We're spellbound when we saw a rainbow, because it's one of three sights that refect the Divine Presence. Therefore, whenever we see one, we should immediately make the blessing of "zocher Habris:"

Baruch ata A'donai, E'loheinu melech ha-olam, zocher habrit v’ne’eman biv’reetoh v’kayam b’ma’amarav.

Blessed are You, Lord our God, King of the Universe, who remembers the covenant, and is faithful to His covenant, and keeps His promise.

בָּרוּךְ אַתָּה ה' אֶ'לוֹהֵינוּ מֶלֶךְ הָעוֹלָם זוֹכֵר הַבְּרִית וְנֶאֱמָן בִּבְרִיתוֹ וְקַיָם בְּמַאֲמָרוֹ