One Who Looks at a Rainbow
Our Sages (Chagiga 16a) state: “The eyes of one who gazes at a rainbow are dimmed, as the verse (Yechezkel 1) states, ‘Like the appearance of a rainbow in the clouds on a rainy day, so is the radiance around and this is the appearance of the glory of Hashem.’” This means that a rainbow is similar to the appearance of the glory of Hashem. It is therefore forbidden to gaze at a rainbow too much, for this is disrespectful towards Hashem.
This matter is seemingly difficult though, for what about a rainbow can possibly resemble the appearance of the glory of Hashem? The Meiri explains that just the appearance of a rainbow is unclear as it looks green from one angle and red from another, so too, when the appearance of Hashem’s glory appeared to the prophet Yechezkel, it was in an unclear manner and was unable to decipher what it was. (For this reason, since there is some similarity between a rainbow and the appearance of Hashem’s glory, a G-d-fearing individual should not gaze at it for too long.)
In any event, halachically speaking, one should not overly gaze at a rainbow. Indeed, Maran Ha’Shulchan Aruch rules (Chapter 229) that it is forbidden to overly gaze at a rainbow. The Magen Avraham quotes the saintly Shelah as saying that gazing at a rainbow too much ruins one’s vision.
Hagaon Harav Yosef Dov Soleveichik zt”l (from Boston) explains that when our Sages taught that the eyesight of one who gazes at a rainbow is dimmed, this is not meant as a punishment; rather, our Sages intended to say that if one cannot discern the glory of Hashem within a rainbow, “his eyes are dimmed,” i.e. he is already considered blind, for he does not know what he is seeing. It is for this reason that our Sages used the term “his eyes are dimmed,” in present tense as opposed to “shall be dimmed,” in future tense. Nevertheless, it seems from the Shelah quoted by the Magen Avraham that the teaching of our Sages simply means that one who gazes at a rainbow is endangering his eyesight.
The Blessing on a Rainbow
One who sees a rainbow recites the blessing: “Baruch Ata Hashem Elokeinu Melech Ha’Olam Zocher Ha’Berit Ne’eman Bivrito Ve’Kayam Be’Ma’amaro.” The meaning of this blessing is that Hashem remembers the covenant He made with us and is loyal to the covenant He made with Noach that although there are many wicked sinners in the world and we are, G-d-forbid, deserving of another great flood upon the earth, nevertheless, Hashem is trustworthy that He shall not bring another flood upon us, as the verse states, “And when the rainbow is seen in the cloud, I shall remember my covenant.” The conclusion of the blessing, “And steadfast in His word,” means that although the rainbow was indeed created during the Six Days of Creation and is part of the laws of nature, Hashem is nevertheless steadfast in His word and since He said he would not send another flood, he shall surely fulfill His word, even without the rainbow.
If the Blessing is Only Recited on a Full Rainbow
Hagaon Mishnah Berura writes in his Be’ur Halacha that it is unclear from the Poskim if one must see the entire rainbow, i.e. a complete half-circle or arch, in order to recite the blessing or if it is sufficient to see even part of the rainbow. Maran Rabbeinu Ovadia Yosef zt”l in his Chazon Ovadia-Berachot quotes the Sefer Ha’Berit who writes that when only a partial rainbow appears in the sky, it is possible that this is not the kind of rainbow a blessing is recited upon to begin with, for only when a rainbow is complete should a blessing be recited; not every time a prism of colors appears in the sky should a blessing be recited. Thus, following the rule of “when in doubt regarding a blessing, do not bless,” one should not recite a blessing on such a rainbow. On the other hand, Hagaon Harav Shalom Mizrahi zt”l (a member of Maran zt”l’s rabbinical court) writes that it seems that there is no distinction and any time one sees any form of a rainbow, one should recite a blessing. However, it is best to be concerned for a doubt regarding blessings and not recite a blessing on a partial rainbow. Nevertheless, one who recites a blessing upon seeing part of a rainbow has on whom to rely.
Summary: One should not overly gaze at a rainbow. Upon seeing a rainbow, one should recite the blessing of “Zocher Ha’Berit Ne’eman Bivrito Ve’Kayam Be’Ma’amaro.” Some say that one should only recite this blessing when seeing a complete rainbow, i.e. the entire arch.