Certain mitzvot are dependent upon light, whether daylight, moonlight, or candlelight. Mitzvot which require daylight include a Kohe
n looking at nega’im (leprous lesions) to determine if they are impure, and a rabbi determining whether a particular stain renders a woman a niddah (menstruant). Additionally, rabbinic courts do not convene at night.
There is one mitzva – Kiddush Levanah (the prayer sanctifying the new moon) – which requires moonlight.
For some mitzvot, we are required to make use of candlelight. For other mitzvot, we are not allowed to make use of the light. For still other mitzvot, a candle is not required, but it still contributes honor and joy.
Mitzvot for which we are required to make use of candlelight include the search for chametz on the night before Pesach. Shabbat and Yom Tov candles are meant to provide useful light. By helping people to avoid tripping and bumping into each other in the dark, the candles contribute to shalom bayit (peace in the home). The blessing over the light of the Havdalah candle is not recited unless one needs the light and derives benefit from it. This is one of the explanations for our custom to hold our hands up to the light and look at our fingernails during Havdalah.
In contrast, one may not derive any benefit from the light of a Chanukah menorah. (This is to make it clear that the candles are being lit to publicize the miracle, and not for any other reason.) In earlier times, when the original Menorah was lit in the Beit HaMikdash, the Kohanim may have avoided using its light. (When guarding the Temple, they would carry torches to light their way.)
Sometimes we light candles to enhance honor and joy. We do this in the synagogue, as well as during celebrations such as weddings, circumcisions, and festive meals.
When studying the laws pertaining to light, an interesting question arises. May we substitute one type of light for another? For example, as we have seen, rabbinic courts convene only during the day. If a room is candle-lit, would the court be permitted to convene at night? Similarly, kosher slaughtering may not be done in the dark. If a room was lit up using a torch, would it then be permissible? Acharonim (15th to 20th century rabbis) disagree about this, with some insisting on sunlight for these activities.
Now let us flip the question around. When candlelight is required, may sunlight or moonlight be used instead? May one search for chametz during daylight hours?
Nowadays, these questions extend to electric lights as well. Some maintain that lightbulbs may be used as Shabbat “candles.” (This does not necessarily mean they can be used for Chanukah candles or a Havdalah candle, since the reasons for the lights in each case are different.) People relate that Rav Chaim Ozer Grodzinski (author of Achiezer) made a point of using incandescent bulbs for Havdalah (others say it was for Shabbat candles). He did this to demonstrate that electricity is considered fire in halacha. People would then understand that turning electric lights on or off on Shabbat is absolutely forbidden.