As we have seen, if a Jew knowingly performs a melakha on Shabbat, neither he nor any other Jew may benefit from it during Shabbat. Even if he did the melakha unknowingly, according to most poskim no one may benefit from it, because the Sages did not want any Jew to benefit from melakha done on Shabbat (SA 318:1). Others maintain that only if the melakha was done knowingly is it forbidden to benefit from it on Shabbat; in contrast, if the melakha was done unknowingly, one may benefit from it. Some allow one to rely on this opinion when necessary (MB 318:7). A non-observant Jew who is aware that it is Shabbat and that the action he is taking may be prohibited is considered to be transgressing knowingly. Even those who are lenient maintain that one may not benefit from his melakha on Shabbat.
Therefore, if a Jew turned on a light on Shabbat, neither he nor any other Jew may benefit from the light. As we have already seen, some maintain that under pressing circumstances, as long as the light was turned on unknowingly, one may benefit from it. Under normal circumstances, though, or if the light was turned on knowingly, one may not benefit from it.
Nevertheless, if one could have engaged in a certain activity with difficulty even without the light, one may also engage in this activity after the light is turned on. For example, if a light in a stairwell was turned on on Shabbat, as long as one could have ascended the stairs in the dark, one may do so in the light. However, one should not run up the stairs, as this would constitute taking advantage of the light. If a bathroom light was turned on on Shabbat, one may still use the bathroom, since one could have done so without light as well. However, if one could not have arranged certain items in one’s home without the light, one may not arrange them using the light. If there was originally enough light to read, even with difficulty, and then an additional light was turned on, one may continue reading even though it has become easier.
If a light in a room was on and one knowingly turned it off, one may sleep in the room. Even though it is now easier to sleep there, it is permitted, since there is no direct benefit from the melakha; it simply removed an impediment to sleep.
If a Jew turned on a radio or music player on Shabbat, one may not benefit by listening to it. However, if it would be inconvenient to leave the room, he need not do so, since the sound was turned on against his wishes, and he does not want to benefit from it. Even if a non-Jew turned on such a device, one still may not benefit from it, both because it is a weekday activity and because it detracts from the honor due Shabbat (above 22:19).
One is not obligated to stay out of his room because his roommate sinned by turning on the heat in their shared room. However, le-khatĥila he should try to prevent his roommate from transgressing. If he was unsuccessful, he should have in mind not to benefit from the prohibited action. He should not move toward the heater in order to warm up. Rather, he should remain in his usual place. If he benefits against his will, he is not transgressing. If he can open a window in order to avoid benefiting from the heat, this is preferable (based on Rema 276:1; AHS ad loc. 4; MB ad loc. 11-13).
Some maintain that one must not even fulfill a mitzva through a melakha that was performed on Shabbat. Others maintain that since mitzvot were not given for our benefit, doing a mitzva cannot be considered deriving benefit from a melakha. In their opinion, if a light was turned on during Shabbat, one may study Torah or pray by its light. One who wishes to follow this leniency has an opinion to rely on. However, if food was cooked on Shabbat, all agree that one may not eat it. Even though eating it would fulfill the mitzva of oneg Shabbat, since the way one fulfills this mitzva is by experiencing pleasure, it would violate the Sages’ decree forbidding benefit from a melakha done on Shabbat. Similarly, if a light was turned on during Shabbat, one may not eat by its light.
. This is the ruling in Ha-tzava Ka-halakha 34:1-2 and Yalkut Yosef 318:36. It would also seem to be the opinion of Har Tzvi, OĤ 1:185. However, the one who turned on the light, the heat, or the radio knowingly, as well as the person who told him to do so, technically must leave the room in order to avoid benefiting from the melakha (MB 276:13; Yalkut Yosef 318:14, n. 20).
. See Sdei Ĥemed (Kuntres Ha-klalim, Ma’arekhet Mem, Klal 95), which quotes dissenting opinions. These are also cited in Ha-tzava Ka-halakha 33:7 and Yalkut Yosef 318:18-20. When dealing with a physical pleasure such as eating, one should be stringent, as it is not clear that any authority would be lenient. See Ĥayei Adam 62:6 and Igrot Moshe, OĤ 1:126. When there is no physical pleasure involved, then since it is a rabbinic dispute, one who wishes to be lenient has an opinion to rely on. This applies even more so if the melakha was done unknowingly, as then one can take into account the authorities who follow R. Meir and permit benefiting from it.