A vampire exposed to sunlight
Sunlight is one of the many popularized weaknesses of vampires.
Origins[]
Sunlight as a weakness to vampires is not actually from folklore, as many types of vampires can survive just fine in the sunlight, and traditional folklore rarely mentions sunlight at all.
In fact, the weakness to sunlight is a modern addition to vampire folklore and literature, which only goes back to the German film Nosferatu, making it one of the newer additions to the vampire lore.
The traditional vampire, for example, would appear to be a normal person by day, and then turn into a vampire by night, effectively making them immune to sunlight. In Bram Stoker's original Dracula novel, Count Dracula himself was not killed by sunlight, but rather, he is simply less powerful during the day.
Artificial Sunlight[]
Soldiers equipped with ultraviloet lights
On rare occasions, some interpretations will make a claim that vampires can be weakened by artificial sunlight, such as sunlamps or ultraviolet radiation. One notable example is in JoJo's Bizarre Adventure, in which soldiers were equipped with large UV lights to destroy vampires.
Another example is in the first Underworld film where the vampire Selene defeats a lycan/werewolf and discovers a gun he was carrying contained UV ammunition or "daylight harnessed as a weapon". In the Blade Trilogy, ultraviolet lights are used against Vampires, either as a means of torture, or simply killing them.