If a person asks his gentile housekeeper on Shabbat to bring him something from a certain room in the house, and she switches on the light so she could find the item with greater ease, may the Jew now derive benefit from the light in that room?
Chacham Ovadia Yosef, in his work Halichot Olam (vol. 3, p. 179), rules that one may make use of the light in this case, because the housekeeper switched on the light for herself, to help her find the object more easily, and not specifically for the Jew. In fact, the work "Shiyurei Kenesset Ha'gedola" records that a certain Rabbi would do this intentionally when he needed light in a room in his house. He would ask his gentile housekeeper to bring him an object from that room, knowing that she would likely kindle a light upon entering the room. He would then be allowed to use the light in the room, since the housekeeper had kindled the light for her own purposes, and not specifically for him.
Another case addressed by the Halachic authorities is where a gentile housekeeper turns on an extra light for the Jew in order to increase the light in the room. In such a case, the Jew may derive benefit from the light, even though the light was kindled specifically for him, because the room already had light and the extra light serves only as an enhancement.
The question arose regarding a case where the housekeeper mistakenly switched off the light, rather than switching on the extra light, and then immediately switched the light back on. Is it then forbidden for the Jew to use the light in the room, given that the room had been dark and the housekeeper switched the light on for him?
The Bei'ur Halacha (supplementary essays to the "Mishna Berura" commentary to the Shulchan Aruch by Rabbi Yisrael Kagan, the "Chafetz Chayim," Lithuania, 1839-1933), in Siman 276 (s.v. "Le'tzorko"), rules that one may make use of the light in such a case. Since the housekeeper switched the light back on to correct her mistake, we consider her to have performed this act for her own purposes, rather than for a Jew's, and one may thus derive benefit from the light.
Summary: One may make use of a light that a gentile kindled on Shabbat for his or her own benefit. Thus, for example, if a gentile housekeeper turns on a light in a room to find an object that her Jewish employer had requested, the Jew may then derive benefit from this light. If a gentile housekeeper mistakenly switched off a light on Shabbat and then immediately switched it back on, one may make use of the light.
