Bare, dark-colored basalt has a thermal conductivity of 3.1 W/(m∙K), whereas light-colored sandstone's thermal conductivity is only 2.4 W/(m∙K). Even though the same amount of radiation is incident on both and their surface temperatures are the same, the temperature gradient within the two materials will differ. For the same patch of area, what is the ratio of the depth in basalt as compared with the depth in sandstone that gives the same temperature difference?