From theoretical considerations, it is known that light from a certain star should reach Earth with intensity l0. However, the path taken by the light from the star to Earth passes through a dust cloud, with absorption coefficient 0.1/light-year. The light reaching Earth has intensity 1/2 l0. How thick is the dust cloud? (The rate of change of light intensity with respect to thickness is proportional to the intensity. One light-year is the distance travelled by light during 1 yr.)