by Rabbi Yossi Marcus
The bonfires celebrate the immense light that was brought into the world by Rabbi Shimon bar Yochai [who passed away on Lag b’Omer], especially on the day of his passing.
The bow commemorates the fact that during Rabbi Shimon’s lifetime no rainbow was ever seen. [Bereishit Rabbah 35:2] Note: This was a good thing because the rainbow appears when the earth deserves punishment. The first time a rainbow appeared was after Noah’s flood, when G-d said that He will no longer destroy the world, rather He would send a sign: the rainbow. During Rabbi Shimon’s lifetime, the world was filled with merit because of him and therefore never saw a rainbow. [Genesis 9:8-17 and Rashi there]
There is a Kabbalistic tradition that on Lag b’Omer a rainbow will appear in a different color, which will symbolize the arrival of the Messianic age [Bnei Yissaschar]