What type of radiation causes a phosphorescent coating inside a fluorescent tube to emit purple light?

The correct answer is ultraviolet radiation because, within a fluorescent tube, the phosphorescent coating is designed to respond to the specific wavelengths emitted by ultraviolet light. When electrical energy is applied to the gas inside the tube, it generates ultraviolet radiation. This ultraviolet light then interacts with the phosphorescent coating on the inside of the tube. The coating absorbs the ultraviolet radiation and re-emits it as visible purple light.

Understanding this process highlights the purpose of the phosphorescent material, which is to convert invisible ultraviolet light into visible light, enabling the tube to illuminate effectively. In contrast, infrared radiation does not have enough energy to excite the phosphorescent material, while X-ray radiation is of a much higher energy level and does not play a role in this context. Visible light, while it can be emitted by the coating, is not what initially causes it to glow; rather, it is the ultraviolet radiation that initiates the process.