interrelationships).

In 1869, Emile Verdet derived a rough estimate for the area within which sunlight falling on the Earth’s surface may be considered spatially coherent. Verdet implicitly assumed, however, that the Earth is in the "far zone" of the Sun, which was conventionally understood to be the distance at which the Sun appears as a point object. Of course, this is not true on the Earth, as we are 10¹⁵ km too close, and this raised the question of whether Verdet’s results, and those which followed, are accurate.

Gbur and colleagues performed new simulations of filtered light emanating from an incoherent spherical source, standing in for the Sun. Surprisingly, they found that the light behaved as if it were in the far zone even when viewed only a few wavelengths from the source. So Verdet’s assumption is correct, but only because the far zone for an incoherent source is much, much closer than expected.

One of Gbur’s co-authors on this study is the University of Rochester’s Emil Wolf, a key developer of the modern theory of coherence, and the other is Oklahoma State University’s Girish S. Agarwal, another influential figure in the development of coherence. (Paper FTuF6 at meeting; also Agarwal, Gbur, and Wolf, Optics Letters, March 1, 2004.)

After publishing their Optics Letters paper, the authors discovered that a little-appreciated paper (Leader, Journal of the Optical Society of America 68, 1978) made similar findings but did not discuss the results in the context of sunlight.