The Sages ordained that bedikat ĥametz be performed by candlelight, because candlelight is focused and effective for searching. During the day, the light of the sun detracts from the brightness of the candle, and it is difficult for the eye to focus on its weak light. Therefore, the Sages ordained searching at night, because at night candlelight is more brilliant and does a good job of illuminating holes and crevices, where the essence of the search is carried out.
One may not search for ĥametz by torchlight, i.e., with a candle that has two or more separate wicks. There are several reasons for this, the main one being that the large flame will cause the searcher to worry about burning something, and as a result not be able to concentrate on the search. If one erred and searched by torchlight, he did not fulfill his obligation. Additionally, one may not search by the light of an oil lamp, because the fear of spilling oil and staining his belongings will deter him from maneuvering the lamp into narrow spaces to get a good look at cracks and crevices. Likewise, one should not use a paraffin candle for bedikat ĥametz. Therefore, the widespread custom is to prefer wax candles, because they do not drip excessively (SA and MB 433:2).
In principle, it is permissible to use a flashlight for bedikat ĥametz; the Sages ordained using a candle because its light is focused, and the light of a flashlight is focused. Moreover, a flashlight has an advantage in that one need not worry about burning things or spilling wax and oil, and if it is a good flashlight, its light is stronger and more focused than a candle’s. Nonetheless, some people are strict and do not search with a flashlight because the Sages derived from Scriptural verses that the search for ĥametz should be performed with a candle (Pesaĥim 7b). Yet a flashlight can also be considered a kind of candle, for the filament is like the flame and the battery is like the oil (She’arim Metzuyanim Be-halakha 111:4).
In practice, the prevailing custom is to search by candlelight, in keeping with the practice of the Sages. However, one who wishes to search with a flashlight may do so with a berakha, and in places where the searcher is concerned that the candle will cause a fire, it is preferable to search with a flashlight (see Sidur Pesaĥ Ke-hilkhato 13:10; Yeĥaveh Da’at 1:4).