Which category of light is considered invisible?

Ultraviolet (UV) light is considered invisible to the human eye. The electromagnetic spectrum is divided into various categories of light based on their wavelengths, and visible light is just a small portion of this spectrum that humans can detect. UV light falls outside of the visible range, with wavelengths shorter than those of visible light.

Infrared light, on the other hand, has longer wavelengths than visible light but is still detectable through heat perception, which humans can sense indirectly. Therefore, while UV light cannot be seen at all, infrared can still be felt, making it not entirely "invisible" in the same sense.

Thus, when considering the nature of UV radiation in the context of light categories, it indeed stands out as a form of radiation that is invisible, confirming that it is the correct answer in this context.