An optical device includes: a plurality of walls disposed to meet along a first axis and having reflecting surfaces facing a space interposed between the walls; and a plurality of partitions, each having a funnel-like shape and each having an outer surface that includes a curved reflecting surface, the partitions disposed in the space interposed between the walls coaxially with the first axis at multiple levels along the first axis; wherein the outer surface of a first of the partitions located at an upper level is configured to receive at least some light through an opening of a second of the partitions located at lower level.