The Gemara states (Shabbat 23a) that not only should anyone who lights a Menorah recite a Berachah, even one who merely sees a lit Menorah (assuming that he cannot light himself) should utter a Berachah, called Birkat HaRo’eh. Tosafot (Sukkah 46a s.v. HaRo’eh) wonder why we did not establish a similar rule for all other Mitzvot, allowing one who merely sees the performance of a Mitzvah to make a Berachah. Tosafot answer that lighting the candles shows Chavivut HaNeis, the “preciousness” of the miracle that occurred. One can express Chavivut HaNeis through a Berachah even if he does not actually light the candles, but only sees them.
However, Tosafot’s answer still leaves a question: what about the Mitzvot of Purim and Pesach? A precious miracle in celebrated on those holidays as well, but we do not provide a Berachah for one who watches others celebrate or drink the four cups! Perhaps due to this issue, Tosafot give a second answer: we have Birkat HaRo’eh on Chanukah in order to allow people who do not have homes to fulfill the Mitzvah of Chanukah candles.
Tosafot acknowledge, though, that this answer also leaves a question: what about Mezuzah? A homeless person cannot take part in that Mitzvah either, yet we do not allow him to make a Berachah for seeing the Mezuzah! Tosafot therefore conclude that the first answer is better. Hence, we must return to our original question: what is so special about the Mitzvah of Neir Chanukah that allows one to fulfill it to degree by merely looking at the Menorah?
Rav Soloveitchik explains that the Mitzvah of Neir Chanukah, unlike all other Mitzvot, is entirely dependent on the onlooker. Even if a person has done the Mitzvah perfectly, if it looks like the Mitzvah was done incorrectly, the Mitzvah is not fulfilled. Since there is such a dependence on the observer, there is a strong connection between the Madlik (the one who lights the Menorah) and the observer. Because of this link, Chazal decided to extend the Mitzvah to the observer as well.
There are two passages in the Gemara that solidify the Rav’s approach. The first appears on Shabbat 23a, where Rav Huna and Rava debate where Menorahs must be placed if a house is situated on a street corner. Rav Huna says that a house with two doors must light a Menorah by both doorways (in that time Menorahs were lit by doorways, as is practiced in many places in Eretz Yisrael today), even if the doors are on the same side of the house. Rava says that a person must light at two doorways only if they are on different sides of the house. This must be done because a person walking by the door that does not have the Menorah might think that the homeowner has not lit a Menorah at all. We see from here that the viewer’s thought process plays a key role in the Mitzvah of Neir Chanukah.
Another passage in the Gemara (Shabbat 22b) questions when the actual obligation to light is fulfilled: does it occur with the Hanachah (the placing of the Menorah) or the lighting itself? Rava says that the Mitzvah of Neir Chanukah is the actual lighting of the Menorah, but the lighting of the Menorah must take place where the Menorah will be placed. The Gemara then asks: if a person lights the Menorah inside and then moves it to the doorway (or nowadays a window), why is the Mitzvah not fulfilled? Rava answers that if passersby see him lighting by a table, they might think the lighting is not for Chanukah. This Gemara also underscores the connection between the Madlik and the observer – it is the latter who determines whether or not the former fulfills his Mitzvah.
-Adapted from a shiur delivered by Rabbi Yosef Adler in TABC