Black hole shadow and Wandering null geodesics

The role of the wandering null geodesic is studied in a black hole spacetime. Based on the continuity of the solution of the geodesic equation, the wandering null geodesics commonly exist and explain the typical phenomena of the optical observation of event horizons. Moreover, a new concept of `black room' is investigated to relate the wandering null geodesic to the black hole shadow more closely.Comment: 15 pages, 3figures, One of main results (prop.2) has been corrected. Some inappropriate sentences are remove