The correct answer is option 4: Fluorescent coating on the glass.

In a fluorescent lamp, the main source of light is not the filament like in an incandescent bulb, but the gas inside the bulb. When an electric current is passed through the gas, it undergoes a process called ionization, where the gas molecules become excited and release energy in the form of ultraviolet (UV) radiation.

However, UV radiation is not visible to the human eye. This is where the fluorescent coating on the glass comes into play. The coating is made up of phosphors, which are substances that can absorb UV radiation and re-emit it as visible light. When the UV radiation from the gas interacts with the phosphors, it causes them to glow and produce visible light, allowing us to see.

Therefore, the secondary source of light in a fluorescent lamp is the fluorescent coating on the glass, as it converts the UV radiation from the gas into visible light. Options 1, 2, and 3 are incorrect as they do not play a role in producing the light in a fluorescent lamp.