Minecraft wiki entry for Light, associated text: If a torch with light level 14 is placed on the floor, the light level of the adjacent floor blocks in all four directions is 13, while the diagonal blocks in all four directions have a light level of 12 (14 minus 1 south, minus 1 east).
If a torch with light level 14 is placed on a wall one block above the floor, then the block on the floor that is one block southeast of and below the torch has a light level of 11 (14 minus 1 south, minus 1 east, minus 1 down).