The continuum radiation in comets is polarized due to scattering of sunlight by the dust particles present in the comet's atmosphere. The degree of polarization and its direction depend upon the size distribution, composition of particles (which can be defined in terms of an average complex refractive index), scattering angle and wavelength of the incident solar radiation. A study of the polarization of continuum radiation in comets is therefore a powerful means to determine the characteristics of the cometary dust grains (Ohman 1941; Blackwell & Willstrop 1957; Bappu et al. 1987; Sen et al. 1991a,b). During Halley's 1986 apparition (and subsequently for more recent comets) a large number of observations of cometary continuum polarization were obtained by several groups (see Joshi et al. 1997).
Phase curves of various comets appear to be similar (Dollfus et al.
1988; Levasseur-Regourd et al. 1996, and references
therein). Such phase curves seem to be typical of fluffy regoliths, or
irregular dust particles with sizes greater than 1 
m (Giese
et al. 1978) and are similar to those expected from aggregates of
interstellar dust (Greenberg & Gustafson 1981). The nature of
the phase curves is expected to provide clues to the physical properties of
the dust.
Levasseur-Regourd et al. (1996) have classified the comets in
two classes on the basis of polarimetric properties. These two classes of
comets are distinct only for the phase angles above 40
. Therefore,
it is important to have polarimetric data covering a wide range of phase
angles, especially above 40 to classify the comet in either
category. Further observations of comets over a large range of phase angles
and continuum wavelengths is necessary before making a definitive statement
about the difference in the properties of cometary dust for the two classes.
Comet Hale-Bopp C/1995 O1 is an intrinsically bright comet and having been
discovered two years before perihelion, it has provided sufficient time
to the astronomers to plan their observations. Furthermore, it was
possible to make observations over a large range of phase angles for this comet
although the largest phase observed was 
.
In this article we report continuum linear polarization observations made during the period from October 1996 to May 1997. On the basis of this data we discuss the nature of dust grains and compare Hale-Bopp characteristics with some other comets.