Ultraviolet

Ultraviolet, also known as UV light, or "Red Dawn" by the Ice Warriors, was light of a wavelength shorter than visible light. Though some creatures like humans were unable to see it, others, like the V'orrn, could. (PROSE: Placebo Effect) Sunlight was ultraviolet. (TV: The Vampires of Venice)

Some deep sea creatures were susceptible to ultraviolet light. The Myrka could be stunned or killed by ultraviolet light. (TV: Warriors of the Deep) The Saturnyns were repelled by ultraviolet light and killed in high amounts. (TV: The Vampires of Venice)

Ultraviolet light could also be dangerous to other lifeforms. The Ice Warriors called ultraviolet light the "Red Dawn", which the Fifth Doctor described as "dying from sunburn". In the 21st century, Earth's thicker layered atmosphere compared to Mars at the same time could filter the ultraviolet rays of the Sun, protecting life there. (AUDIO: Red Dawn) Millennia before, the ultraviolet light on Mars increased to fatal levels after the Key to Time was removed from the planet, causing climatic changes. (AUDIO: The Judgement of Isskar)

On Solos, ultraviolet rays reacted with a nitrogen isotope in the soil to produce a mist toxic to humans. (TV: The Mutants)

Ultraviolet radiation could be converted into the power source called black light. (TV: The Mysterious Planet)

In the London Underground, the Third Doctor used a torch with an ultraviolet beam to track the thief Hingrad, having placed a radioactive trace element over his shoes. (COMIC: Secret of the Tower)