A black light emits a blue-purplish glow, even though it's called "black" light.
While the term "black light" might suggest that it emits black light, that's not entirely accurate. A black light bulb primarily emits ultraviolet (UV) light, which is invisible to the human eye. However, black lights also emit a small amount of visible light, primarily in the blue and violet range. This is what you see as the characteristic blue-purplish glow when a black light is turned on in a darkened room.
The reason for this blue-purplish glow is due to the phosphors coating the inside of the bulb. These phosphors are designed to emit visible light when exposed to UV radiation. The specific type of phosphor used determines the precise color emitted, which is typically in the blue-purple spectrum.
Therefore, when you observe a black light, you're not seeing the UV light itself, but rather the visible light emitted by the phosphors as a result of UV excitation. This visible light is what gives black lights their distinct appearance.
