Mehta et al. Reply: Garcıa-Camara et al. [1] have raisedcertain points against the assumptions made in our Letter[2] to interpret zero forward scattering observed in disper-sion of micron sized magnetic spheres in a ferrofluid.

All these points are already discussed in the Letter. Sincethe Letter has to be concise, we could not elaborate further.It appears that our sentence "We attribute this effect to thezero forward scattering by magnetic spheres as predictedby Kerker et al." prompted them to write this Comment. Ifone reads this sentence vis-a-vis the previous two, then themeaning is clear. In the Letter, we have shown that theeffect is observable only when micron sized magneticscatterers are dispersed in a ferrofluid and the dispersionis subjected to the critical field. Kerker et al. have devel-oped a theory for electromagnetic scattering by magneticspheres and have predicted several unusual effects. In thetheory, the surrounding medium was nonmagnetic, iso-tropic and effect of the field was not incorporated; never-theless, it is a proper starting point. We made this pointclear in the Letter. We never claimed that the effect per seoccurs "solely" due to scattering by magnetic spheres.

Justification of the use of the Rayleigh-Gans approxi-mation is also discussed in the Letter. A ferrofluid iscomposed of nanomagnetic particles. Since the size ofthe nanomagnetic particles is much smaller than the wave-length of light used in the experiment and the fluid is dilute,one can treat the ferrofluid as an effective continuousmedium [3,4]. Obviously, " as well as � of the ferrofluidwill be larger than that of the host medium, i.e., kerosene.Moreover, both are field dependent. Consequently, relativeto the ferrofluid medium, "scatt and �scatt of the micronsized scatterers will also be field dependent. This has madeit possible to tune the effect. Since we are observing theforward scattering, the form factor can be taken to be 1 andpattern can be explained on the basis of Rayleigh theory asa first approximation [5]. Rigorous theory may be devel-oped by evaluating the amplitude functions an and bn asfunctions of the field. There is no question of ‘‘defending’’our statement, particularly when we were aware of thelimitations of our model. Vehemence is detrimental to theprogress of science. If a model is wrong, then it should berejected, and if it is approximate, it should be refined. In thespirit, we carried out further investigations and studiedback scattering pattern in the field. We found that it alsodisappears at the same critical field (Fig. 1). These resultsare published elsewhere [6]. The case when " � � � �2leads to singularity in the expressions of a1 and b1 [7] andis not applicable in the current case.

R. V. Mehta,† Rajesh Patel, Rucha Desai, R. V. Upadhyay,and Kinnari Parekh*

Department of PhysicsBhavnagar UniversityBhavnagar, 364002, India

Received 31 July 2006; published 27 April 2007DOI: 10.1103/PhysRevLett.98.179702PACS numbers: 78.20.Ls, 42.70.Qs, 47.65.Cb, 75.50.Mm

*Current address: Physics Department, Faculty of Science,M. S. University of Baroda, Vadodara, 390002, India.

†Corresponding author: rvm@bhavuni.edu[1] B. Garcıa-Camara , F. Moreno, F. Gonzalez, and J. M.

Saiz, preceding Comment, Phys. Rev. Lett. 98, 179701(2007).

[2] R. V. Mehta, R. Patel, R. Desai, R. V. Upadhyay, andK. Parekh, Phys. Rev. Lett. 96, 127402 (2006).

[3] P. Sheng, Introduction to Wave Scattering, Localizationand Mesoscopic Phenomena (Academic, San Diego,1995).

[4] R. E. Rosensweig, Ferrohydrodynamics (Cambridge Univ.Press, Cambridge, 1986).

[5] H. C. van de Hulst, Light Scattering by Small Particles(Wiley, New York, 1957).

[6] R. V. Mehta, R. Patel, and R. V. Upadhyay, Phys. Rev. B74, 195127 (2006).

[7] M. Kerker, D. S. Wang, and L. Giles, J. Opt. Soc. Am. 73,765 (1983).

FIG. 1. Backward scattering patterns for 3�m sized magnetitespheres (6% by vol.) in the ferrofluid (diluted 78% by vol.)(a) H � 0 Oe, (b) h � 50 Oe, (c) 100 Oe, (d) H � 200 Oe,(e) h � 250 Oe, and (f) H � 500 Oe. Complete vanishing ofbackward scattering light is evident in (c).

PRL 98, 179702 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending27 APRIL 2007

0031-9007=07=98(17)=179702(1) 179702-1 © 2007 The American Physical Society