  The world lines of null particles admit arbitrary parametrizations. In the
presence of a family of observers one may introduce along a null world line an
extension of the so-called Cattaneo's relative standard time parameter (valid
for massive particles) which plays a special role. Another possibility is to
use the coordinate time itself as a parameter. The relation between relative
standard time and coordinate time allows for the introduction of an
observer-dependent optical path and associated refraction index. Both these
quantities are studied here working out explicit examples concerning familiar
null orbits and observers in black hole spacetimes.
