Under normal circumstances, daylight can be harmful to Vampires. And depending on factors (i.e. the intensity of the light, length of exposure to it, along with how old and powerful the vampire in question). the effect may range from merely causing the vampire to sizzle or steam, to incinerating them instantly on the spot. Vampires that are still in transition can resist the effects of direct exposure, albeit will be be severely incovenienced by it.
The reason vampires are naturally weak against the sun, actually started at the time of the Originals, when the witch Esther used the sun's life aspect and the White Oak tree's immortality to make her family live forever and be able to protect themselves. Sunlight is harmful but not fatal to Originals, however, Esther created the very first daylight rings.
Some vampires have charmed daylight jewelry with Lapis Lazuli; these rings must be forged by witches because the Lapis itself is simply a stone. It protects them from the sunlight. So long as the vampire wears it in the sun, he or she can't die from exposure. Both Stefan and Damon Salvatore are in possession of rings containing Lapis Lazuli which protect them from the harmful effects of the sun. However, without such a ring sunlight will burn a vampire on contact. During Damon's imprisonment, Stefan deprives him of his ring so when he later tries attacking Caroline and runs into a shaft of sunlight, he's left with nasty facial burns following exposure of approximately 3 seconds.
Slater's cafe and home contained tempered glass windows to block UV rays, making them safe environments for vampires. This shows the harmful element is natural UV light, but it is unclear if artificial UV light would have the same effect. Furthermore, it's unclear if the lapis lazuli rings protect against only natural sunlight or UV rays in general.
In Ancient Greek Myth, the sun was under the rule of two gods, the first being was Helios who was kind and fair and was the First Lord of the Sun. Under his rule, the sun was seen as a symbol of riches and gold, its sunlight gave life to the living and the crop and Helios drove the sun the sky in his chariot with his fiery horses which he named: Pyrois, Aeos, Aethon, and Phlegon.
The second was Apollo who was the God of Sunlight and under his rule, the sun was seen as a symbol of fire and destruction, under his rein, the sun burnt down hoses and killed the crop.
- In the series, Isobel Flemming is the first vampire seen to actually die from sun exposure.
- In the novels, Katherine von Swartzschild and Elena Gilbert are the first vampires to die of sun exposure.
- Known vampires that have been damaged by sunlight are: Alaric Saltzman, Damon Salvatore, Stefan Salvatore, Ben McKittrick, Caroline Forbes, Rose, Katherine Pierce, Isobel Flemming, Rebekah, Elena Gilbert, Slater, Alexander's prey and Lexi Branson.