Falling water drops from a dripping faucet, illuminated from above, exhibit a row of bright strips of light, a few centimeters apart at a fixed distance below the faucet. Flash photographs of the drops show that they are oblate in shape when the flashes occur, and the bright flashes of light originate from the edge of the drop that is on the opposite of the overhead light source. Here we show that the spots result from the same internal reflection that gives rise to the rainbow in a cloud of spherical drops. The periodic flashes reflect the capillary oscillations of the liquid drop between alternating prolate and oblate shapes, and the dramatic enhancement in the oblate phase results from a combination of several optical effects. Ray tracing analysis shows that the flashes occur when the rainbow angle is $42°$ in spherical drops but sweeps over a wide range between $35°$ and $65°$ for typical ellipsoidal drops, and the intensity of the caustic is strongly enhanced in the oblate phase. This phenomenon can be seen in all brightly lit water sprays with millimeter size drops and is responsible for their white color.

© 2009 Optical Society of America

1. René Descartes, Discourse on Method (Cambridge University Press, 1637).
2. C. E. Bohren and D. R. Huffman, Absorption and Scattering or Light by Small Particles (Wiley-Interscience, 1983).
3. P. L. Marston, “Rainbow phenomena and the detection of nonsphericity in drops,” Appl. Opt. 19, 680-689 (1980). [CrossRef] [PubMed]
4. J. W. S. Rayleigh, “On the stability of cylindrical fluid surfaces,” Phil. Mag. 34, 177 -181 (1892).
5. Ph. Lenard, “Über die Schwingungen fallender Tropfen,” Ann. Phys. Chem. 30, 209 (1877).
6. F. E. Volz, “Some aspects of the optics of the rainbow and the physics of rain,” Physics of Precipitation, H. Weickmann, ed. (American Geophysical Union, 1960), Monograph 5, pp. 280-286.
7. P.-G. de Gennes, F. Brochard-Wyart, D. Qur, and A. Reisinger, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves (Springer, 2004).
8. S. Chandrasekhar, “The oscillations of a viscous liquid globe,” Proc. London Math. Soc. s3-9, 141-149 (1959). [CrossRef]
9. D. H. Peregrine, G. Shoker, and A. Symon, “The bifurcation of liquid bridges,” J. Fluid Mech. 212, 25-39 (1990). [CrossRef]
10. R. Li, X. Han, L. Shi, K. F. Ren, and Huifen Jiang, “Debye series for Gaussian beam scattering by a multilayered sphere,” Appl. Opt. 46, 4804-4812 (2007). [CrossRef] [PubMed]
11. R. T. Wang and H. C. van de Hulst, “Rainbows: Mie computations and the Airy approximation,” Appl. Opt. 30, 106-117 (1991). [CrossRef] [PubMed]
12. R. Clift, J. R. Grace, and M. E. Weber, Bubbles Drops and Particles (Academic, 1978), p. 185.

Rayleigh, J. W. S.
Bohren, C. E.
Brochard-Wyart, F.
Chandrasekhar, S.
Clift, R.
de Gennes, P.-G.
Descartes, René
Grace, J. R.
Han, X.
Huffman, D. R.
Jiang, Huifen
Lenard, Ph.
Li, R.
Marston, P. L.
Peregrine, D. H.
Qur, D.
Reisinger, A.
Ren, K. F.
Shi, L.
Shoker, G.
Symon, A.
van de Hulst, H. C.
Volz, F. E.
Wang, R. T.
Weber, M. E.

Ann. Phys. Chem.

• Ph. Lenard, “Über die Schwingungen fallender Tropfen,” Ann. Phys. Chem. 30, 209 (1877).

Appl. Opt.

• P. L. Marston, “Rainbow phenomena and the detection of nonsphericity in drops,” Appl. Opt. 19, 680-689 (1980). [CrossRef] [PubMed]
• R. Li, X. Han, L. Shi, K. F. Ren, and Huifen Jiang, “Debye series for Gaussian beam scattering by a multilayered sphere,” Appl. Opt. 46, 4804-4812 (2007). [CrossRef] [PubMed]
• R. T. Wang and H. C. van de Hulst, “Rainbows: Mie computations and the Airy approximation,” Appl. Opt. 30, 106-117 (1991). [CrossRef] [PubMed]

J. Fluid Mech.

• D. H. Peregrine, G. Shoker, and A. Symon, “The bifurcation of liquid bridges,” J. Fluid Mech. 212, 25-39 (1990). [CrossRef]

Phil. Mag.

• J. W. S. Rayleigh, “On the stability of cylindrical fluid surfaces,” Phil. Mag. 34, 177 -181 (1892).

Proc. London Math. Soc.

• S. Chandrasekhar, “The oscillations of a viscous liquid globe,” Proc. London Math. Soc. s3-9, 141-149 (1959). [CrossRef]

Other

• F. E. Volz, “Some aspects of the optics of the rainbow and the physics of rain,” Physics of Precipitation, H. Weickmann, ed. (American Geophysical Union, 1960), Monograph 5, pp. 280-286.
• P.-G. de Gennes, F. Brochard-Wyart, D. Qur, and A. Reisinger, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves (Springer, 2004).
• René Descartes, Discourse on Method (Cambridge University Press, 1637).
• C. E. Bohren and D. R. Huffman, Absorption and Scattering or Light by Small Particles (Wiley-Interscience, 1983).
• R. Clift, J. R. Grace, and M. E. Weber, Bubbles Drops and Particles (Academic, 1978), p. 185.

2007, Li, Appl. Opt.
1991, Wang, Appl. Opt.
1990, Peregrine, J. Fluid Mech.
1980, Marston, Appl. Opt.
1959, Chandrasekhar, Proc. London Math. Soc.
1892, Rayleigh, Phil. Mag.
1877, Lenard, Ann. Phys. Chem.

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