  In this paper we theoretically describe the absorption of hexagonal boron
nitride (hBN) single layer. We develop the necessary formalism and present an
efficient method for solving the Wannier equation for excitons. We give
predictions for the absorption of hBN on quartz and on graphite. We compare our
predictions with recently published results [Elias {\it et al.}, Nat. Comm.
{\bf 10}, 2639 (2019)] for a monolayer of hBN on graphite. The spontaneous
radiative lifetime of excitons in hBN is also computed. We argue that the
optical properties of hBN in the ultraviolet are very useful for the study of
peptides and other biomolecules.
